Model.js

define([
        '../Core/BoundingSphere',
        '../Core/Cartesian2',
        '../Core/Cartesian3',
        '../Core/Cartesian4',
        '../Core/Cartographic',
        '../Core/Check',
        '../Core/clone',
        '../Core/Color',
        '../Core/combine',
        '../Core/createGuid',
        '../Core/defaultValue',
        '../Core/defined',
        '../Core/defineProperties',
        '../Core/destroyObject',
        '../Core/DeveloperError',
        '../Core/DistanceDisplayCondition',
        '../Core/FeatureDetection',
        '../Core/getAbsoluteUri',
        '../Core/getMagic',
        '../Core/getStringFromTypedArray',
        '../Core/IndexDatatype',
        '../Core/isArray',
        '../Core/loadCRN',
        '../Core/loadImageFromTypedArray',
        '../Core/loadKTX',
        '../Core/Math',
        '../Core/Matrix3',
        '../Core/Matrix4',
        '../Core/PixelFormat',
        '../Core/PrimitiveType',
        '../Core/Quaternion',
        '../Core/Resource',
        '../Core/Transforms',
        '../Core/WebGLConstants',
        '../Renderer/Buffer',
        '../Renderer/BufferUsage',
        '../Renderer/DrawCommand',
        '../Renderer/Pass',
        '../Renderer/RenderState',
        '../Renderer/Sampler',
        '../Renderer/ShaderProgram',
        '../Renderer/ShaderSource',
        '../Renderer/Texture',
        '../Renderer/TextureMinificationFilter',
        '../Renderer/TextureWrap',
        '../Renderer/VertexArray',
        '../ThirdParty/GltfPipeline/addDefaults',
        '../ThirdParty/GltfPipeline/addPipelineExtras',
        '../ThirdParty/GltfPipeline/ForEach',
        '../ThirdParty/GltfPipeline/getAccessorByteStride',
        '../ThirdParty/GltfPipeline/hasExtension',
        '../ThirdParty/GltfPipeline/numberOfComponentsForType',
        '../ThirdParty/GltfPipeline/parseGlb',
        '../ThirdParty/GltfPipeline/removePipelineExtras',
        '../ThirdParty/GltfPipeline/updateVersion',
        '../ThirdParty/when',
        './Axis',
        './BlendingState',
        './ClippingPlaneCollection',
        './ColorBlendMode',
        './DracoLoader',
        './getClipAndStyleCode',
        './getClippingFunction',
        './HeightReference',
        './JobType',
        './ModelAnimationCache',
        './ModelAnimationCollection',
        './ModelLoadResources',
        './ModelMaterial',
        './ModelMesh',
        './ModelNode',
        './ModelUtility',
        './OctahedralProjectedCubeMap',
        './processModelMaterialsCommon',
        './processPbrMaterials',
        './SceneMode',
        './ShadowMode'
    ], function(
        BoundingSphere,
        Cartesian2,
        Cartesian3,
        Cartesian4,
        Cartographic,
        Check,
        clone,
        Color,
        combine,
        createGuid,
        defaultValue,
        defined,
        defineProperties,
        destroyObject,
        DeveloperError,
        DistanceDisplayCondition,
        FeatureDetection,
        getAbsoluteUri,
        getMagic,
        getStringFromTypedArray,
        IndexDatatype,
        isArray,
        loadCRN,
        loadImageFromTypedArray,
        loadKTX,
        CesiumMath,
        Matrix3,
        Matrix4,
        PixelFormat,
        PrimitiveType,
        Quaternion,
        Resource,
        Transforms,
        WebGLConstants,
        Buffer,
        BufferUsage,
        DrawCommand,
        Pass,
        RenderState,
        Sampler,
        ShaderProgram,
        ShaderSource,
        Texture,
        TextureMinificationFilter,
        TextureWrap,
        VertexArray,
        addDefaults,
        addPipelineExtras,
        ForEach,
        getAccessorByteStride,
        hasExtension,
        numberOfComponentsForType,
        parseGlb,
        removePipelineExtras,
        updateVersion,
        when,
        Axis,
        BlendingState,
        ClippingPlaneCollection,
        ColorBlendMode,
        DracoLoader,
        getClipAndStyleCode,
        getClippingFunction,
        HeightReference,
        JobType,
        ModelAnimationCache,
        ModelAnimationCollection,
        ModelLoadResources,
        ModelMaterial,
        ModelMesh,
        ModelNode,
        ModelUtility,
        OctahedralProjectedCubeMap,
        processModelMaterialsCommon,
        processPbrMaterials,
        SceneMode,
        ShadowMode) {
    'use strict';

    // Bail out if the browser doesn't support typed arrays, to prevent the setup function
    // from failing, since we won't be able to create a WebGL context anyway.
    if (!FeatureDetection.supportsTypedArrays()) {
        return {};
    }

    var boundingSphereCartesian3Scratch = new Cartesian3();

    var ModelState = ModelUtility.ModelState;

    // glTF MIME types discussed in https://github.com/KhronosGroup/glTF/issues/412 and https://github.com/KhronosGroup/glTF/issues/943
    var defaultModelAccept = 'model/gltf-binary,model/gltf+json;q=0.8,application/json;q=0.2,*/*;q=0.01';

    var articulationEpsilon = CesiumMath.EPSILON16;

    ///////////////////////////////////////////////////////////////////////////

    function setCachedGltf(model, cachedGltf) {
        model._cachedGltf = cachedGltf;
    }

    // glTF JSON can be big given embedded geometry, textures, and animations, so we
    // cache it across all models using the same url/cache-key.  This also reduces the
    // slight overhead in assigning defaults to missing values.
    //
    // Note that this is a global cache, compared to renderer resources, which
    // are cached per context.
    function CachedGltf(options) {
        this._gltf = options.gltf;
        this.ready = options.ready;
        this.modelsToLoad = [];
        this.count = 0;
    }

    defineProperties(CachedGltf.prototype, {
        gltf : {
            set : function(value) {
                this._gltf = value;
            },

            get : function() {
                return this._gltf;
            }
        }
    });

    CachedGltf.prototype.makeReady = function(gltfJson) {
        this.gltf = gltfJson;

        var models = this.modelsToLoad;
        var length = models.length;
        for (var i = 0; i < length; ++i) {
            var m = models[i];
            if (!m.isDestroyed()) {
                setCachedGltf(m, this);
            }
        }
        this.modelsToLoad = undefined;
        this.ready = true;
    };

    var gltfCache = {};
    var uriToGuid = {};
    ///////////////////////////////////////////////////////////////////////////

    /**
     * A 3D model based on glTF, the runtime asset format for WebGL, OpenGL ES, and OpenGL.
     * <p>
     * Cesium includes support for geometry and materials, glTF animations, and glTF skinning.
     * In addition, individual glTF nodes are pickable with {@link Scene#pick} and animatable
     * with {@link Model#getNode}.  glTF cameras and lights are not currently supported.
     * </p>
     * <p>
     * An external glTF asset is created with {@link Model.fromGltf}.  glTF JSON can also be
     * created at runtime and passed to this constructor function.  In either case, the
     * {@link Model#readyPromise} is resolved when the model is ready to render, i.e.,
     * when the external binary, image, and shader files are downloaded and the WebGL
     * resources are created.
     * </p>
     * <p>
     * Cesium supports glTF assets with the following extensions:
     * <ul>
     * <li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Khronos/KHR_binary_glTF/README.md|KHR_binary_glTF (glTF 1.0)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Khronos/KHR_materials_common/README.md|KHR_materials_common (glTF 1.0)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Vendor/WEB3D_quantized_attributes/README.md|WEB3D_quantized_attributes (glTF 1.0)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Vendor/AGI_articulations/README.md|AGI_articulations}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/pull/1302|KHR_blend (draft)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_draco_mesh_compression/README.md|KHR_draco_mesh_compression}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_pbrSpecularGlossiness/README.md|KHR_materials_pbrSpecularGlossiness}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_unlit/README.md|KHR_materials_unlit}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_techniques_webgl/README.md|KHR_techniques_webgl}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_texture_transform/README.md|KHR_texture_transform}
     * </li>
     * </ul>
     * </p>
     * <p>
     * For high-precision rendering, Cesium supports the {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Vendor/CESIUM_RTC/README.md|CESIUM_RTC} extension, which introduces the
     * CESIUM_RTC_MODELVIEW parameter semantic that says the node is in WGS84 coordinates translated
     * relative to a local origin.
     * </p>
     *
     * @alias Model
     * @constructor
     *
     * @param {Object} [options] Object with the following properties:
     * @param {Object|ArrayBuffer|Uint8Array} [options.gltf] A glTF JSON object, or a binary glTF buffer.
     * @param {Resource|String} [options.basePath=''] The base path that paths in the glTF JSON are relative to.
     * @param {Boolean} [options.show=true] Determines if the model primitive will be shown.
     * @param {Matrix4} [options.modelMatrix=Matrix4.IDENTITY] The 4x4 transformation matrix that transforms the model from model to world coordinates.
     * @param {Number} [options.scale=1.0] A uniform scale applied to this model.
     * @param {Number} [options.minimumPixelSize=0.0] The approximate minimum pixel size of the model regardless of zoom.
     * @param {Number} [options.maximumScale] The maximum scale size of a model. An upper limit for minimumPixelSize.
     * @param {Object} [options.id] A user-defined object to return when the model is picked with {@link Scene#pick}.
     * @param {Boolean} [options.allowPicking=true] When <code>true</code>, each glTF mesh and primitive is pickable with {@link Scene#pick}.
     * @param {Boolean} [options.incrementallyLoadTextures=true] Determine if textures may continue to stream in after the model is loaded.
     * @param {Boolean} [options.asynchronous=true] Determines if model WebGL resource creation will be spread out over several frames or block until completion once all glTF files are loaded.
     * @param {Boolean} [options.clampAnimations=true] Determines if the model's animations should hold a pose over frames where no keyframes are specified.
     * @param {ShadowMode} [options.shadows=ShadowMode.ENABLED] Determines whether the model casts or receives shadows from each light source.
     * @param {Boolean} [options.debugShowBoundingVolume=false] For debugging only. Draws the bounding sphere for each draw command in the model.
     * @param {Boolean} [options.debugWireframe=false] For debugging only. Draws the model in wireframe.
     * @param {HeightReference} [options.heightReference=HeightReference.NONE] Determines how the model is drawn relative to terrain.
     * @param {Scene} [options.scene] Must be passed in for models that use the height reference property.
     * @param {DistanceDisplayCondition} [options.distanceDisplayCondition] The condition specifying at what distance from the camera that this model will be displayed.
     * @param {Color} [options.color=Color.WHITE] A color that blends with the model's rendered color.
     * @param {ColorBlendMode} [options.colorBlendMode=ColorBlendMode.HIGHLIGHT] Defines how the color blends with the model.
     * @param {Number} [options.colorBlendAmount=0.5] Value used to determine the color strength when the <code>colorBlendMode</code> is <code>MIX</code>. A value of 0.0 results in the model's rendered color while a value of 1.0 results in a solid color, with any value in-between resulting in a mix of the two.
     * @param {Color} [options.silhouetteColor=Color.RED] The silhouette color. If more than 256 models have silhouettes enabled, there is a small chance that overlapping models will have minor artifacts.
     * @param {Number} [options.silhouetteSize=0.0] The size of the silhouette in pixels.
     * @param {ClippingPlaneCollection} [options.clippingPlanes] The {@link ClippingPlaneCollection} used to selectively disable rendering the model.
     * @param {Boolean} [options.dequantizeInShader=true] Determines if a {@link https://github.com/google/draco|Draco} encoded model is dequantized on the GPU. This decreases total memory usage for encoded models.
     * @param {Cartesian2} [options.imageBasedLightingFactor=Cartesian2(1.0, 1.0)] Scales diffuse and specular image-based lighting from the earth, sky, atmosphere and star skybox.
     * @param {Cartesian3} [options.lightColor] The color and intensity of the sunlight used to shade the model.
     * @param {Number} [options.luminanceAtZenith=0.5] The sun's luminance at the zenith in kilo candela per meter squared to use for this model's procedural environment map.
     * @param {Cartesian3[]} [options.sphericalHarmonicCoefficients] The third order spherical harmonic coefficients used for the diffuse color of image-based lighting.
     * @param {String} [options.specularEnvironmentMaps] A URL to a KTX file that contains a cube map of the specular lighting and the convoluted specular mipmaps.
     *
     * @see Model.fromGltf
     *
     * @demo {@link https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=3D%20Models.html|Cesium Sandcastle Models Demo}
     */
    function Model(options) {
        options = defaultValue(options, defaultValue.EMPTY_OBJECT);

        var cacheKey = options.cacheKey;
        this._cacheKey = cacheKey;
        this._cachedGltf = undefined;
        this._releaseGltfJson = defaultValue(options.releaseGltfJson, false);

        var cachedGltf;
        if (defined(cacheKey) && defined(gltfCache[cacheKey]) && gltfCache[cacheKey].ready) {
            // glTF JSON is in cache and ready
            cachedGltf = gltfCache[cacheKey];
            ++cachedGltf.count;
        } else {
            // glTF was explicitly provided, e.g., when a user uses the Model constructor directly
            var gltf = options.gltf;

            if (defined(gltf)) {
                if (gltf instanceof ArrayBuffer) {
                    gltf = new Uint8Array(gltf);
                }

                if (gltf instanceof Uint8Array) {
                    // Binary glTF
                    var parsedGltf = parseGlb(gltf);

                    cachedGltf = new CachedGltf({
                        gltf : parsedGltf,
                        ready : true
                    });
                } else {
                    // Normal glTF (JSON)
                    cachedGltf = new CachedGltf({
                        gltf : options.gltf,
                        ready : true
                    });
                }

                cachedGltf.count = 1;

                if (defined(cacheKey)) {
                    gltfCache[cacheKey] = cachedGltf;
                }
            }
        }
        setCachedGltf(this, cachedGltf);

        var basePath = defaultValue(options.basePath, '');
        this._resource = Resource.createIfNeeded(basePath);

        /**
         * Determines if the model primitive will be shown.
         *
         * @type {Boolean}
         *
         * @default true
         */
        this.show = defaultValue(options.show, true);

        /**
         * The silhouette color.
         *
         * @type {Color}
         *
         * @default Color.RED
         */
        this.silhouetteColor = defaultValue(options.silhouetteColor, Color.RED);
        this._silhouetteColor = new Color();
        this._silhouetteColorPreviousAlpha = 1.0;
        this._normalAttributeName = undefined;

        /**
         * The size of the silhouette in pixels.
         *
         * @type {Number}
         *
         * @default 0.0
         */
        this.silhouetteSize = defaultValue(options.silhouetteSize, 0.0);

        /**
         * The 4x4 transformation matrix that transforms the model from model to world coordinates.
         * When this is the identity matrix, the model is drawn in world coordinates, i.e., Earth's WGS84 coordinates.
         * Local reference frames can be used by providing a different transformation matrix, like that returned
         * by {@link Transforms.eastNorthUpToFixedFrame}.
         *
         * @type {Matrix4}
         *
         * @default {@link Matrix4.IDENTITY}
         *
         * @example
         * var origin = Cesium.Cartesian3.fromDegrees(-95.0, 40.0, 200000.0);
         * m.modelMatrix = Cesium.Transforms.eastNorthUpToFixedFrame(origin);
         */
        this.modelMatrix = Matrix4.clone(defaultValue(options.modelMatrix, Matrix4.IDENTITY));
        this._modelMatrix = Matrix4.clone(this.modelMatrix);
        this._clampedModelMatrix = undefined;

        /**
         * A uniform scale applied to this model before the {@link Model#modelMatrix}.
         * Values greater than <code>1.0</code> increase the size of the model; values
         * less than <code>1.0</code> decrease.
         *
         * @type {Number}
         *
         * @default 1.0
         */
        this.scale = defaultValue(options.scale, 1.0);
        this._scale = this.scale;

        /**
         * The approximate minimum pixel size of the model regardless of zoom.
         * This can be used to ensure that a model is visible even when the viewer
         * zooms out.  When <code>0.0</code>, no minimum size is enforced.
         *
         * @type {Number}
         *
         * @default 0.0
         */
        this.minimumPixelSize = defaultValue(options.minimumPixelSize, 0.0);
        this._minimumPixelSize = this.minimumPixelSize;

        /**
         * The maximum scale size for a model. This can be used to give
         * an upper limit to the {@link Model#minimumPixelSize}, ensuring that the model
         * is never an unreasonable scale.
         *
         * @type {Number}
         */
        this.maximumScale = options.maximumScale;
        this._maximumScale = this.maximumScale;

        /**
         * User-defined object returned when the model is picked.
         *
         * @type Object
         *
         * @default undefined
         *
         * @see Scene#pick
         */
        this.id = options.id;
        this._id = options.id;

        /**
         * Returns the height reference of the model
         *
         * @type {HeightReference}
         *
         * @default HeightReference.NONE
         */
        this.heightReference = defaultValue(options.heightReference, HeightReference.NONE);
        this._heightReference = this.heightReference;
        this._heightChanged = false;
        this._removeUpdateHeightCallback = undefined;
        var scene = options.scene;
        this._scene = scene;
        if (defined(scene) && defined(scene.terrainProviderChanged)) {
            this._terrainProviderChangedCallback = scene.terrainProviderChanged.addEventListener(function() {
                this._heightChanged = true;
            }, this);
        }

        /**
         * Used for picking primitives that wrap a model.
         *
         * @private
         */
        this._pickObject = options.pickObject;
        this._allowPicking = defaultValue(options.allowPicking, true);

        this._ready = false;
        this._readyPromise = when.defer();

        /**
         * The currently playing glTF animations.
         *
         * @type {ModelAnimationCollection}
         */
        this.activeAnimations = new ModelAnimationCollection(this);

        /**
         * Determines if the model's animations should hold a pose over frames where no keyframes are specified.
         *
         * @type {Boolean}
         */
        this.clampAnimations = defaultValue(options.clampAnimations, true);

        this._defaultTexture = undefined;
        this._incrementallyLoadTextures = defaultValue(options.incrementallyLoadTextures, true);
        this._asynchronous = defaultValue(options.asynchronous, true);

        /**
         * Determines whether the model casts or receives shadows from each light source.
         *
         * @type {ShadowMode}
         *
         * @default ShadowMode.ENABLED
         */
        this.shadows = defaultValue(options.shadows, ShadowMode.ENABLED);
        this._shadows = this.shadows;

        /**
         * A color that blends with the model's rendered color.
         *
         * @type {Color}
         *
         * @default Color.WHITE
         */
        this.color = Color.clone(defaultValue(options.color, Color.WHITE));
        this._colorPreviousAlpha = 1.0;

        /**
         * Defines how the color blends with the model.
         *
         * @type {ColorBlendMode}
         *
         * @default ColorBlendMode.HIGHLIGHT
         */
        this.colorBlendMode = defaultValue(options.colorBlendMode, ColorBlendMode.HIGHLIGHT);

        /**
         * Value used to determine the color strength when the <code>colorBlendMode</code> is <code>MIX</code>.
         * A value of 0.0 results in the model's rendered color while a value of 1.0 results in a solid color, with
         * any value in-between resulting in a mix of the two.
         *
         * @type {Number}
         *
         * @default 0.5
         */
        this.colorBlendAmount = defaultValue(options.colorBlendAmount, 0.5);

        this._colorShadingEnabled = false;

        this._clippingPlanes = undefined;
        this.clippingPlanes = options.clippingPlanes;
        // Used for checking if shaders need to be regenerated due to clipping plane changes.
        this._clippingPlanesState = 0;
        // If defined, use this matrix to position the clipping planes instead of the modelMatrix.
        // This is so that when models are part of a tileset they all get clipped relative
        // to the root tile.
        this.clippingPlanesOriginMatrix = undefined;

        /**
         * This property is for debugging only; it is not for production use nor is it optimized.
         * <p>
         * Draws the bounding sphere for each draw command in the model.  A glTF primitive corresponds
         * to one draw command.  A glTF mesh has an array of primitives, often of length one.
         * </p>
         *
         * @type {Boolean}
         *
         * @default false
         */
        this.debugShowBoundingVolume = defaultValue(options.debugShowBoundingVolume, false);
        this._debugShowBoundingVolume = false;

        /**
         * This property is for debugging only; it is not for production use nor is it optimized.
         * <p>
         * Draws the model in wireframe.
         * </p>
         *
         * @type {Boolean}
         *
         * @default false
         */
        this.debugWireframe = defaultValue(options.debugWireframe, false);
        this._debugWireframe = false;

        this._distanceDisplayCondition = options.distanceDisplayCondition;

        // Undocumented options
        this._addBatchIdToGeneratedShaders = options.addBatchIdToGeneratedShaders;
        this._precreatedAttributes = options.precreatedAttributes;
        this._vertexShaderLoaded = options.vertexShaderLoaded;
        this._fragmentShaderLoaded = options.fragmentShaderLoaded;
        this._uniformMapLoaded = options.uniformMapLoaded;
        this._pickIdLoaded = options.pickIdLoaded;
        this._ignoreCommands = defaultValue(options.ignoreCommands, false);
        this._requestType = options.requestType;
        this._upAxis = defaultValue(options.upAxis, Axis.Y);
        this._gltfForwardAxis = Axis.Z;
        this._forwardAxis = options.forwardAxis;

        /**
         * @private
         * @readonly
         */
        this.cull = defaultValue(options.cull, true);

        /**
         * @private
         * @readonly
         */
        this.opaquePass = defaultValue(options.opaquePass, Pass.OPAQUE);

        this._computedModelMatrix = new Matrix4(); // Derived from modelMatrix and scale
        this._clippingPlaneModelViewMatrix = Matrix4.clone(Matrix4.IDENTITY); // Derived from modelMatrix, scale, and the current view matrix
        this._initialRadius = undefined;           // Radius without model's scale property, model-matrix scale, animations, or skins
        this._boundingSphere = undefined;
        this._scaledBoundingSphere = new BoundingSphere();
        this._state = ModelState.NEEDS_LOAD;
        this._loadResources = undefined;

        this._mode = undefined;

        this._perNodeShowDirty = false;            // true when the Cesium API was used to change a node's show property
        this._cesiumAnimationsDirty = false;       // true when the Cesium API, not a glTF animation, changed a node transform
        this._dirty = false;                       // true when the model was transformed this frame
        this._maxDirtyNumber = 0;                  // Used in place of a dirty boolean flag to avoid an extra graph traversal

        this._runtime = {
            animations : undefined,
            articulationsByName : undefined,
            articulationsByStageKey : undefined,
            stagesByKey : undefined,
            rootNodes : undefined,
            nodes : undefined,            // Indexed with the node's index
            nodesByName : undefined,      // Indexed with name property in the node
            skinnedNodes : undefined,
            meshesByName : undefined,     // Indexed with the name property in the mesh
            materialsByName : undefined,  // Indexed with the name property in the material
            materialsById : undefined     // Indexed with the material's index
        };

        this._uniformMaps = {};           // Not cached since it can be targeted by glTF animation
        this._extensionsUsed = undefined;     // Cached used glTF extensions
        this._extensionsRequired = undefined; // Cached required glTF extensions
        this._quantizedUniforms = {};     // Quantized uniforms for each program for WEB3D_quantized_attributes
        this._programPrimitives = {};
        this._rendererResources = {       // Cached between models with the same url/cache-key
            buffers : {},
            vertexArrays : {},
            programs : {},
            sourceShaders : {},
            silhouettePrograms : {},
            textures : {},
            samplers : {},
            renderStates : {}
        };
        this._cachedRendererResources = undefined;
        this._loadRendererResourcesFromCache = false;

        this._dequantizeInShader = defaultValue(options.dequantizeInShader, true);
        this._decodedData = {};

        this._cachedGeometryByteLength = 0;
        this._cachedTexturesByteLength = 0;
        this._geometryByteLength = 0;
        this._texturesByteLength = 0;
        this._trianglesLength = 0;

        // Hold references for shader reconstruction.
        // Hold these separately because _cachedGltf may get released (this.releaseGltfJson)
        this._sourceTechniques = {};
        this._sourcePrograms = {};
        this._quantizedVertexShaders = {};

        this._nodeCommands = [];
        this._pickIds = [];

        // CESIUM_RTC extension
        this._rtcCenter = undefined;    // reference to either 3D or 2D
        this._rtcCenterEye = undefined; // in eye coordinates
        this._rtcCenter3D = undefined;  // in world coordinates
        this._rtcCenter2D = undefined;  // in projected world coordinates

        this._sourceVersion = undefined;
        this._sourceKHRTechniquesWebGL = undefined;

        this._imageBasedLightingFactor = new Cartesian2(1.0, 1.0);
        Cartesian2.clone(options.imageBasedLightingFactor, this._imageBasedLightingFactor);
        this._lightColor = Cartesian3.clone(options.lightColor);

        this._luminanceAtZenith = undefined;
        this.luminanceAtZenith = defaultValue(options.luminanceAtZenith, 0.5);

        this._sphericalHarmonicCoefficients = options.sphericalHarmonicCoefficients;
        this._specularEnvironmentMaps = options.specularEnvironmentMaps;
        this._shouldUpdateSpecularMapAtlas = true;
        this._specularEnvironmentMapAtlas = undefined;

        this._useDefaultSphericalHarmonics = false;
        this._useDefaultSpecularMaps = false;

        this._shouldRegenerateShaders = false;
    }

    defineProperties(Model.prototype, {
        /**
         * The object for the glTF JSON, including properties with default values omitted
         * from the JSON provided to this model.
         *
         * @memberof Model.prototype
         *
         * @type {Object}
         * @readonly
         *
         * @default undefined
         */
        gltf : {
            get : function() {
                return defined(this._cachedGltf) ? this._cachedGltf.gltf : undefined;
            }
        },

        /**
         * When <code>true</code>, the glTF JSON is not stored with the model once the model is
         * loaded (when {@link Model#ready} is <code>true</code>).  This saves memory when
         * geometry, textures, and animations are embedded in the .gltf file, which is the
         * default for the {@link https://cesiumjs.org/convertmodel.html|Cesium model converter}.
         * This is especially useful for cases like 3D buildings, where each .gltf model is unique
         * and caching the glTF JSON is not effective.
         *
         * @memberof Model.prototype
         *
         * @type {Boolean}
         * @readonly
         *
         * @default false
         *
         * @private
         */
        releaseGltfJson : {
            get : function() {
                return this._releaseGltfJson;
            }
        },

        /**
         * The key identifying this model in the model cache for glTF JSON, renderer resources, and animations.
         * Caching saves memory and improves loading speed when several models with the same url are created.
         * <p>
         * This key is automatically generated when the model is created with {@link Model.fromGltf}.  If the model
         * is created directly from glTF JSON using the {@link Model} constructor, this key can be manually
         * provided; otherwise, the model will not be changed.
         * </p>
         *
         * @memberof Model.prototype
         *
         * @type {String}
         * @readonly
         *
         * @private
         */
        cacheKey : {
            get : function() {
                return this._cacheKey;
            }
        },

        /**
         * The base path that paths in the glTF JSON are relative to.  The base
         * path is the same path as the path containing the .gltf file
         * minus the .gltf file, when binary, image, and shader files are
         * in the same directory as the .gltf.  When this is <code>''</code>,
         * the app's base path is used.
         *
         * @memberof Model.prototype
         *
         * @type {String}
         * @readonly
         *
         * @default ''
         */
        basePath : {
            get : function() {
                return this._resource.url;
            }
        },

        /**
         * The model's bounding sphere in its local coordinate system.  This does not take into
         * account glTF animations and skins nor does it take into account {@link Model#minimumPixelSize}.
         *
         * @memberof Model.prototype
         *
         * @type {BoundingSphere}
         * @readonly
         *
         * @default undefined
         *
         * @exception {DeveloperError} The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.
         *
         * @example
         * // Center in WGS84 coordinates
         * var center = Cesium.Matrix4.multiplyByPoint(model.modelMatrix, model.boundingSphere.center, new Cesium.Cartesian3());
         */
        boundingSphere : {
            get : function() {
                //>>includeStart('debug', pragmas.debug);
                if (this._state !== ModelState.LOADED) {
                    throw new DeveloperError('The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.');
                }
                //>>includeEnd('debug');

                var modelMatrix = this.modelMatrix;
                if ((this.heightReference !== HeightReference.NONE) && this._clampedModelMatrix) {
                    modelMatrix = this._clampedModelMatrix;
                }

                var nonUniformScale = Matrix4.getScale(modelMatrix, boundingSphereCartesian3Scratch);
                var scale = defined(this.maximumScale) ? Math.min(this.maximumScale, this.scale) : this.scale;
                Cartesian3.multiplyByScalar(nonUniformScale, scale, nonUniformScale);

                var scaledBoundingSphere = this._scaledBoundingSphere;
                scaledBoundingSphere.center = Cartesian3.multiplyComponents(this._boundingSphere.center, nonUniformScale, scaledBoundingSphere.center);
                scaledBoundingSphere.radius = Cartesian3.maximumComponent(nonUniformScale) * this._initialRadius;

                if (defined(this._rtcCenter)) {
                    Cartesian3.add(this._rtcCenter, scaledBoundingSphere.center, scaledBoundingSphere.center);
                }

                return scaledBoundingSphere;
            }
        },

        /**
         * When <code>true</code>, this model is ready to render, i.e., the external binary, image,
         * and shader files were downloaded and the WebGL resources were created.  This is set to
         * <code>true</code> right before {@link Model#readyPromise} is resolved.
         *
         * @memberof Model.prototype
         *
         * @type {Boolean}
         * @readonly
         *
         * @default false
         */
        ready : {
            get : function() {
                return this._ready;
            }
        },

        /**
         * Gets the promise that will be resolved when this model is ready to render, i.e., when the external binary, image,
         * and shader files were downloaded and the WebGL resources were created.
         * <p>
         * This promise is resolved at the end of the frame before the first frame the model is rendered in.
         * </p>
         *
         * @memberof Model.prototype
         * @type {Promise.<Model>}
         * @readonly
         *
         * @example
         * // Play all animations at half-speed when the model is ready to render
         * Cesium.when(model.readyPromise).then(function(model) {
         *   model.activeAnimations.addAll({
         *     multiplier : 0.5
         *   });
         * }).otherwise(function(error){
         *   window.alert(error);
         * });
         *
         * @see Model#ready
         */
        readyPromise : {
            get : function() {
                return this._readyPromise.promise;
            }
        },

        /**
         * Determines if model WebGL resource creation will be spread out over several frames or
         * block until completion once all glTF files are loaded.
         *
         * @memberof Model.prototype
         *
         * @type {Boolean}
         * @readonly
         *
         * @default true
         */
        asynchronous : {
            get : function() {
                return this._asynchronous;
            }
        },

        /**
         * When <code>true</code>, each glTF mesh and primitive is pickable with {@link Scene#pick}.  When <code>false</code>, GPU memory is saved.
         *
         * @memberof Model.prototype
         *
         * @type {Boolean}
         * @readonly
         *
         * @default true
         */
        allowPicking : {
            get : function() {
                return this._allowPicking;
            }
        },

        /**
         * Determine if textures may continue to stream in after the model is loaded.
         *
         * @memberof Model.prototype
         *
         * @type {Boolean}
         * @readonly
         *
         * @default true
         */
        incrementallyLoadTextures : {
            get : function() {
                return this._incrementallyLoadTextures;
            }
        },

        /**
         * Return the number of pending texture loads.
         *
         * @memberof Model.prototype
         *
         * @type {Number}
         * @readonly
         */
        pendingTextureLoads : {
            get : function() {
                return defined(this._loadResources) ? this._loadResources.pendingTextureLoads : 0;
            }
        },

        /**
         * Returns true if the model was transformed this frame
         *
         * @memberof Model.prototype
         *
         * @type {Boolean}
         * @readonly
         *
         * @private
         */
        dirty : {
            get : function() {
                return this._dirty;
            }
        },

        /**
         * Gets or sets the condition specifying at what distance from the camera that this model will be displayed.
         * @memberof Model.prototype
         * @type {DistanceDisplayCondition}
         * @default undefined
         */
        distanceDisplayCondition : {
            get : function() {
                return this._distanceDisplayCondition;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (defined(value) && value.far <= value.near) {
                    throw new DeveloperError('far must be greater than near');
                }
                //>>includeEnd('debug');
                this._distanceDisplayCondition = DistanceDisplayCondition.clone(value, this._distanceDisplayCondition);
            }
        },

        extensionsUsed : {
            get : function() {
                if (!defined(this._extensionsUsed)) {
                    this._extensionsUsed = ModelUtility.getUsedExtensions(this.gltf);
                }
                return this._extensionsUsed;
            }
        },

        extensionsRequired : {
            get : function() {
                if (!defined(this._extensionsRequired)) {
                    this._extensionsRequired = ModelUtility.getRequiredExtensions(this.gltf);
                }
                return this._extensionsRequired;
            }
        },

        /**
         * Gets the model's up-axis.
         * By default models are y-up according to the glTF spec, however geo-referenced models will typically be z-up.
         *
         * @memberof Model.prototype
         *
         * @type {Number}
         * @default Axis.Y
         * @readonly
         *
         * @private
         */
        upAxis : {
            get : function() {
                return this._upAxis;
            }
        },

        /**
         * Gets the model's forward axis.
         * By default, glTF 2.0 models are z-forward according to the glTF spec, however older
         * glTF (1.0, 0.8) models used x-forward.  Note that only Axis.X and Axis.Z are supported.
         *
         * @memberof Model.prototype
         *
         * @type {Number}
         * @default Axis.Z
         * @readonly
         *
         * @private
         */
        forwardAxis : {
            get : function() {
                if (defined(this._forwardAxis)) {
                    return this._forwardAxis;
                }
                return this._gltfForwardAxis;
            }
        },

        /**
         * Gets the model's triangle count.
         *
         * @private
         */
        trianglesLength : {
            get : function() {
                return this._trianglesLength;
            }
        },

        /**
         * Gets the model's geometry memory in bytes. This includes all vertex and index buffers.
         *
         * @private
         */
        geometryByteLength : {
            get : function() {
                return this._geometryByteLength;
            }
        },

        /**
         * Gets the model's texture memory in bytes.
         *
         * @private
         */
        texturesByteLength : {
            get : function() {
                return this._texturesByteLength;
            }
        },

        /**
         * Gets the model's cached geometry memory in bytes. This includes all vertex and index buffers.
         *
         * @private
         */
        cachedGeometryByteLength : {
            get : function() {
                return this._cachedGeometryByteLength;
            }
        },

        /**
         * Gets the model's cached texture memory in bytes.
         *
         * @private
         */
        cachedTexturesByteLength : {
            get : function() {
                return this._cachedTexturesByteLength;
            }
        },

        /**
         * The {@link ClippingPlaneCollection} used to selectively disable rendering the model.
         *
         * @memberof Model.prototype
         *
         * @type {ClippingPlaneCollection}
         */
        clippingPlanes : {
            get : function() {
                return this._clippingPlanes;
            },
            set : function(value) {
                if (value === this._clippingPlanes) {
                    return;
                }
                // Handle destroying, checking of unknown, checking for existing ownership
                ClippingPlaneCollection.setOwner(value, this, '_clippingPlanes');
            }
        },

        /**
         * @private
         */
        pickIds : {
            get : function() {
                return this._pickIds;
            }
        },

        /**
         * Cesium adds lighting from the earth, sky, atmosphere, and star skybox. This cartesian is used to scale the final
         * diffuse and specular lighting contribution from those sources to the final color. A value of 0.0 will disable those light sources.
         *
         * @memberof Model.prototype
         *
         * @type {Cartesian2}
         * @default Cartesian2(1.0, 1.0)
         */
        imageBasedLightingFactor : {
            get : function() {
                return this._imageBasedLightingFactor;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                Check.typeOf.object('imageBasedLightingFactor', value);
                Check.typeOf.number.greaterThanOrEquals('imageBasedLightingFactor.x', value.x, 0.0);
                Check.typeOf.number.lessThanOrEquals('imageBasedLightingFactor.x', value.x, 1.0);
                Check.typeOf.number.greaterThanOrEquals('imageBasedLightingFactor.y', value.y, 0.0);
                Check.typeOf.number.lessThanOrEquals('imageBasedLightingFactor.y', value.y, 1.0);
                //>>includeEnd('debug');
                var imageBasedLightingFactor = this._imageBasedLightingFactor;
                if ((value === imageBasedLightingFactor) || Cartesian2.equals(value, imageBasedLightingFactor)) {
                    return;
                }
                this._shouldRegenerateShaders = this._shouldRegenerateShaders || (this._imageBasedLightingFactor.x > 0.0 && value.x === 0.0) || (this._imageBasedLightingFactor.x === 0.0 && value.x > 0.0);
                this._shouldRegenerateShaders = this._shouldRegenerateShaders || (this._imageBasedLightingFactor.y > 0.0 && value.y === 0.0) || (this._imageBasedLightingFactor.y === 0.0 && value.y > 0.0);
                Cartesian2.clone(value, this._imageBasedLightingFactor);
            }
        },

        /**
         * The color and intensity of the sunlight used to shade the model.
         * <p>
         * For example, disabling additional light sources by setting <code>model.imageBasedLightingFactor = new Cesium.Cartesian2(0.0, 0.0)</code> will make the
         * model much darker. Here, increasing the intensity of the light source will make the model brighter.
         * </p>
         *
         * @memberof Model.prototype
         *
         * @type {Cartesian3}
         * @default undefined
         */
        lightColor : {
            get : function() {
                return this._lightColor;
            },
            set : function(value) {
                var lightColor = this._lightColor;
                if (value === lightColor || Cartesian3.equals(value, lightColor)) {
                    return;
                }
                this._shouldRegenerateShaders = this._shouldRegenerateShaders || (defined(lightColor) && !defined(value)) || (defined(value) && !defined(lightColor));
                this._lightColor = Cartesian3.clone(value, lightColor);
            }
        },

        /**
         * The sun's luminance at the zenith in kilo candela per meter squared to use for this model's procedural environment map.
         * This is used when {@link Model#specularEnvironmentMaps} and {@link Model#sphericalHarmonicCoefficients} are not defined.
         *
         * @memberof Model.prototype
         *
         * @demo {@link https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Image-Based Lighting.html|Sandcastle Image Based Lighting Demo}
         * @type {Number}
         * @default 0.5
         */
        luminanceAtZenith : {
            get : function() {
                return this._luminanceAtZenith;
            },
            set : function(value) {
                var lum = this._luminanceAtZenith;
                if (value === lum) {
                    return;
                }
                this._shouldRegenerateShaders = this._shouldRegenerateShaders || (defined(lum) && !defined(value)) || (defined(value) && !defined(lum));
                this._luminanceAtZenith = value;
            }
        },

        /**
         * The third order spherical harmonic coefficients used for the diffuse color of image-based lighting. When <code>undefined</code>, a diffuse irradiance
         * computed from the atmosphere color is used.
         * <p>
         * There are nine <code>Cartesian3</code> coefficients.
         * The order of the coefficients is: L<sub>00</sub>, L<sub>1-1</sub>, L<sub>10</sub>, L<sub>11</sub>, L<sub>2-2</sub>, L<sub>2-1</sub>, L<sub>20</sub>, L<sub>21</sub>, L<sub>22</sub>
         * </p>
         *
         * These values can be obtained by preprocessing the environment map using the <code>cmgen</code> tool of
         * {@link https://github.com/google/filament/releases | Google's Filament project}. This will also generate a KTX file that can be
         * supplied to {@link Model#specularEnvironmentMaps}.
         *
         * @memberof Model.prototype
         *
         * @type {Cartesian3[]}
         * @demo {@link https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Image-Based Lighting.html|Sandcastle Image Based Lighting Demo}
         * @see {@link https://graphics.stanford.edu/papers/envmap/envmap.pdf|An Efficient Representation for Irradiance Environment Maps}
         */
        sphericalHarmonicCoefficients : {
            get : function() {
                return this._sphericalHarmonicCoefficients;
            },
            set : function(value) {
                //>>includeStart('debug', pragmas.debug);
                if (defined(value) && (!isArray(value) || value.length !== 9)) {
                    throw new DeveloperError('sphericalHarmonicCoefficients must be an array of 9 Cartesian3 values.');
                }
                //>>includeEnd('debug');
                if (value === this._sphericalHarmonicCoefficients) {
                    return;
                }
                this._sphericalHarmonicCoefficients = value;
                this._shouldRegenerateShaders = true;
            }
        },

        /**
         * A URL to a KTX file that contains a cube map of the specular lighting and the convoluted specular mipmaps.
         *
         * @memberof Model.prototype
         * @demo {@link https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Image-Based Lighting.html|Sandcastle Image Based Lighting Demo}
         * @type {String}
         * @see Model#sphericalHarmonicCoefficients
         */
        specularEnvironmentMaps : {
            get : function() {
                return this._specularEnvironmentMaps;
            },
            set : function(value) {
                this._shouldUpdateSpecularMapAtlas = this._shouldUpdateSpecularMapAtlas || value !== this._specularEnvironmentMaps;
                this._specularEnvironmentMaps = value;
            }
        }
    });

    function silhouetteSupported(context) {
        return context.stencilBuffer;
    }

    function isColorShadingEnabled(model) {
        return !Color.equals(model.color, Color.WHITE) || (model.colorBlendMode !== ColorBlendMode.HIGHLIGHT);
    }

    function isClippingEnabled(model) {
        var clippingPlanes = model._clippingPlanes;
        return defined(clippingPlanes) && clippingPlanes.enabled && clippingPlanes.length !== 0;
    }

    /**
     * Determines if silhouettes are supported.
     *
     * @param {Scene} scene The scene.
     * @returns {Boolean} <code>true</code> if silhouettes are supported; otherwise, returns <code>false</code>
     */
    Model.silhouetteSupported = function(scene) {
        return silhouetteSupported(scene.context);
    };

    function containsGltfMagic(uint8Array) {
        var magic = getMagic(uint8Array);
        return magic === 'glTF';
    }

    /**
     * <p>
     * Creates a model from a glTF asset.  When the model is ready to render, i.e., when the external binary, image,
     * and shader files are downloaded and the WebGL resources are created, the {@link Model#readyPromise} is resolved.
     * </p>
     * <p>
     * The model can be a traditional glTF asset with a .gltf extension or a Binary glTF using the .glb extension.
     * </p>
     * <p>
     * Cesium supports glTF assets with the following extensions:
     * <ul>
     * <li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Khronos/KHR_binary_glTF/README.md|KHR_binary_glTF (glTF 1.0)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Khronos/KHR_materials_common/README.md|KHR_materials_common (glTF 1.0)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Vendor/WEB3D_quantized_attributes/README.md|WEB3D_quantized_attributes (glTF 1.0)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Vendor/AGI_articulations/README.md|AGI_articulations}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/pull/1302|KHR_blend (draft)}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_draco_mesh_compression/README.md|KHR_draco_mesh_compression}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_pbrSpecularGlossiness/README.md|KHR_materials_pbrSpecularGlossiness}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_unlit/README.md|KHR_materials_unlit}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_techniques_webgl/README.md|KHR_techniques_webgl}
     * </li><li>
     * {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_texture_transform/README.md|KHR_texture_transform}
     * </li>
     * </ul>
     * </p>
     * <p>
     * For high-precision rendering, Cesium supports the {@link https://github.com/KhronosGroup/glTF/blob/master/extensions/1.0/Vendor/CESIUM_RTC/README.md|CESIUM_RTC} extension, which introduces the
     * CESIUM_RTC_MODELVIEW parameter semantic that says the node is in WGS84 coordinates translated
     * relative to a local origin.
     * </p>
     *
     * @param {Object} options Object with the following properties:
     * @param {Resource|String} options.url The url to the .gltf file.
     * @param {Resource|String} [options.basePath] The base path that paths in the glTF JSON are relative to.
     * @param {Boolean} [options.show=true] Determines if the model primitive will be shown.
     * @param {Matrix4} [options.modelMatrix=Matrix4.IDENTITY] The 4x4 transformation matrix that transforms the model from model to world coordinates.
     * @param {Number} [options.scale=1.0] A uniform scale applied to this model.
     * @param {Number} [options.minimumPixelSize=0.0] The approximate minimum pixel size of the model regardless of zoom.
     * @param {Number} [options.maximumScale] The maximum scale for the model.
     * @param {Object} [options.id] A user-defined object to return when the model is picked with {@link Scene#pick}.
     * @param {Boolean} [options.allowPicking=true] When <code>true</code>, each glTF mesh and primitive is pickable with {@link Scene#pick}.
     * @param {Boolean} [options.incrementallyLoadTextures=true] Determine if textures may continue to stream in after the model is loaded.
     * @param {Boolean} [options.asynchronous=true] Determines if model WebGL resource creation will be spread out over several frames or block until completion once all glTF files are loaded.
     * @param {Boolean} [options.clampAnimations=true] Determines if the model's animations should hold a pose over frames where no keyframes are specified.
     * @param {ShadowMode} [options.shadows=ShadowMode.ENABLED] Determines whether the model casts or receives shadows from each light source.
     * @param {Boolean} [options.debugShowBoundingVolume=false] For debugging only. Draws the bounding sphere for each {@link DrawCommand} in the model.
     * @param {Boolean} [options.debugWireframe=false] For debugging only. Draws the model in wireframe.
     * @param {HeightReference} [options.heightReference=HeightReference.NONE] Determines how the model is drawn relative to terrain.
     * @param {Scene} [options.scene] Must be passed in for models that use the height reference property.
     * @param {DistanceDisplayCondition} [options.distanceDisplayCondition] The condition specifying at what distance from the camera that this model will be displayed.
     * @param {Color} [options.color=Color.WHITE] A color that blends with the model's rendered color.
     * @param {ColorBlendMode} [options.colorBlendMode=ColorBlendMode.HIGHLIGHT] Defines how the color blends with the model.
     * @param {Number} [options.colorBlendAmount=0.5] Value used to determine the color strength when the <code>colorBlendMode</code> is <code>MIX</code>. A value of 0.0 results in the model's rendered color while a value of 1.0 results in a solid color, with any value in-between resulting in a mix of the two.
     * @param {Color} [options.silhouetteColor=Color.RED] The silhouette color. If more than 256 models have silhouettes enabled, there is a small chance that overlapping models will have minor artifacts.
     * @param {Number} [options.silhouetteSize=0.0] The size of the silhouette in pixels.
     * @param {ClippingPlaneCollection} [options.clippingPlanes] The {@link ClippingPlaneCollection} used to selectively disable rendering the model.
     * @param {Boolean} [options.dequantizeInShader=true] Determines if a {@link https://github.com/google/draco|Draco} encoded model is dequantized on the GPU. This decreases total memory usage for encoded models.
     *
     * @returns {Model} The newly created model.
     *
     * @example
     * // Example 1. Create a model from a glTF asset
     * var model = scene.primitives.add(Cesium.Model.fromGltf({
     *   url : './duck/duck.gltf'
     * }));
     *
     * @example
     * // Example 2. Create model and provide all properties and events
     * var origin = Cesium.Cartesian3.fromDegrees(-95.0, 40.0, 200000.0);
     * var modelMatrix = Cesium.Transforms.eastNorthUpToFixedFrame(origin);
     *
     * var model = scene.primitives.add(Cesium.Model.fromGltf({
     *   url : './duck/duck.gltf',
     *   show : true,                     // default
     *   modelMatrix : modelMatrix,
     *   scale : 2.0,                     // double size
     *   minimumPixelSize : 128,          // never smaller than 128 pixels
     *   maximumScale: 20000,             // never larger than 20000 * model size (overrides minimumPixelSize)
     *   allowPicking : false,            // not pickable
     *   debugShowBoundingVolume : false, // default
     *   debugWireframe : false
     * }));
     *
     * model.readyPromise.then(function(model) {
     *   // Play all animations when the model is ready to render
     *   model.activeAnimations.addAll();
     * });
     */
    Model.fromGltf = function(options) {
        //>>includeStart('debug', pragmas.debug);
        if (!defined(options) || !defined(options.url)) {
            throw new DeveloperError('options.url is required');
        }
        //>>includeEnd('debug');

        var url = options.url;
        options = clone(options);

        // Create resource for the model file
        var modelResource = Resource.createIfNeeded(url);

        // Setup basePath to get dependent files
        var basePath = defaultValue(options.basePath, modelResource.clone());
        var resource = Resource.createIfNeeded(basePath);

        // If no cache key is provided, use a GUID.
        // Check using a URI to GUID dictionary that we have not already added this model.
        var cacheKey = defaultValue(options.cacheKey, uriToGuid[getAbsoluteUri(modelResource.url)]);
        if (!defined(cacheKey)) {
            cacheKey = createGuid();
            uriToGuid[getAbsoluteUri(modelResource.url)] = cacheKey;
        }

        if (defined(options.basePath) && !defined(options.cacheKey)) {
            cacheKey += resource.url;
        }
        options.cacheKey = cacheKey;
        options.basePath = resource;

        var model = new Model(options);

        var cachedGltf = gltfCache[cacheKey];
        if (!defined(cachedGltf)) {
            cachedGltf = new CachedGltf({
                ready : false
            });
            cachedGltf.count = 1;
            cachedGltf.modelsToLoad.push(model);
            setCachedGltf(model, cachedGltf);
            gltfCache[cacheKey] = cachedGltf;

            // Add Accept header if we need it
            if (!defined(modelResource.headers.Accept)) {
                modelResource.headers.Accept = defaultModelAccept;
            }

            modelResource.fetchArrayBuffer().then(function(arrayBuffer) {
                var array = new Uint8Array(arrayBuffer);
                if (containsGltfMagic(array)) {
                    // Load binary glTF
                    var parsedGltf = parseGlb(array);
                    cachedGltf.makeReady(parsedGltf);
                } else {
                    // Load text (JSON) glTF
                    var json = getStringFromTypedArray(array);
                    cachedGltf.makeReady(JSON.parse(json));
                }
            }).otherwise(ModelUtility.getFailedLoadFunction(model, 'model', modelResource.url));
        } else if (!cachedGltf.ready) {
            // Cache hit but the fetchArrayBuffer() or fetchText() request is still pending
            ++cachedGltf.count;
            cachedGltf.modelsToLoad.push(model);
        }
        // else if the cached glTF is defined and ready, the
        // model constructor will pick it up using the cache key.

        return model;
    };

    /**
     * For the unit tests to verify model caching.
     *
     * @private
     */
    Model._gltfCache = gltfCache;

    function getRuntime(model, runtimeName, name) {
        //>>includeStart('debug', pragmas.debug);
        if (model._state !== ModelState.LOADED) {
            throw new DeveloperError('The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.');
        }

        if (!defined(name)) {
            throw new DeveloperError('name is required.');
        }
        //>>includeEnd('debug');

        return (model._runtime[runtimeName])[name];
    }

    /**
     * Returns the glTF node with the given <code>name</code> property.  This is used to
     * modify a node's transform for animation outside of glTF animations.
     *
     * @param {String} name The glTF name of the node.
     * @returns {ModelNode} The node or <code>undefined</code> if no node with <code>name</code> exists.
     *
     * @exception {DeveloperError} The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.
     *
     * @example
     * // Apply non-uniform scale to node LOD3sp
     * var node = model.getNode('LOD3sp');
     * node.matrix = Cesium.Matrix4.fromScale(new Cesium.Cartesian3(5.0, 1.0, 1.0), node.matrix);
     */
    Model.prototype.getNode = function(name) {
        var node = getRuntime(this, 'nodesByName', name);
        return defined(node) ? node.publicNode : undefined;
    };

    /**
     * Returns the glTF mesh with the given <code>name</code> property.
     *
     * @param {String} name The glTF name of the mesh.
     *
     * @returns {ModelMesh} The mesh or <code>undefined</code> if no mesh with <code>name</code> exists.
     *
     * @exception {DeveloperError} The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.
     */
    Model.prototype.getMesh = function(name) {
        return getRuntime(this, 'meshesByName', name);
    };

    /**
     * Returns the glTF material with the given <code>name</code> property.
     *
     * @param {String} name The glTF name of the material.
     * @returns {ModelMaterial} The material or <code>undefined</code> if no material with <code>name</code> exists.
     *
     * @exception {DeveloperError} The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.
     */
    Model.prototype.getMaterial = function(name) {
        return getRuntime(this, 'materialsByName', name);
    };

    /**
     * Sets the current value of an articulation stage.  After setting one or multiple stage values, call
     * Model.applyArticulations() to cause the node matrices to be recalculated.
     *
     * @param {String} articulationStageKey The name of the articulation, a space, and the name of the stage.
     * @param {Number} value The numeric value of this stage of the articulation.
     *
     * @exception {DeveloperError} The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.
     *
     * @see Model#applyArticulations
     */
    Model.prototype.setArticulationStage = function(articulationStageKey, value) {
        //>>includeStart('debug', pragmas.debug);
        Check.typeOf.number('value', value);
        //>>includeEnd('debug');

        var stage = getRuntime(this, 'stagesByKey', articulationStageKey);
        var articulation = getRuntime(this, 'articulationsByStageKey', articulationStageKey);
        if (defined(stage) && defined(articulation)) {
            value = CesiumMath.clamp(value, stage.minimumValue, stage.maximumValue);
            if (!CesiumMath.equalsEpsilon(stage.currentValue, value, articulationEpsilon)) {
                stage.currentValue = value;
                articulation.isDirty = true;
            }
        }
    };

    var scratchArticulationCartesian = new Cartesian3();
    var scratchArticulationRotation = new Matrix3();

    /**
     * Modifies a Matrix4 by applying a transformation for a given value of a stage.  Note this is different usage
     * from the typical <code>result</code> parameter, in that the incoming value of <code>result</code> is
     * meaningful.  Various stages of an articulation can be multiplied together, so their
     * transformations are all merged into a composite Matrix4 representing them all.
     *
     * @param {object} stage The stage of an articulation that is being evaluated.
     * @param {Matrix4} result The matrix to be modified.
     * @returns {Matrix4} A matrix transformed as requested by the articulation stage.
     *
     * @private
     */
    function applyArticulationStageMatrix(stage, result) {
        //>>includeStart('debug', pragmas.debug);
        Check.typeOf.object('stage', stage);
        Check.typeOf.object('result', result);
        //>>includeEnd('debug');

        var value = stage.currentValue;
        var cartesian = scratchArticulationCartesian;
        var rotation;
        switch (stage.type) {
            case 'xRotate':
                rotation = Matrix3.fromRotationX(CesiumMath.toRadians(value), scratchArticulationRotation);
                Matrix4.multiplyByMatrix3(result, rotation, result);
                break;
            case 'yRotate':
                rotation = Matrix3.fromRotationY(CesiumMath.toRadians(value), scratchArticulationRotation);
                Matrix4.multiplyByMatrix3(result, rotation, result);
                break;
            case 'zRotate':
                rotation = Matrix3.fromRotationZ(CesiumMath.toRadians(value), scratchArticulationRotation);
                Matrix4.multiplyByMatrix3(result, rotation, result);
                break;
            case 'xTranslate':
                cartesian.x = value;
                cartesian.y = 0.0;
                cartesian.z = 0.0;
                Matrix4.multiplyByTranslation(result, cartesian, result);
                break;
            case 'yTranslate':
                cartesian.x = 0.0;
                cartesian.y = value;
                cartesian.z = 0.0;
                Matrix4.multiplyByTranslation(result, cartesian, result);
                break;
            case 'zTranslate':
                cartesian.x = 0.0;
                cartesian.y = 0.0;
                cartesian.z = value;
                Matrix4.multiplyByTranslation(result, cartesian, result);
                break;
            case 'xScale':
                cartesian.x = value;
                cartesian.y = 1.0;
                cartesian.z = 1.0;
                Matrix4.multiplyByScale(result, cartesian, result);
                break;
            case 'yScale':
                cartesian.x = 1.0;
                cartesian.y = value;
                cartesian.z = 1.0;
                Matrix4.multiplyByScale(result, cartesian, result);
                break;
            case 'zScale':
                cartesian.x = 1.0;
                cartesian.y = 1.0;
                cartesian.z = value;
                Matrix4.multiplyByScale(result, cartesian, result);
                break;
            case 'uniformScale':
                Matrix4.multiplyByUniformScale(result, value, result);
                break;
            default:
                break;
        }
        return result;
    }

    var scratchApplyArticulationTransform = new Matrix4();

    /**
     * Applies any modified articulation stages to the matrix of each node that participates
     * in any articulation.  Note that this will overwrite any nodeTransformations on participating nodes.
     *
     * @exception {DeveloperError} The model is not loaded.  Use Model.readyPromise or wait for Model.ready to be true.
     */
    Model.prototype.applyArticulations = function() {
        var articulationsByName = this._runtime.articulationsByName;
        for (var articulationName in articulationsByName) {
            if (articulationsByName.hasOwnProperty(articulationName)) {
                var articulation = articulationsByName[articulationName];
                if (articulation.isDirty) {
                    articulation.isDirty = false;
                    var numNodes = articulation.nodes.length;
                    for (var n = 0; n < numNodes; ++n) {
                        var node = articulation.nodes[n];
                        var transform = Matrix4.clone(node.originalMatrix, scratchApplyArticulationTransform);

                        var numStages = articulation.stages.length;
                        for (var s = 0; s < numStages; ++s) {
                            var stage = articulation.stages[s];
                            transform = applyArticulationStageMatrix(stage, transform);
                        }
                        node.matrix = transform;
                    }
                }
            }
        }
    };

    ///////////////////////////////////////////////////////////////////////////

    function addBuffersToLoadResources(model) {
        var gltf = model.gltf;
        var loadResources = model._loadResources;
        ForEach.buffer(gltf, function(buffer, id) {
            loadResources.buffers[id] = buffer.extras._pipeline.source;
        });
    }

    function bufferLoad(model, id) {
        return function(arrayBuffer) {
            var loadResources = model._loadResources;
            var buffer = new Uint8Array(arrayBuffer);
            --loadResources.pendingBufferLoads;
            model.gltf.buffers[id].extras._pipeline.source = buffer;
        };
    }

    function parseBufferViews(model) {
        var bufferViews = model.gltf.bufferViews;
        var vertexBuffersToCreate = model._loadResources.vertexBuffersToCreate;

        // Only ARRAY_BUFFER here.  ELEMENT_ARRAY_BUFFER created below.
        ForEach.bufferView(model.gltf, function(bufferView, id) {
            if (bufferView.target === WebGLConstants.ARRAY_BUFFER) {
                vertexBuffersToCreate.enqueue(id);
            }
        });

        var indexBuffersToCreate = model._loadResources.indexBuffersToCreate;
        var indexBufferIds = {};

        // The Cesium Renderer requires knowing the datatype for an index buffer
        // at creation type, which is not part of the glTF bufferview so loop
        // through glTF accessors to create the bufferview's index buffer.
        ForEach.accessor(model.gltf, function(accessor) {
            var bufferViewId = accessor.bufferView;
            if (!defined(bufferViewId)) {
                return;
            }

            var bufferView = bufferViews[bufferViewId];
            if ((bufferView.target === WebGLConstants.ELEMENT_ARRAY_BUFFER) && !defined(indexBufferIds[bufferViewId])) {
                indexBufferIds[bufferViewId] = true;
                indexBuffersToCreate.enqueue({
                    id : bufferViewId,
                    componentType : accessor.componentType
                });
            }
        });
    }

    function parseTechniques(model) {
        // retain references to gltf techniques
        var gltf = model.gltf;
        if (!hasExtension(gltf, 'KHR_techniques_webgl')) {
            return;
        }

        var sourcePrograms = model._sourcePrograms;
        var sourceTechniques = model._sourceTechniques;
        var programs = gltf.extensions.KHR_techniques_webgl.programs;

        ForEach.technique(gltf, function(technique, techniqueId) {
            sourceTechniques[techniqueId] = clone(technique);

            var programId = technique.program;
            if (!defined(sourcePrograms[programId])) {
                sourcePrograms[programId] = clone(programs[programId]);
            }
        });
    }

    function shaderLoad(model, type, id) {
        return function(source) {
            var loadResources = model._loadResources;
            loadResources.shaders[id] = {
                source : source,
                type : type,
                bufferView : undefined
            };
            --loadResources.pendingShaderLoads;
            model._rendererResources.sourceShaders[id] = source;
        };
    }

    function parseShaders(model) {
        var gltf = model.gltf;
        var buffers = gltf.buffers;
        var bufferViews = gltf.bufferViews;
        var sourceShaders = model._rendererResources.sourceShaders;
        ForEach.shader(gltf, function(shader, id) {
            // Shader references either uri (external or base64-encoded) or bufferView
            if (defined(shader.bufferView)) {
                var bufferViewId = shader.bufferView;
                var bufferView = bufferViews[bufferViewId];
                var bufferId = bufferView.buffer;
                var buffer = buffers[bufferId];
                var source = getStringFromTypedArray(buffer.extras._pipeline.source, bufferView.byteOffset, bufferView.byteLength);
                sourceShaders[id] = source;
            } else if (defined(shader.extras._pipeline.source)) {
                sourceShaders[id] = shader.extras._pipeline.source;
            } else {
                ++model._loadResources.pendingShaderLoads;

                var shaderResource = model._resource.getDerivedResource({
                    url: shader.uri
                });

                shaderResource.fetchText()
                    .then(shaderLoad(model, shader.type, id))
                    .otherwise(ModelUtility.getFailedLoadFunction(model, 'shader', shaderResource.url));
            }
        });
    }

    function parsePrograms(model) {
        var sourceTechniques = model._sourceTechniques;
        for (var techniqueId in sourceTechniques) {
            if (sourceTechniques.hasOwnProperty(techniqueId)) {
                var technique = sourceTechniques[techniqueId];
                model._loadResources.programsToCreate.enqueue({
                    programId: technique.program,
                    techniqueId: techniqueId
                });
            }
        }
    }

    function parseArticulations(model) {
        var articulationsByName = {};
        var articulationsByStageKey = {};
        var runtimeStagesByKey = {};

        model._runtime.articulationsByName = articulationsByName;
        model._runtime.articulationsByStageKey = articulationsByStageKey;
        model._runtime.stagesByKey = runtimeStagesByKey;

        var gltf = model.gltf;
        if (!hasExtension(gltf, 'AGI_articulations') || !defined(gltf.extensions) || !defined(gltf.extensions.AGI_articulations)) {
            return;
        }

        var gltfArticulations = gltf.extensions.AGI_articulations.articulations;
        if (!defined(gltfArticulations)) {
            return;
        }

        var numArticulations = gltfArticulations.length;
        for (var i = 0; i < numArticulations; ++i) {
            var articulation = clone(gltfArticulations[i]);
            articulation.nodes = [];
            articulation.isDirty = true;
            articulationsByName[articulation.name] = articulation;

            var numStages = articulation.stages.length;
            for (var s = 0; s < numStages; ++s) {
                var stage = articulation.stages[s];
                stage.currentValue = stage.initialValue;

                var stageKey = articulation.name + ' ' + stage.name;
                articulationsByStageKey[stageKey] = articulation;
                runtimeStagesByKey[stageKey] = stage;
            }
        }
    }

    function imageLoad(model, textureId) {
        return function(image) {
            var loadResources = model._loadResources;
            --loadResources.pendingTextureLoads;
            loadResources.texturesToCreate.enqueue({
                id : textureId,
                image : image,
                bufferView : image.bufferView,
                width : image.width,
                height : image.height,
                internalFormat : image.internalFormat
            });
        };
    }

    var ktxRegex = /(^data:image\/ktx)|(\.ktx$)/i;
    var crnRegex = /(^data:image\/crn)|(\.crn$)/i;

    function parseTextures(model, context, supportsWebP) {
        var gltf = model.gltf;
        var images = gltf.images;
        var uri;
        ForEach.texture(gltf, function(texture, id) {
            var imageId = texture.source;

            if (defined(texture.extensions) && defined(texture.extensions.EXT_texture_webp) && supportsWebP) {
                imageId = texture.extensions.EXT_texture_webp.source;
            }

            var gltfImage = images[imageId];
            var extras = gltfImage.extras;

            var bufferViewId = gltfImage.bufferView;
            var mimeType = gltfImage.mimeType;
            uri = gltfImage.uri;

            // First check for a compressed texture
            if (defined(extras) && defined(extras.compressedImage3DTiles)) {
                var crunch = extras.compressedImage3DTiles.crunch;
                var s3tc = extras.compressedImage3DTiles.s3tc;
                var pvrtc = extras.compressedImage3DTiles.pvrtc1;
                var etc1 = extras.compressedImage3DTiles.etc1;

                if (context.s3tc && defined(crunch)) {
                    mimeType = crunch.mimeType;
                    if (defined(crunch.bufferView)) {
                        bufferViewId = crunch.bufferView;
                    } else {
                        uri = crunch.uri;
                    }
                } else if (context.s3tc && defined(s3tc)) {
                    mimeType = s3tc.mimeType;
                    if (defined(s3tc.bufferView)) {
                        bufferViewId = s3tc.bufferView;
                    } else {
                        uri = s3tc.uri;
                    }
                } else if (context.pvrtc && defined(pvrtc)) {
                    mimeType = pvrtc.mimeType;
                    if (defined(pvrtc.bufferView)) {
                        bufferViewId = pvrtc.bufferView;
                    } else {
                        uri = pvrtc.uri;
                    }
                } else if (context.etc1 && defined(etc1)) {
                    mimeType = etc1.mimeType;
                    if (defined(etc1.bufferView)) {
                        bufferViewId = etc1.bufferView;
                    } else {
                        uri = etc1.uri;
                    }
                }
            }

            // Image references either uri (external or base64-encoded) or bufferView
            if (defined(bufferViewId)) {
                model._loadResources.texturesToCreateFromBufferView.enqueue({
                    id : id,
                    image : undefined,
                    bufferView : bufferViewId,
                    mimeType : mimeType
                });
            } else {
                ++model._loadResources.pendingTextureLoads;

                var imageResource = model._resource.getDerivedResource({
                    url : uri
                });

                var promise;
                if (ktxRegex.test(uri)) {
                    promise = loadKTX(imageResource);
                } else if (crnRegex.test(uri)) {
                    promise = loadCRN(imageResource);
                } else {
                    promise = imageResource.fetchImage();
                }
                promise.then(imageLoad(model, id, imageId)).otherwise(ModelUtility.getFailedLoadFunction(model, 'image', imageResource.url));
            }
        });
    }

    var scratchArticulationStageInitialTransform = new Matrix4();

    function parseNodes(model) {
        var runtimeNodes = {};
        var runtimeNodesByName = {};
        var skinnedNodes = [];

        var skinnedNodesIds = model._loadResources.skinnedNodesIds;
        var articulationsByName = model._runtime.articulationsByName;

        ForEach.node(model.gltf, function(node, id) {
            var runtimeNode = {
                // Animation targets
                matrix : undefined,
                translation : undefined,
                rotation : undefined,
                scale : undefined,

                // Per-node show inherited from parent
                computedShow : true,

                // Computed transforms
                transformToRoot : new Matrix4(),
                computedMatrix : new Matrix4(),
                dirtyNumber : 0,                    // The frame this node was made dirty by an animation; for graph traversal

                // Rendering
                commands : [],                      // empty for transform, light, and camera nodes

                // Skinned node
                inverseBindMatrices : undefined,    // undefined when node is not skinned
                bindShapeMatrix : undefined,        // undefined when node is not skinned or identity
                joints : [],                        // empty when node is not skinned
                computedJointMatrices : [],         // empty when node is not skinned

                // Joint node
                jointName : node.jointName,         // undefined when node is not a joint

                weights : [],

                // Graph pointers
                children : [],                      // empty for leaf nodes
                parents : [],                       // empty for root nodes

                // Publicly-accessible ModelNode instance to modify animation targets
                publicNode : undefined
            };
            runtimeNode.publicNode = new ModelNode(model, node, runtimeNode, id, ModelUtility.getTransform(node));

            runtimeNodes[id] = runtimeNode;
            runtimeNodesByName[node.name] = runtimeNode;

            if (defined(node.skin)) {
                skinnedNodesIds.push(id);
                skinnedNodes.push(runtimeNode);
            }

            if (defined(node.extensions) && defined(node.extensions.AGI_articulations)) {
                var articulationName = node.extensions.AGI_articulations.articulationName;
                if (defined(articulationName)) {
                    var transform = Matrix4.clone(runtimeNode.publicNode.originalMatrix, scratchArticulationStageInitialTransform);
                    var articulation = articulationsByName[articulationName];
                    articulation.nodes.push(runtimeNode.publicNode);

                    var numStages = articulation.stages.length;
                    for (var s = 0; s < numStages; ++s) {
                        var stage = articulation.stages[s];
                        transform = applyArticulationStageMatrix(stage, transform);
                    }
                    runtimeNode.publicNode.matrix = transform;
                }
            }
        });

        model._runtime.nodes = runtimeNodes;
        model._runtime.nodesByName = runtimeNodesByName;
        model._runtime.skinnedNodes = skinnedNodes;
    }

    function parseMaterials(model) {
        var gltf = model.gltf;
        var techniques = model._sourceTechniques;

        var runtimeMaterialsByName = {};
        var runtimeMaterialsById = {};
        var uniformMaps = model._uniformMaps;

        ForEach.material(gltf, function(material, materialId) {
            // Allocated now so ModelMaterial can keep a reference to it.
            uniformMaps[materialId] = {
                uniformMap : undefined,
                values : undefined,
                jointMatrixUniformName : undefined,
                morphWeightsUniformName : undefined
            };

            var modelMaterial = new ModelMaterial(model, material, materialId);

            if (defined(material.extensions) && defined(material.extensions.KHR_techniques_webgl)) {
                var techniqueId = material.extensions.KHR_techniques_webgl.technique;
                modelMaterial._technique = techniqueId;
                modelMaterial._program = techniques[techniqueId].program;

                ForEach.materialValue(material, function(value, uniformName) {
                    if (!defined(modelMaterial._values)) {
                        modelMaterial._values = {};
                    }

                    modelMaterial._values[uniformName] = clone(value);
                });
            }

            runtimeMaterialsByName[material.name] = modelMaterial;
            runtimeMaterialsById[materialId] = modelMaterial;
        });

        model._runtime.materialsByName = runtimeMaterialsByName;
        model._runtime.materialsById = runtimeMaterialsById;
    }

    function parseMeshes(model) {
        var runtimeMeshesByName = {};
        var runtimeMaterialsById = model._runtime.materialsById;

        ForEach.mesh(model.gltf, function(mesh, meshId) {
            runtimeMeshesByName[mesh.name] = new ModelMesh(mesh, runtimeMaterialsById, meshId);
            if (defined(model.extensionsUsed.WEB3D_quantized_attributes) || model._dequantizeInShader) {
                // Cache primitives according to their program
                ForEach.meshPrimitive(mesh, function(primitive, primitiveId) {
                    var programId = getProgramForPrimitive(model, primitive);
                    var programPrimitives = model._programPrimitives[programId];
                    if (!defined(programPrimitives)) {
                        programPrimitives = {};
                        model._programPrimitives[programId] = programPrimitives;
                    }
                    programPrimitives[meshId + '.primitive.' + primitiveId] = primitive;
                });
            }
        });

        model._runtime.meshesByName = runtimeMeshesByName;
    }

    ///////////////////////////////////////////////////////////////////////////

    var CreateVertexBufferJob = function() {
        this.id = undefined;
        this.model = undefined;
        this.context = undefined;
    };

    CreateVertexBufferJob.prototype.set = function(id, model, context) {
        this.id = id;
        this.model = model;
        this.context = context;
    };

    CreateVertexBufferJob.prototype.execute = function() {
        createVertexBuffer(this.id, this.model, this.context);
    };

    ///////////////////////////////////////////////////////////////////////////

    function createVertexBuffer(bufferViewId, model, context) {
        var loadResources = model._loadResources;
        var bufferViews = model.gltf.bufferViews;
        var bufferView = bufferViews[bufferViewId];

        // Use bufferView created at runtime
        if (!defined(bufferView)) {
            bufferView = loadResources.createdBufferViews[bufferViewId];
        }

        var vertexBuffer = Buffer.createVertexBuffer({
            context : context,
            typedArray : loadResources.getBuffer(bufferView),
            usage : BufferUsage.STATIC_DRAW
        });
        vertexBuffer.vertexArrayDestroyable = false;
        model._rendererResources.buffers[bufferViewId] = vertexBuffer;
        model._geometryByteLength += vertexBuffer.sizeInBytes;
    }

    ///////////////////////////////////////////////////////////////////////////

    var CreateIndexBufferJob = function() {
        this.id = undefined;
        this.componentType = undefined;
        this.model = undefined;
        this.context = undefined;
    };

    CreateIndexBufferJob.prototype.set = function(id, componentType, model, context) {
        this.id = id;
        this.componentType = componentType;
        this.model = model;
        this.context = context;
    };

    CreateIndexBufferJob.prototype.execute = function() {
        createIndexBuffer(this.id, this.componentType, this.model, this.context);
    };

    ///////////////////////////////////////////////////////////////////////////

    function createIndexBuffer(bufferViewId, componentType, model, context) {
        var loadResources = model._loadResources;
        var bufferViews = model.gltf.bufferViews;
        var bufferView = bufferViews[bufferViewId];

        // Use bufferView created at runtime
        if (!defined(bufferView)) {
            bufferView = loadResources.createdBufferViews[bufferViewId];
        }

        var indexBuffer = Buffer.createIndexBuffer({
            context : context,
            typedArray : loadResources.getBuffer(bufferView),
            usage : BufferUsage.STATIC_DRAW,
            indexDatatype : componentType
        });
        indexBuffer.vertexArrayDestroyable = false;
        model._rendererResources.buffers[bufferViewId] = indexBuffer;
        model._geometryByteLength += indexBuffer.sizeInBytes;
    }

    var scratchVertexBufferJob = new CreateVertexBufferJob();
    var scratchIndexBufferJob = new CreateIndexBufferJob();

    function createBuffers(model, frameState) {
        var loadResources = model._loadResources;

        if (loadResources.pendingBufferLoads !== 0) {
            return;
        }

        var context = frameState.context;
        var vertexBuffersToCreate = loadResources.vertexBuffersToCreate;
        var indexBuffersToCreate = loadResources.indexBuffersToCreate;
        var i;

        if (model.asynchronous) {
            while (vertexBuffersToCreate.length > 0) {
                scratchVertexBufferJob.set(vertexBuffersToCreate.peek(), model, context);
                if (!frameState.jobScheduler.execute(scratchVertexBufferJob, JobType.BUFFER)) {
                    break;
                }
                vertexBuffersToCreate.dequeue();
            }

            while (indexBuffersToCreate.length > 0) {
                i = indexBuffersToCreate.peek();
                scratchIndexBufferJob.set(i.id, i.componentType, model, context);
                if (!frameState.jobScheduler.execute(scratchIndexBufferJob, JobType.BUFFER)) {
                    break;
                }
                indexBuffersToCreate.dequeue();
            }
        } else {
            while (vertexBuffersToCreate.length > 0) {
                createVertexBuffer(vertexBuffersToCreate.dequeue(), model, context);
            }

            while (indexBuffersToCreate.length > 0) {
                i = indexBuffersToCreate.dequeue();
                createIndexBuffer(i.id, i.componentType, model, context);
            }
        }
    }

    function getProgramForPrimitive(model, primitive) {
        var material = model._runtime.materialsById[primitive.material];
        if (!defined(material)) {
            return;
        }

        return material._program;
    }

    function modifyShaderForQuantizedAttributes(shader, programName, model) {
        var primitive;
        var primitives = model._programPrimitives[programName];

        // If no primitives were cached for this program, there's no need to modify the shader
        if (!defined(primitives)) {
            return shader;
        }

        var primitiveId;
        for (primitiveId in primitives) {
            if (primitives.hasOwnProperty(primitiveId)) {
                primitive = primitives[primitiveId];
                if (getProgramForPrimitive(model, primitive) === programName) {
                    break;
                }
            }
        }

        // This is not needed after the program is processed, free the memory
        model._programPrimitives[programName] = undefined;

        var result;
        if (model.extensionsUsed.WEB3D_quantized_attributes) {
            result = ModelUtility.modifyShaderForQuantizedAttributes(model.gltf, primitive, shader);
            model._quantizedUniforms[programName] = result.uniforms;
        } else {
            var decodedData = model._decodedData[primitiveId];
            if (defined(decodedData)) {
                result = ModelUtility.modifyShaderForDracoQuantizedAttributes(model.gltf, primitive, shader, decodedData.attributes);
            } else {
                return shader;
            }
        }

        return result.shader;
    }

    function modifyShaderForColor(shader) {
        shader = ShaderSource.replaceMain(shader, 'gltf_blend_main');
        shader +=
            'uniform vec4 gltf_color; \n' +
            'uniform float gltf_colorBlend; \n' +
            'void main() \n' +
            '{ \n' +
            '    gltf_blend_main(); \n' +
            '    gl_FragColor.rgb = mix(gl_FragColor.rgb, gltf_color.rgb, gltf_colorBlend); \n' +
            '    float highlight = ceil(gltf_colorBlend); \n' +
            '    gl_FragColor.rgb *= mix(gltf_color.rgb, vec3(1.0), highlight); \n' +
            '    gl_FragColor.a *= gltf_color.a; \n' +
            '} \n';

        return shader;
    }

    function modifyShader(shader, programName, callback) {
        if (defined(callback)) {
            shader = callback(shader, programName);
        }
        return shader;
    }

    var CreateProgramJob = function() {
        this.programToCreate = undefined;
        this.model = undefined;
        this.context = undefined;
    };

    CreateProgramJob.prototype.set = function(programToCreate, model, context) {
        this.programToCreate = programToCreate;
        this.model = model;
        this.context = context;
    };

    CreateProgramJob.prototype.execute = function() {
        createProgram(this.programToCreate, this.model, this.context);
    };

    ///////////////////////////////////////////////////////////////////////////

    // When building programs for the first time, do not include modifiers for clipping planes and color
    // since this is the version of the program that will be cached for use with other Models.
    function createProgram(programToCreate, model, context) {
        var programId = programToCreate.programId;
        var techniqueId = programToCreate.techniqueId;
        var program = model._sourcePrograms[programId];
        var shaders = model._rendererResources.sourceShaders;

        var vs = shaders[program.vertexShader];
        var fs = shaders[program.fragmentShader];

        var quantizedVertexShaders = model._quantizedVertexShaders;
        var toClipCoordinatesGLSL = model._toClipCoordinatesGLSL[programId];

        if (model.extensionsUsed.WEB3D_quantized_attributes || model._dequantizeInShader) {
            var quantizedVS = quantizedVertexShaders[programId];
            if (!defined(quantizedVS)) {
                quantizedVS = modifyShaderForQuantizedAttributes(vs, programId, model);
                quantizedVertexShaders[programId] = quantizedVS;
            }
            vs = quantizedVS;
        }

        var drawVS = modifyShader(vs, programId, model._vertexShaderLoaded);
        var drawFS = modifyShader(fs, programId, model._fragmentShaderLoaded);

        // Internet Explorer seems to have problems with discard (for clipping planes) after too many levels of indirection:
        // https://github.com/AnalyticalGraphicsInc/cesium/issues/6575.
        // For IE log depth code is defined out anyway due to unsupported WebGL extensions, so the wrappers can be omitted.
        if (!FeatureDetection.isInternetExplorer()) {
            drawVS = ModelUtility.modifyVertexShaderForLogDepth(drawVS, toClipCoordinatesGLSL);
            drawFS = ModelUtility.modifyFragmentShaderForLogDepth(drawFS);
        }

        if (!defined(model._uniformMapLoaded)) {
            drawFS = 'uniform vec4 czm_pickColor;\n' + drawFS;
        }

        var useIBL = model._imageBasedLightingFactor.x > 0.0 || model._imageBasedLightingFactor.y > 0.0;
        if (useIBL) {
            drawFS = '#define USE_IBL_LIGHTING \n\n' + drawFS;
        }

        if (defined(model._lightColor)) {
            drawFS = '#define USE_CUSTOM_LIGHT_COLOR \n\n' + drawFS;
        }

        if (model._sourceVersion !== '2.0' || model._sourceKHRTechniquesWebGL) {
            drawFS = ShaderSource.replaceMain(drawFS, 'non_gamma_corrected_main');
            drawFS =
                drawFS +
                '\n' +
                'void main() { \n' +
                '    non_gamma_corrected_main(); \n' +
                '    gl_FragColor = czm_gammaCorrect(gl_FragColor); \n' +
                '} \n';
        }

        if (OctahedralProjectedCubeMap.isSupported(context)) {
            var usesSH = defined(model._sphericalHarmonicCoefficients) || model._useDefaultSphericalHarmonics;
            var usesSM = (defined(model._specularEnvironmentMapAtlas) && model._specularEnvironmentMapAtlas.ready) || model._useDefaultSpecularMaps;
            var addMatrix = usesSH || usesSM || useIBL;
            if (addMatrix) {
                drawFS = 'uniform mat4 gltf_clippingPlanesMatrix; \n' + drawFS;
            }

            if (defined(model._sphericalHarmonicCoefficients)) {
                drawFS = '#define DIFFUSE_IBL \n' + '#define CUSTOM_SPHERICAL_HARMONICS \n' + 'uniform vec3 gltf_sphericalHarmonicCoefficients[9]; \n' + drawFS;
            } else if (model._useDefaultSphericalHarmonics) {
                drawFS = '#define DIFFUSE_IBL \n' + drawFS;
            }

            if (defined(model._specularEnvironmentMapAtlas) && model._specularEnvironmentMapAtlas.ready) {
                drawFS = '#define SPECULAR_IBL \n' + '#define CUSTOM_SPECULAR_IBL \n' + 'uniform sampler2D gltf_specularMap; \n' + 'uniform vec2 gltf_specularMapSize; \n' + 'uniform float gltf_maxSpecularLOD; \n' + drawFS;
            } else if (model._useDefaultSpecularMaps) {
                drawFS = '#define SPECULAR_IBL \n' + drawFS;
            }
        }

        if (defined(model._luminanceAtZenith)) {
            drawFS = '#define USE_SUN_LUMINANCE \n' + 'uniform float gltf_luminanceAtZenith;\n' + drawFS;
        }

        createAttributesAndProgram(programId, techniqueId, drawFS, drawVS, model, context);
    }

    function recreateProgram(programToCreate, model, context) {
        var programId = programToCreate.programId;
        var techniqueId = programToCreate.techniqueId;
        var program = model._sourcePrograms[programId];
        var shaders = model._rendererResources.sourceShaders;

        var quantizedVertexShaders = model._quantizedVertexShaders;
        var toClipCoordinatesGLSL = model._toClipCoordinatesGLSL[programId];

        var clippingPlaneCollection = model.clippingPlanes;
        var addClippingPlaneCode = isClippingEnabled(model);

        var vs = shaders[program.vertexShader];
        var fs = shaders[program.fragmentShader];

        if (model.extensionsUsed.WEB3D_quantized_attributes || model._dequantizeInShader) {
            vs = quantizedVertexShaders[programId];
        }

        var finalFS = fs;
        if (isColorShadingEnabled(model)) {
            finalFS = Model._modifyShaderForColor(finalFS);
        }
        if (addClippingPlaneCode) {
            finalFS = modifyShaderForClippingPlanes(finalFS, clippingPlaneCollection, context);
        }

        var drawVS = modifyShader(vs, programId, model._vertexShaderLoaded);
        var drawFS = modifyShader(finalFS, programId, model._fragmentShaderLoaded);

        if (!FeatureDetection.isInternetExplorer()) {
            drawVS = ModelUtility.modifyVertexShaderForLogDepth(drawVS, toClipCoordinatesGLSL);
            drawFS = ModelUtility.modifyFragmentShaderForLogDepth(drawFS);
        }

        if (!defined(model._uniformMapLoaded)) {
            drawFS = 'uniform vec4 czm_pickColor;\n' + drawFS;
        }

        var useIBL = model._imageBasedLightingFactor.x > 0.0 || model._imageBasedLightingFactor.y > 0.0;
        if (useIBL) {
            drawFS = '#define USE_IBL_LIGHTING \n\n' + drawFS;
        }

        if (defined(model._lightColor)) {
            drawFS = '#define USE_CUSTOM_LIGHT_COLOR \n\n' + drawFS;
        }

        if (model._sourceVersion !== '2.0' || model._sourceKHRTechniquesWebGL) {
            drawFS = ShaderSource.replaceMain(drawFS, 'non_gamma_corrected_main');
            drawFS =
                drawFS +
                '\n' +
                'void main() { \n' +
                '    non_gamma_corrected_main(); \n' +
                '    gl_FragColor = czm_gammaCorrect(gl_FragColor); \n' +
                '} \n';
        }

        if (OctahedralProjectedCubeMap.isSupported(context)) {
            var usesSH = defined(model._sphericalHarmonicCoefficients) || model._useDefaultSphericalHarmonics;
            var usesSM = (defined(model._specularEnvironmentMapAtlas) && model._specularEnvironmentMapAtlas.ready) || model._useDefaultSpecularMaps;
            var addMatrix = !addClippingPlaneCode && (usesSH || usesSM || useIBL);
            if (addMatrix) {
                drawFS = 'uniform mat4 gltf_clippingPlanesMatrix; \n' + drawFS;
            }

            if (defined(model._sphericalHarmonicCoefficients)) {
                drawFS = '#define DIFFUSE_IBL \n' + '#define CUSTOM_SPHERICAL_HARMONICS \n' + 'uniform vec3 gltf_sphericalHarmonicCoefficients[9]; \n' + drawFS;
            } else if (model._useDefaultSphericalHarmonics) {
                drawFS = '#define DIFFUSE_IBL \n' + drawFS;
            }

            if (defined(model._specularEnvironmentMapAtlas) && model._specularEnvironmentMapAtlas.ready) {
                drawFS = '#define SPECULAR_IBL \n' + '#define CUSTOM_SPECULAR_IBL \n' + 'uniform sampler2D gltf_specularMap; \n' + 'uniform vec2 gltf_specularMapSize; \n' + 'uniform float gltf_maxSpecularLOD; \n' + drawFS;
            } else if (model._useDefaultSpecularMaps) {
                drawFS = '#define SPECULAR_IBL \n' + drawFS;
            }
        }

        if (defined(model._luminanceAtZenith)) {
            drawFS = '#define USE_SUN_LUMINANCE \n' + 'uniform float gltf_luminanceAtZenith;\n' + drawFS;
        }

        createAttributesAndProgram(programId, techniqueId, drawFS, drawVS, model, context);
    }

    function createAttributesAndProgram(programId, techniqueId, drawFS, drawVS, model, context) {
        var technique = model._sourceTechniques[techniqueId];
        var attributeLocations = ModelUtility.createAttributeLocations(technique, model._precreatedAttributes);

        model._rendererResources.programs[programId] = ShaderProgram.fromCache({
            context : context,
            vertexShaderSource : drawVS,
            fragmentShaderSource : drawFS,
            attributeLocations : attributeLocations
        });
    }

    var scratchCreateProgramJob = new CreateProgramJob();

    function createPrograms(model, frameState) {
        var loadResources = model._loadResources;
        var programsToCreate = loadResources.programsToCreate;

        if (loadResources.pendingShaderLoads !== 0) {
            return;
        }

        // PERFORMANCE_IDEA: this could be more fine-grained by looking
        // at the shader's bufferView's to determine the buffer dependencies.
        if (loadResources.pendingBufferLoads !== 0) {
            return;
        }

        var context = frameState.context;

        if (model.asynchronous) {
            while (programsToCreate.length > 0) {
                scratchCreateProgramJob.set(programsToCreate.peek(), model, context);
                if (!frameState.jobScheduler.execute(scratchCreateProgramJob, JobType.PROGRAM)) {
                    break;
                }
                programsToCreate.dequeue();
            }
        } else {
            // Create all loaded programs this frame
            while (programsToCreate.length > 0) {
                createProgram(programsToCreate.dequeue(), model, context);
            }
        }
    }

    function getOnImageCreatedFromTypedArray(loadResources, gltfTexture) {
        return function(image) {
            loadResources.texturesToCreate.enqueue({
                id : gltfTexture.id,
                image : image,
                bufferView : undefined
            });

            --loadResources.pendingBufferViewToImage;
        };
    }

    function loadTexturesFromBufferViews(model) {
        var loadResources = model._loadResources;

        if (loadResources.pendingBufferLoads !== 0) {
            return;
        }

        while (loadResources.texturesToCreateFromBufferView.length > 0) {
            var gltfTexture = loadResources.texturesToCreateFromBufferView.dequeue();

            var gltf = model.gltf;
            var bufferView = gltf.bufferViews[gltfTexture.bufferView];
            var imageId = gltf.textures[gltfTexture.id].source;

            var onerror = ModelUtility.getFailedLoadFunction(model, 'image', 'id: ' + gltfTexture.id + ', bufferView: ' + gltfTexture.bufferView);

            if (gltfTexture.mimeType === 'image/ktx') {
                loadKTX(loadResources.getBuffer(bufferView)).then(imageLoad(model, gltfTexture.id, imageId)).otherwise(onerror);
                ++model._loadResources.pendingTextureLoads;
            } else if (gltfTexture.mimeType === 'image/crn') {
                loadCRN(loadResources.getBuffer(bufferView)).then(imageLoad(model, gltfTexture.id, imageId)).otherwise(onerror);
                ++model._loadResources.pendingTextureLoads;
            } else {
                var onload = getOnImageCreatedFromTypedArray(loadResources, gltfTexture);
                loadImageFromTypedArray({
                    uint8Array: loadResources.getBuffer(bufferView),
                    format: gltfTexture.mimeType,
                    flipY: false
                })
                    .then(onload).otherwise(onerror);
                ++loadResources.pendingBufferViewToImage;
            }
        }
    }

    function createSamplers(model) {
        var loadResources = model._loadResources;
        if (loadResources.createSamplers) {
            loadResources.createSamplers = false;

            var rendererSamplers = model._rendererResources.samplers;
            ForEach.sampler(model.gltf, function(sampler, samplerId) {
                rendererSamplers[samplerId] = new Sampler({
                    wrapS: sampler.wrapS,
                    wrapT: sampler.wrapT,
                    minificationFilter: sampler.minFilter,
                    magnificationFilter: sampler.magFilter
                });
            });
        }
    }

    ///////////////////////////////////////////////////////////////////////////

    var CreateTextureJob = function() {
        this.gltfTexture = undefined;
        this.model = undefined;
        this.context = undefined;
    };

    CreateTextureJob.prototype.set = function(gltfTexture, model, context) {
        this.gltfTexture = gltfTexture;
        this.model = model;
        this.context = context;
    };

    CreateTextureJob.prototype.execute = function() {
        createTexture(this.gltfTexture, this.model, this.context);
    };

    ///////////////////////////////////////////////////////////////////////////

    function createTexture(gltfTexture, model, context) {
        var textures = model.gltf.textures;
        var texture = textures[gltfTexture.id];

        var rendererSamplers = model._rendererResources.samplers;
        var sampler = rendererSamplers[texture.sampler];
        if (!defined(sampler)) {
            sampler = new Sampler({
                wrapS : TextureWrap.REPEAT,
                wrapT : TextureWrap.REPEAT
            });
        }

        var usesTextureTransform = false;
        var materials = model.gltf.materials;
        var materialsLength = materials.length;
        for (var i = 0; i < materialsLength; ++i) {
            var material = materials[i];
            if (defined(material.extensions) && defined(material.extensions.KHR_techniques_webgl)) {
                var values = material.extensions.KHR_techniques_webgl.values;
                for (var valueName in values) {
                    if (values.hasOwnProperty(valueName) && valueName.indexOf('Texture') !== -1) {
                        var value = values[valueName];
                        if (value.index === gltfTexture.id && defined(value.extensions) && defined(value.extensions.KHR_texture_transform)) {
                            usesTextureTransform = true;
                            break;
                        }
                    }
                }
            }
            if (usesTextureTransform) {
                break;
            }
        }

        var wrapS = sampler.wrapS;
        var wrapT = sampler.wrapT;
        var minFilter = sampler.minificationFilter;

        if (usesTextureTransform && minFilter !== TextureMinificationFilter.LINEAR && minFilter !== TextureMinificationFilter.NEAREST) {
            if (minFilter === TextureMinificationFilter.NEAREST_MIPMAP_NEAREST || minFilter === TextureMinificationFilter.NEAREST_MIPMAP_LINEAR) {
                minFilter = TextureMinificationFilter.NEAREST;
            } else {
                minFilter = TextureMinificationFilter.LINEAR;
            }

            sampler = new Sampler({
                wrapS : sampler.wrapS,
                wrapT : sampler.wrapT,
                textureMinificationFilter : minFilter,
                textureMagnificationFilter : sampler.magnificationFilter
            });
        }

        var internalFormat = gltfTexture.internalFormat;

        var mipmap =
            (!(defined(internalFormat) && PixelFormat.isCompressedFormat(internalFormat))) &&
            ((minFilter === TextureMinificationFilter.NEAREST_MIPMAP_NEAREST) ||
             (minFilter === TextureMinificationFilter.NEAREST_MIPMAP_LINEAR) ||
             (minFilter === TextureMinificationFilter.LINEAR_MIPMAP_NEAREST) ||
             (minFilter === TextureMinificationFilter.LINEAR_MIPMAP_LINEAR));
        var requiresNpot = mipmap ||
           (wrapS === TextureWrap.REPEAT) ||
           (wrapS === TextureWrap.MIRRORED_REPEAT) ||
           (wrapT === TextureWrap.REPEAT) ||
           (wrapT === TextureWrap.MIRRORED_REPEAT);

        var tx;
        var source = gltfTexture.image;

        if (defined(internalFormat)) {
            tx = new Texture({
                context : context,
                source : {
                    arrayBufferView : gltfTexture.bufferView
                },
                width : gltfTexture.width,
                height : gltfTexture.height,
                pixelFormat : internalFormat,
                sampler : sampler
            });
        } else if (defined(source)) {
            var npot = !CesiumMath.isPowerOfTwo(source.width) || !CesiumMath.isPowerOfTwo(source.height);

            if (requiresNpot && npot) {
                // WebGL requires power-of-two texture dimensions for mipmapping and REPEAT/MIRRORED_REPEAT wrap modes.
                var canvas = document.createElement('canvas');
                canvas.width = CesiumMath.nextPowerOfTwo(source.width);
                canvas.height = CesiumMath.nextPowerOfTwo(source.height);
                var canvasContext = canvas.getContext('2d');
                canvasContext.drawImage(source, 0, 0, source.width, source.height, 0, 0, canvas.width, canvas.height);
                source = canvas;
            }

            tx = new Texture({
                context : context,
                source : source,
                pixelFormat : texture.internalFormat,
                pixelDatatype : texture.type,
                sampler : sampler,
                flipY : false
            });
            // GLTF_SPEC: Support TEXTURE_CUBE_MAP.  https://github.com/KhronosGroup/glTF/issues/40
            if (mipmap) {
                tx.generateMipmap();
            }
        }
        if (defined(tx)) {
            model._rendererResources.textures[gltfTexture.id] = tx;
            model._texturesByteLength += tx.sizeInBytes;
        }
    }

    var scratchCreateTextureJob = new CreateTextureJob();

    function createTextures(model, frameState) {
        var context = frameState.context;
        var texturesToCreate = model._loadResources.texturesToCreate;

        if (model.asynchronous) {
            while (texturesToCreate.length > 0) {
                scratchCreateTextureJob.set(texturesToCreate.peek(), model, context);
                if (!frameState.jobScheduler.execute(scratchCreateTextureJob, JobType.TEXTURE)) {
                    break;
                }
                texturesToCreate.dequeue();
            }
        } else {
            // Create all loaded textures this frame
            while (texturesToCreate.length > 0) {
                createTexture(texturesToCreate.dequeue(), model, context);
            }
        }
    }

    function getAttributeLocations(model, primitive) {
        var techniques = model._sourceTechniques;

        // Retrieve the compiled shader program to assign index values to attributes
        var attributeLocations = {};

        var location;
        var index;
        var material = model._runtime.materialsById[primitive.material];
        if (!defined(material)) {
            return attributeLocations;
        }

        var technique = techniques[material._technique];
        if (!defined(technique)) {
            return attributeLocations;
        }

        var attributes = technique.attributes;
        var program = model._rendererResources.programs[technique.program];
        var programVertexAttributes = program.vertexAttributes;
        var programAttributeLocations = program._attributeLocations;

        // Note: WebGL shader compiler may have optimized and removed some attributes from programVertexAttributes
        for (location in programVertexAttributes) {
            if (programVertexAttributes.hasOwnProperty(location)) {
                var attribute = attributes[location];
                if (defined(attribute)) {
                    index = programAttributeLocations[location];
                    attributeLocations[attribute.semantic] = index;
                }
            }
        }

        // Always add pre-created attributes.
        // Some pre-created attributes, like per-instance pickIds, may be compiled out of the draw program
        // but should be included in the list of attribute locations for the pick program.
        // This is safe to do since programVertexAttributes and programAttributeLocations are equivalent except
        // that programVertexAttributes optimizes out unused attributes.
        var precreatedAttributes = model._precreatedAttributes;
        if (defined(precreatedAttributes)) {
            for (location in precreatedAttributes) {
                if (precreatedAttributes.hasOwnProperty(location)) {
                    index = programAttributeLocations[location];
                    attributeLocations[location] = index;
                }
            }
        }

        return attributeLocations;
    }

    function mapJointNames(forest, nodes) {
        var length = forest.length;
        var jointNodes = {};
        for (var i = 0; i < length; ++i) {
            var stack = [forest[i]]; // Push root node of tree

            while (stack.length > 0) {
                var id = stack.pop();
                var n = nodes[id];

                if (defined(n)) {
                    jointNodes[id] = id;
                }

                var children = n.children;
                if (defined(children)) {
                    var childrenLength = children.length;
                    for (var k = 0; k < childrenLength; ++k) {
                        stack.push(children[k]);
                    }
                }
            }
        }
        return jointNodes;
    }

    function createJoints(model, runtimeSkins) {
        var gltf = model.gltf;
        var skins = gltf.skins;
        var nodes = gltf.nodes;
        var runtimeNodes = model._runtime.nodes;

        var skinnedNodesIds = model._loadResources.skinnedNodesIds;
        var length = skinnedNodesIds.length;
        for (var j = 0; j < length; ++j) {
            var id = skinnedNodesIds[j];
            var skinnedNode = runtimeNodes[id];
            var node = nodes[id];

            var runtimeSkin = runtimeSkins[node.skin];
            skinnedNode.inverseBindMatrices = runtimeSkin.inverseBindMatrices;
            skinnedNode.bindShapeMatrix = runtimeSkin.bindShapeMatrix;

            // 1. Find nodes with the names in node.skeletons (the node's skeletons)
            // 2. These nodes form the root nodes of the forest to search for each joint in skin.jointNames.  This search uses jointName, not the node's name.
            // 3. Search for the joint name among the gltf node hierarchy instead of the runtime node hierarchy. Child links aren't set up yet for runtime nodes.
            var forest = [];
            var skin = skins[node.skin];
            if (defined(skin.skeleton)) {
                forest.push(skin.skeleton);
            }

            var mappedJointNames = mapJointNames(forest, nodes);
            var gltfJointNames = skins[node.skin].joints;
            var jointNamesLength = gltfJointNames.length;
            for (var i = 0; i < jointNamesLength; ++i) {
                var jointName = gltfJointNames[i];
                var nodeId = mappedJointNames[jointName];
                var jointNode = runtimeNodes[nodeId];
                skinnedNode.joints.push(jointNode);
            }
        }
    }

    function createSkins(model) {
        var loadResources = model._loadResources;

        if (loadResources.pendingBufferLoads !== 0) {
            return;
        }

        if (!loadResources.createSkins) {
            return;
        }
        loadResources.createSkins = false;

        var gltf = model.gltf;
        var accessors = gltf.accessors;
        var runtimeSkins = {};

        ForEach.skin(gltf, function(skin, id) {
            var accessor = accessors[skin.inverseBindMatrices];

            var bindShapeMatrix;
            if (!Matrix4.equals(skin.bindShapeMatrix, Matrix4.IDENTITY)) {
                bindShapeMatrix = Matrix4.clone(skin.bindShapeMatrix);
            }

            runtimeSkins[id] = {
                inverseBindMatrices : ModelAnimationCache.getSkinInverseBindMatrices(model, accessor),
                bindShapeMatrix : bindShapeMatrix // not used when undefined
            };
        });

        createJoints(model, runtimeSkins);
    }

    function getChannelEvaluator(model, runtimeNode, targetPath, spline) {
        return function(localAnimationTime) {
            //  Workaround for https://github.com/KhronosGroup/glTF/issues/219

            //if (targetPath === 'translation') {
            //    return;
            //}
            if (defined(spline)) {
                localAnimationTime = model.clampAnimations ? spline.clampTime(localAnimationTime) : spline.wrapTime(localAnimationTime);
                runtimeNode[targetPath] = spline.evaluate(localAnimationTime, runtimeNode[targetPath]);
                runtimeNode.dirtyNumber = model._maxDirtyNumber;
            }
        };
    }

    function createRuntimeAnimations(model) {
        var loadResources = model._loadResources;

        if (!loadResources.finishedPendingBufferLoads()) {
            return;
        }

        if (!loadResources.createRuntimeAnimations) {
            return;
        }
        loadResources.createRuntimeAnimations = false;

        model._runtime.animations = [];

        var runtimeNodes = model._runtime.nodes;
        var accessors = model.gltf.accessors;

        ForEach.animation(model.gltf, function (animation, i) {
            var channels = animation.channels;
            var samplers = animation.samplers;

            // Find start and stop time for the entire animation
            var startTime = Number.MAX_VALUE;
            var stopTime = -Number.MAX_VALUE;

            var channelsLength = channels.length;
            var channelEvaluators = new Array(channelsLength);

            for (var j = 0; j < channelsLength; ++j) {
                var channel = channels[j];
                var target = channel.target;
                var path = target.path;
                var sampler = samplers[channel.sampler];
                var input = ModelAnimationCache.getAnimationParameterValues(model, accessors[sampler.input]);
                var output = ModelAnimationCache.getAnimationParameterValues(model, accessors[sampler.output]);

                startTime = Math.min(startTime, input[0]);
                stopTime = Math.max(stopTime, input[input.length - 1]);

                var spline = ModelAnimationCache.getAnimationSpline(model, i, animation, channel.sampler, sampler, input, path, output);

                // GLTF_SPEC: Support more targets like materials. https://github.com/KhronosGroup/glTF/issues/142
                channelEvaluators[j] = getChannelEvaluator(model, runtimeNodes[target.node], target.path, spline);
            }

            model._runtime.animations[i] = {
                name : animation.name,
                startTime : startTime,
                stopTime : stopTime,
                channelEvaluators : channelEvaluators
            };
        });
    }

    function createVertexArrays(model, context) {
        var loadResources = model._loadResources;
        if (!loadResources.finishedBuffersCreation() || !loadResources.finishedProgramCreation()
                || !loadResources.createVertexArrays) {
            return;
        }
        loadResources.createVertexArrays = false;

        var rendererBuffers = model._rendererResources.buffers;
        var rendererVertexArrays = model._rendererResources.vertexArrays;
        var gltf = model.gltf;
        var accessors = gltf.accessors;
        ForEach.mesh(gltf, function(mesh, meshId) {
            ForEach.meshPrimitive(mesh, function(primitive, primitiveId) {
                var attributes = [];
                var attributeLocation;
                var attributeLocations = getAttributeLocations(model, primitive);
                var decodedData = model._decodedData[meshId + '.primitive.' + primitiveId];
                ForEach.meshPrimitiveAttribute(primitive, function(accessorId, attributeName) {
                    // Skip if the attribute is not used by the material, e.g., because the asset
                    // was exported with an attribute that wasn't used and the asset wasn't optimized.
                    attributeLocation = attributeLocations[attributeName];
                    if (defined(attributeLocation)) {
                        // Use attributes of previously decoded draco geometry
                        if (defined(decodedData)) {
                            var decodedAttributes = decodedData.attributes;
                            if (decodedAttributes.hasOwnProperty(attributeName)) {
                                var decodedAttribute = decodedAttributes[attributeName];
                                attributes.push({
                                    index: attributeLocation,
                                    vertexBuffer: rendererBuffers[decodedAttribute.bufferView],
                                    componentsPerAttribute: decodedAttribute.componentsPerAttribute,
                                    componentDatatype: decodedAttribute.componentDatatype,
                                    normalize: decodedAttribute.normalized,
                                    offsetInBytes: decodedAttribute.byteOffset,
                                    strideInBytes: decodedAttribute.byteStride
                                });

                                return;
                            }
                        }

                        var a = accessors[accessorId];
                        var normalize = defined(a.normalized) && a.normalized;
                        attributes.push({
                            index: attributeLocation,
                            vertexBuffer: rendererBuffers[a.bufferView],
                            componentsPerAttribute: numberOfComponentsForType(a.type),
                            componentDatatype: a.componentType,
                            normalize: normalize,
                            offsetInBytes: a.byteOffset,
                            strideInBytes: getAccessorByteStride(gltf, a)
                        });
                    }
                });

                // Add pre-created attributes
                var attribute;
                var attributeName;
                var precreatedAttributes = model._precreatedAttributes;
                if (defined(precreatedAttributes)) {
                    for (attributeName in precreatedAttributes) {
                        if (precreatedAttributes.hasOwnProperty(attributeName)) {
                            attributeLocation = attributeLocations[attributeName];
                            if (defined(attributeLocation)) {
                                attribute = precreatedAttributes[attributeName];
                                attribute.index = attributeLocation;
                                attributes.push(attribute);
                            }
                        }
                    }
                }

                var indexBuffer;
                if (defined(primitive.indices)) {
                    var accessor = accessors[primitive.indices];
                    var bufferView = accessor.bufferView;

                    // Use buffer of previously decoded draco geometry
                    if (defined(decodedData)) {
                        bufferView = decodedData.bufferView;
                    }

                    indexBuffer = rendererBuffers[bufferView];
                }
                rendererVertexArrays[meshId + '.primitive.' + primitiveId] = new VertexArray({
                    context: context,
                    attributes: attributes,
                    indexBuffer: indexBuffer
                });
            });
        });
    }

    function createRenderStates(model) {
        var loadResources = model._loadResources;
        if (loadResources.createRenderStates) {
            loadResources.createRenderStates = false;

            ForEach.material(model.gltf, function (material, materialId) {
                createRenderStateForMaterial(model, material, materialId);
            });
        }
    }

    function createRenderStateForMaterial(model, material, materialId) {
        var rendererRenderStates = model._rendererResources.renderStates;

        var blendEquationSeparate = [
            WebGLConstants.FUNC_ADD,
            WebGLConstants.FUNC_ADD
        ];
        var blendFuncSeparate = [
            WebGLConstants.ONE,
            WebGLConstants.ONE_MINUS_SRC_ALPHA,
            WebGLConstants.ONE,
            WebGLConstants.ONE_MINUS_SRC_ALPHA
        ];

        if (defined(material.extensions) && defined(material.extensions.KHR_blend)) {
            blendEquationSeparate = material.extensions.KHR_blend.blendEquation;
            blendFuncSeparate = material.extensions.KHR_blend.blendFactors;
        }

        var enableCulling = !material.doubleSided;
        var blendingEnabled = (material.alphaMode === 'BLEND');
        rendererRenderStates[materialId] = RenderState.fromCache({
            cull : {
                enabled : enableCulling
            },
            depthTest : {
                enabled : true
            },
            depthMask : !blendingEnabled,
            blending : {
                enabled : blendingEnabled,
                equationRgb : blendEquationSeparate[0],
                equationAlpha : blendEquationSeparate[1],
                functionSourceRgb : blendFuncSeparate[0],
                functionDestinationRgb : blendFuncSeparate[1],
                functionSourceAlpha : blendFuncSeparate[2],
                functionDestinationAlpha : blendFuncSeparate[3]
            }
        });
    }

    ///////////////////////////////////////////////////////////////////////////

    var gltfUniformsFromNode = {
        MODEL : function(uniformState, model, runtimeNode) {
            return function() {
                return runtimeNode.computedMatrix;
            };
        },
        VIEW : function(uniformState, model, runtimeNode) {
            return function() {
                return uniformState.view;
            };
        },
        PROJECTION : function(uniformState, model, runtimeNode) {
            return function() {
                return uniformState.projection;
            };
        },
        MODELVIEW : function(uniformState, model, runtimeNode) {
            var mv = new Matrix4();
            return function() {
                return Matrix4.multiplyTransformation(uniformState.view, runtimeNode.computedMatrix, mv);
            };
        },
        CESIUM_RTC_MODELVIEW : function(uniformState, model, runtimeNode) {
            // CESIUM_RTC extension
            var mvRtc = new Matrix4();
            return function() {
                Matrix4.multiplyTransformation(uniformState.view, runtimeNode.computedMatrix, mvRtc);
                return Matrix4.setTranslation(mvRtc, model._rtcCenterEye, mvRtc);
            };
        },
        MODELVIEWPROJECTION : function(uniformState, model, runtimeNode) {
            var mvp = new Matrix4();
            return function() {
                Matrix4.multiplyTransformation(uniformState.view, runtimeNode.computedMatrix, mvp);
                return Matrix4.multiply(uniformState._projection, mvp, mvp);
            };
        },
        MODELINVERSE : function(uniformState, model, runtimeNode) {
            var mInverse = new Matrix4();
            return function() {
                return Matrix4.inverse(runtimeNode.computedMatrix, mInverse);
            };
        },
        VIEWINVERSE : function(uniformState, model) {
            return function() {
                return uniformState.inverseView;
            };
        },
        PROJECTIONINVERSE : function(uniformState, model, runtimeNode) {
            return function() {
                return uniformState.inverseProjection;
            };
        },
        MODELVIEWINVERSE : function(uniformState, model, runtimeNode) {
            var mv = new Matrix4();
            var mvInverse = new Matrix4();
            return function() {
                Matrix4.multiplyTransformation(uniformState.view, runtimeNode.computedMatrix, mv);
                return Matrix4.inverse(mv, mvInverse);
            };
        },
        MODELVIEWPROJECTIONINVERSE : function(uniformState, model, runtimeNode) {
            var mvp = new Matrix4();
            var mvpInverse = new Matrix4();
            return function() {
                Matrix4.multiplyTransformation(uniformState.view, runtimeNode.computedMatrix, mvp);
                Matrix4.multiply(uniformState._projection, mvp, mvp);
                return Matrix4.inverse(mvp, mvpInverse);
            };
        },
        MODELINVERSETRANSPOSE : function(uniformState, model, runtimeNode) {
            var mInverse = new Matrix4();
            var mInverseTranspose = new Matrix3();
            return function() {
                Matrix4.inverse(runtimeNode.computedMatrix, mInverse);
                Matrix4.getRotation(mInverse, mInverseTranspose);
                return Matrix3.transpose(mInverseTranspose, mInverseTranspose);
            };
        },
        MODELVIEWINVERSETRANSPOSE : function(uniformState, model, runtimeNode) {
            var mv = new Matrix4();
            var mvInverse = new Matrix4();
            var mvInverseTranspose = new Matrix3();
            return function() {
                Matrix4.multiplyTransformation(uniformState.view, runtimeNode.computedMatrix, mv);
                Matrix4.inverse(mv, mvInverse);
                Matrix4.getRotation(mvInverse, mvInverseTranspose);
                return Matrix3.transpose(mvInverseTranspose, mvInverseTranspose);
            };
        },
        VIEWPORT : function(uniformState, model, runtimeNode) {
            return function() {
                return uniformState.viewportCartesian4;
            };
        }
    };

    function getUniformFunctionFromSource(source, model, semantic, uniformState) {
        var runtimeNode = model._runtime.nodes[source];
        return gltfUniformsFromNode[semantic](uniformState, model, runtimeNode);
    }

    function createUniformsForMaterial(model, material, technique, instanceValues, context, textures, defaultTexture) {
        var uniformMap = {};
        var uniformValues = {};
        var jointMatrixUniformName;
        var morphWeightsUniformName;

        ForEach.techniqueUniform(technique, function(uniform, uniformName) {
            // GLTF_SPEC: This does not take into account uniform arrays,
            // indicated by uniforms with a count property.
            //
            // https://github.com/KhronosGroup/glTF/issues/258

            // GLTF_SPEC: In this implementation, material parameters with a
            // semantic or targeted via a source (for animation) are not
            // targetable for material animations.  Is this too strict?
            //
            // https://github.com/KhronosGroup/glTF/issues/142

            var uv;
            if (defined(instanceValues) && defined(instanceValues[uniformName])) {
                // Parameter overrides by the instance technique
                uv = ModelUtility.createUniformFunction(uniform.type, instanceValues[uniformName], textures, defaultTexture);
                uniformMap[uniformName] = uv.func;
                uniformValues[uniformName] = uv;
            } else if (defined(uniform.node)) {
                uniformMap[uniformName] = getUniformFunctionFromSource(uniform.node, model, uniform.semantic, context.uniformState);
            } else if (defined(uniform.semantic)) {
                if (uniform.semantic === 'JOINTMATRIX') {
                    jointMatrixUniformName = uniformName;
                } else if (uniform.semantic === 'MORPHWEIGHTS') {
                    morphWeightsUniformName = uniformName;
                } else if (uniform.semantic === 'ALPHACUTOFF') {
                    // The material's alphaCutoff value uses a uniform with semantic ALPHACUTOFF.
                    // A uniform with this semantic will ignore the instance or default values.
                    var alphaMode = material.alphaMode;
                    if (defined(alphaMode) && alphaMode === 'MASK') {
                        var alphaCutoffValue = defaultValue(material.alphaCutoff, 0.5);
                        uv = ModelUtility.createUniformFunction(uniform.type, alphaCutoffValue, textures, defaultTexture);
                        uniformMap[uniformName] = uv.func;
                        uniformValues[uniformName] = uv;
                    }
                } else {
                    // Map glTF semantic to Cesium automatic uniform
                    uniformMap[uniformName] = ModelUtility.getGltfSemanticUniforms()[uniform.semantic](context.uniformState, model);
                }
            } else if (defined(uniform.value)) {
                // Technique value that isn't overridden by a material
                var uv2 = ModelUtility.createUniformFunction(uniform.type, uniform.value, textures, defaultTexture);
                uniformMap[uniformName] = uv2.func;
                uniformValues[uniformName] = uv2;
            }
        });

        return {
            map : uniformMap,
            values : uniformValues,
            jointMatrixUniformName : jointMatrixUniformName,
            morphWeightsUniformName : morphWeightsUniformName
        };
    }

    function createUniformMaps(model, context) {
        var loadResources = model._loadResources;

        if (!loadResources.finishedProgramCreation()) {
            return;
        }

        if (!loadResources.createUniformMaps) {
            return;
        }
        loadResources.createUniformMaps = false;

        var gltf = model.gltf;
        var techniques = model._sourceTechniques;
        var uniformMaps = model._uniformMaps;

        var textures = model._rendererResources.textures;
        var defaultTexture = model._defaultTexture;

        ForEach.material(gltf, function (material, materialId) {
            var modelMaterial = model._runtime.materialsById[materialId];
            var technique = techniques[modelMaterial._technique];
            var instanceValues = modelMaterial._values;

            var uniforms = createUniformsForMaterial(model, material, technique, instanceValues, context, textures, defaultTexture);

            var u = uniformMaps[materialId];
            u.uniformMap = uniforms.map;                          // uniform name -> function for the renderer
            u.values = uniforms.values;                           // material parameter name -> ModelMaterial for modifying the parameter at runtime
            u.jointMatrixUniformName = uniforms.jointMatrixUniformName;
            u.morphWeightsUniformName = uniforms.morphWeightsUniformName;
        });
    }

    function createUniformsForDracoQuantizedAttributes(decodedData) {
        return ModelUtility.createUniformsForDracoQuantizedAttributes(decodedData.attributes);
    }

    function createUniformsForQuantizedAttributes(model, primitive) {
        var programId = getProgramForPrimitive(model, primitive);
        var quantizedUniforms = model._quantizedUniforms[programId];
        return ModelUtility.createUniformsForQuantizedAttributes(model.gltf, primitive, quantizedUniforms);
    }

    function createPickColorFunction(color) {
        return function() {
            return color;
        };
    }

    function createJointMatricesFunction(runtimeNode) {
        return function() {
            return runtimeNode.computedJointMatrices;
        };
    }

    function createMorphWeightsFunction(runtimeNode) {
        return function() {
            return runtimeNode.weights;
        };
    }

    function createSilhouetteColorFunction(model) {
        return function() {
            return model.silhouetteColor;
        };
    }

    function createSilhouetteSizeFunction(model) {
        return function() {
            return model.silhouetteSize;
        };
    }

    function createColorFunction(model) {
        return function() {
            return model.color;
        };
    }

    var scratchClippingPlaneMatrix = new Matrix4();
    function createClippingPlanesMatrixFunction(model) {
        return function() {
            var clippingPlanes = model.clippingPlanes;
            if (!defined(clippingPlanes) && !defined(model._sphericalHarmonicCoefficients) && !defined(model._specularEnvironmentMaps)) {
                return Matrix4.IDENTITY;
            }
            var modelMatrix = defined(clippingPlanes) ? clippingPlanes.modelMatrix : Matrix4.IDENTITY;
            return Matrix4.multiply(model._clippingPlaneModelViewMatrix, modelMatrix, scratchClippingPlaneMatrix);
        };
    }

    function createClippingPlanesFunction(model) {
        return function() {
            var clippingPlanes = model.clippingPlanes;
            return (!defined(clippingPlanes) || !clippingPlanes.enabled) ? model._defaultTexture : clippingPlanes.texture;
        };
    }

    function createClippingPlanesEdgeStyleFunction(model) {
        return function() {
            var clippingPlanes = model.clippingPlanes;
            if (!defined(clippingPlanes)) {
                return Color.WHITE.withAlpha(0.0);
            }

            var style = Color.clone(clippingPlanes.edgeColor);
            style.alpha = clippingPlanes.edgeWidth;
            return style;
        };
    }

    function createColorBlendFunction(model) {
        return function() {
            return ColorBlendMode.getColorBlend(model.colorBlendMode, model.colorBlendAmount);
        };
    }

    function createIBLFactorFunction(model) {
        return function() {
            return model._imageBasedLightingFactor;
        };
    }

    function createLightColorFunction(model) {
        return function() {
            return model._lightColor;
        };
    }

    function createLuminanceAtZenithFunction(model) {
        return function() {
            return model.luminanceAtZenith;
        };
    }

    function createSphericalHarmonicCoefficientsFunction(model) {
        return function() {
            return model._sphericalHarmonicCoefficients;
        };
    }

    function createSpecularEnvironmentMapFunction(model) {
        return function() {
            return model._specularEnvironmentMapAtlas.texture;
        };
    }

    function createSpecularEnvironmentMapSizeFunction(model) {
        return function() {
            return model._specularEnvironmentMapAtlas.texture.dimensions;
        };
    }

    function createSpecularEnvironmentMapLOD(model) {
        return function() {
            return model._specularEnvironmentMapAtlas.maximumMipmapLevel;
        };
    }

    function triangleCountFromPrimitiveIndices(primitive, indicesCount) {
        switch (primitive.mode) {
            case PrimitiveType.TRIANGLES:
                return (indicesCount / 3);
            case PrimitiveType.TRIANGLE_STRIP:
            case PrimitiveType.TRIANGLE_FAN:
                return Math.max(indicesCount - 2, 0);
            default:
                return 0;
        }
    }

    function createCommand(model, gltfNode, runtimeNode, context, scene3DOnly) {
        var nodeCommands = model._nodeCommands;
        var pickIds = model._pickIds;
        var allowPicking = model.allowPicking;
        var runtimeMeshesByName = model._runtime.meshesByName;

        var resources = model._rendererResources;
        var rendererVertexArrays = resources.vertexArrays;
        var rendererPrograms = resources.programs;
        var rendererRenderStates = resources.renderStates;
        var uniformMaps = model._uniformMaps;

        var gltf = model.gltf;
        var accessors = gltf.accessors;
        var gltfMeshes = gltf.meshes;

        var id = gltfNode.mesh;
        var mesh = gltfMeshes[id];

        var primitives = mesh.primitives;
        var length = primitives.length;

        // The glTF node hierarchy is a DAG so a node can have more than one
        // parent, so a node may already have commands.  If so, append more
        // since they will have a different model matrix.

        for (var i = 0; i < length; ++i) {
            var primitive = primitives[i];
            var ix = accessors[primitive.indices];
            var material = model._runtime.materialsById[primitive.material];
            var programId = material._program;
            var decodedData = model._decodedData[id + '.primitive.' + i];

            var boundingSphere;
            var positionAccessor = primitive.attributes.POSITION;
            if (defined(positionAccessor)) {
                var minMax = ModelUtility.getAccessorMinMax(gltf, positionAccessor);
                boundingSphere = BoundingSphere.fromCornerPoints(Cartesian3.fromArray(minMax.min), Cartesian3.fromArray(minMax.max));
            }

            var vertexArray = rendererVertexArrays[id + '.primitive.' + i];
            var offset;
            var count;

            // Use indices of the previously decoded Draco geometry.
            if (defined(decodedData)) {
                count = decodedData.numberOfIndices;
                offset = 0;
            } else if (defined(ix)) {
                count = ix.count;
                offset = (ix.byteOffset / IndexDatatype.getSizeInBytes(ix.componentType)); // glTF has offset in bytes.  Cesium has offsets in indices
            } else {
                var positions = accessors[primitive.attributes.POSITION];
                count = positions.count;
                offset = 0;
            }

            // Update model triangle count using number of indices
            model._trianglesLength += triangleCountFromPrimitiveIndices(primitive, count);

            var um = uniformMaps[primitive.material];
            var uniformMap = um.uniformMap;
            if (defined(um.jointMatrixUniformName)) {
                var jointUniformMap = {};
                jointUniformMap[um.jointMatrixUniformName] = createJointMatricesFunction(runtimeNode);

                uniformMap = combine(uniformMap, jointUniformMap);
            }
            if (defined(um.morphWeightsUniformName)) {
                var morphWeightsUniformMap = {};
                morphWeightsUniformMap[um.morphWeightsUniformName] = createMorphWeightsFunction(runtimeNode);

                uniformMap = combine(uniformMap, morphWeightsUniformMap);
            }

            uniformMap = combine(uniformMap, {
                gltf_color : createColorFunction(model),
                gltf_colorBlend : createColorBlendFunction(model),
                gltf_clippingPlanes : createClippingPlanesFunction(model),
                gltf_clippingPlanesEdgeStyle : createClippingPlanesEdgeStyleFunction(model),
                gltf_clippingPlanesMatrix : createClippingPlanesMatrixFunction(model),
                gltf_iblFactor : createIBLFactorFunction(model),
                gltf_lightColor : createLightColorFunction(model),
                gltf_sphericalHarmonicCoefficients : createSphericalHarmonicCoefficientsFunction(model),
                gltf_specularMap : createSpecularEnvironmentMapFunction(model),
                gltf_specularMapSize : createSpecularEnvironmentMapSizeFunction(model),
                gltf_maxSpecularLOD : createSpecularEnvironmentMapLOD(model),
                gltf_luminanceAtZenith : createLuminanceAtZenithFunction(model)
            });

            // Allow callback to modify the uniformMap
            if (defined(model._uniformMapLoaded)) {
                uniformMap = model._uniformMapLoaded(uniformMap, programId, runtimeNode);
            }

            // Add uniforms for decoding quantized attributes if used
            var quantizedUniformMap = {};
            if (model.extensionsUsed.WEB3D_quantized_attributes) {
                quantizedUniformMap = createUniformsForQuantizedAttributes(model, primitive);
            } else if (model._dequantizeInShader && defined(decodedData)) {
                quantizedUniformMap = createUniformsForDracoQuantizedAttributes(decodedData);
            }
            uniformMap = combine(uniformMap, quantizedUniformMap);

            var rs = rendererRenderStates[primitive.material];
            var isTranslucent = rs.blending.enabled;

            var owner = model._pickObject;
            if (!defined(owner)) {
                owner = {
                    primitive : model,
                    id : model.id,
                    node : runtimeNode.publicNode,
                    mesh : runtimeMeshesByName[mesh.name]
                };
            }

            var castShadows = ShadowMode.castShadows(model._shadows);
            var receiveShadows = ShadowMode.receiveShadows(model._shadows);

            var pickId;
            if (allowPicking && !defined(model._uniformMapLoaded)) {
                pickId = context.createPickId(owner);
                pickIds.push(pickId);
                var pickUniforms = {
                    czm_pickColor : createPickColorFunction(pickId.color)
                };
                uniformMap = combine(uniformMap, pickUniforms);
            }

            if (allowPicking) {
                if (defined(model._pickIdLoaded) && defined(model._uniformMapLoaded)) {
                    pickId = model._pickIdLoaded();
                } else {
                    pickId = 'czm_pickColor';
                }
            }

            var command = new DrawCommand({
                boundingVolume : new BoundingSphere(), // updated in update()
                cull : model.cull,
                modelMatrix : new Matrix4(),           // computed in update()
                primitiveType : primitive.mode,
                vertexArray : vertexArray,
                count : count,
                offset : offset,
                shaderProgram : rendererPrograms[programId],
                castShadows : castShadows,
                receiveShadows : receiveShadows,
                uniformMap : uniformMap,
                renderState : rs,
                owner : owner,
                pass : isTranslucent ? Pass.TRANSLUCENT : model.opaquePass,
                pickId : pickId
            });

            var command2D;
            if (!scene3DOnly) {
                command2D = DrawCommand.shallowClone(command);
                command2D.boundingVolume = new BoundingSphere(); // updated in update()
                command2D.modelMatrix = new Matrix4();           // updated in update()
            }

            var nodeCommand = {
                show : true,
                boundingSphere : boundingSphere,
                command : command,
                command2D : command2D,
                // Generated on demand when silhouette size is greater than 0.0 and silhouette alpha is greater than 0.0
                silhouetteModelCommand : undefined,
                silhouetteModelCommand2D : undefined,
                silhouetteColorCommand : undefined,
                silhouetteColorCommand2D : undefined,
                // Generated on demand when color alpha is less than 1.0
                translucentCommand : undefined,
                translucentCommand2D : undefined,
                // For updating node commands on shader reconstruction
                programId : programId
            };
            runtimeNode.commands.push(nodeCommand);
            nodeCommands.push(nodeCommand);
        }
    }

    function createRuntimeNodes(model, context, scene3DOnly) {
        var loadResources = model._loadResources;

        if (!loadResources.finishedEverythingButTextureCreation()) {
            return;
        }

        if (!loadResources.createRuntimeNodes) {
            return;
        }
        loadResources.createRuntimeNodes = false;

        var rootNodes = [];
        var runtimeNodes = model._runtime.nodes;

        var gltf = model.gltf;
        var nodes = gltf.nodes;
        var skins = gltf.skins;

        var scene = gltf.scenes[gltf.scene];
        var sceneNodes = scene.nodes;
        var length = sceneNodes.length;

        var stack = [];
        var seen = {};

        for (var i = 0; i < length; ++i) {
            stack.push({
                parentRuntimeNode : undefined,
                gltfNode : nodes[sceneNodes[i]],
                id : sceneNodes[i]
            });

            var skeletonIds = [];
            while (stack.length > 0) {
                var n = stack.pop();
                seen[n.id] = true;
                var parentRuntimeNode = n.parentRuntimeNode;
                var gltfNode = n.gltfNode;

                // Node hierarchy is a DAG so a node can have more than one parent so it may already exist
                var runtimeNode = runtimeNodes[n.id];
                if (runtimeNode.parents.length === 0) {
                    if (defined(gltfNode.matrix)) {
                        runtimeNode.matrix = Matrix4.fromColumnMajorArray(gltfNode.matrix);
                    } else {
                        // TRS converted to Cesium types
                        var rotation = gltfNode.rotation;
                        runtimeNode.translation = Cartesian3.fromArray(gltfNode.translation);
                        runtimeNode.rotation = Quaternion.unpack(rotation);
                        runtimeNode.scale = Cartesian3.fromArray(gltfNode.scale);
                    }
                }

                if (defined(parentRuntimeNode)) {
                    parentRuntimeNode.children.push(runtimeNode);
                    runtimeNode.parents.push(parentRuntimeNode);
                } else {
                    rootNodes.push(runtimeNode);
                }

                if (defined(gltfNode.mesh)) {
                    createCommand(model, gltfNode, runtimeNode, context, scene3DOnly);
                }

                var children = gltfNode.children;
                if (defined(children)) {
                    var childrenLength = children.length;
                    for (var j = 0; j < childrenLength; j++) {
                        var childId = children[j];
                        if (!seen[childId]) {
                            stack.push({
                                parentRuntimeNode : runtimeNode,
                                gltfNode : nodes[childId],
                                id : children[j]
                            });
                        }
                    }
                }

                var skin = gltfNode.skin;
                if (defined(skin)) {
                    skeletonIds.push(skins[skin].skeleton);
                }

                if (stack.length === 0) {
                    for (var k = 0; k < skeletonIds.length; k++) {
                        var skeleton = skeletonIds[k];
                        if (!seen[skeleton]) {
                            stack.push({
                                parentRuntimeNode : undefined,
                                gltfNode : nodes[skeleton],
                                id : skeleton
                            });
                        }
                    }
                }
            }
        }

        model._runtime.rootNodes = rootNodes;
        model._runtime.nodes = runtimeNodes;
    }

    function getGeometryByteLength(buffers) {
        var memory = 0;
        for (var id in buffers) {
            if (buffers.hasOwnProperty(id)) {
                memory += buffers[id].sizeInBytes;
            }
        }
        return memory;
    }

    function getTexturesByteLength(textures) {
        var memory = 0;
        for (var id in textures) {
            if (textures.hasOwnProperty(id)) {
                memory += textures[id].sizeInBytes;
            }
        }
        return memory;
    }

    function createResources(model, frameState) {
        var context = frameState.context;
        var scene3DOnly = frameState.scene3DOnly;
        var quantizedVertexShaders = model._quantizedVertexShaders;
        var toClipCoordinates = model._toClipCoordinatesGLSL = {};
        var techniques = model._sourceTechniques;
        var programs = model._sourcePrograms;

        var resources = model._rendererResources;
        var shaders = resources.sourceShaders;
        if (model._loadRendererResourcesFromCache) {
            shaders = resources.sourceShaders = model._cachedRendererResources.sourceShaders;
        }

        for (var techniqueId in techniques) {
            if (techniques.hasOwnProperty(techniqueId)) {
                var programId = techniques[techniqueId].program;
                var program = programs[programId];
                var shader = shaders[program.vertexShader];

                ModelUtility.checkSupportedGlExtensions(program.glExtensions, context);

                if (model.extensionsUsed.WEB3D_quantized_attributes || model._dequantizeInShader) {
                    var quantizedVS = quantizedVertexShaders[programId];
                    if (!defined(quantizedVS)) {
                        quantizedVS = modifyShaderForQuantizedAttributes(shader, programId, model);
                        quantizedVertexShaders[programId] = quantizedVS;
                    }
                    shader = quantizedVS;
                }

                shader = modifyShader(shader, programId, model._vertexShaderLoaded);
                toClipCoordinates[programId] = ModelUtility.toClipCoordinatesGLSL(model.gltf, shader);
            }
        }

        if (model._loadRendererResourcesFromCache) {
            var cachedResources = model._cachedRendererResources;

            resources.buffers = cachedResources.buffers;
            resources.vertexArrays = cachedResources.vertexArrays;
            resources.programs = cachedResources.programs;
            resources.silhouettePrograms = cachedResources.silhouettePrograms;
            resources.textures = cachedResources.textures;
            resources.samplers = cachedResources.samplers;
            resources.renderStates = cachedResources.renderStates;

            // Vertex arrays are unique to this model, create instead of using the cache.
            if (defined(model._precreatedAttributes)) {
                createVertexArrays(model, context);
            }

            model._cachedGeometryByteLength += getGeometryByteLength(cachedResources.buffers);
            model._cachedTexturesByteLength += getTexturesByteLength(cachedResources.textures);
        } else {
            createBuffers(model, frameState); // using glTF bufferViews
            createPrograms(model, frameState);
            createSamplers(model, context);
            loadTexturesFromBufferViews(model);
            createTextures(model, frameState);
        }

        createSkins(model);
        createRuntimeAnimations(model);

        if (!model._loadRendererResourcesFromCache) {
            createVertexArrays(model, context); // using glTF meshes
            createRenderStates(model); // using glTF materials/techniques/states
            // Long-term, we might not cache render states if they could change
            // due to an animation, e.g., a uniform going from opaque to transparent.
            // Could use copy-on-write if it is worth it.  Probably overkill.
        }

        createUniformMaps(model, context);               // using glTF materials/techniques
        createRuntimeNodes(model, context, scene3DOnly); // using glTF scene
    }

    ///////////////////////////////////////////////////////////////////////////

    function getNodeMatrix(node, result) {
        var publicNode = node.publicNode;
        var publicMatrix = publicNode.matrix;

        if (publicNode.useMatrix && defined(publicMatrix)) {
            // Public matrix overrides original glTF matrix and glTF animations
            Matrix4.clone(publicMatrix, result);
        } else if (defined(node.matrix)) {
            Matrix4.clone(node.matrix, result);
        } else {
            Matrix4.fromTranslationQuaternionRotationScale(node.translation, node.rotation, node.scale, result);
            // Keep matrix returned by the node in-sync if the node is targeted by an animation.  Only TRS nodes can be targeted.
            publicNode.setMatrix(result);
        }
    }

    var scratchNodeStack = [];
    var scratchComputedTranslation = new Cartesian4();
    var scratchComputedMatrixIn2D = new Matrix4();

    function updateNodeHierarchyModelMatrix(model, modelTransformChanged, justLoaded, projection) {
        var maxDirtyNumber = model._maxDirtyNumber;

        var rootNodes = model._runtime.rootNodes;
        var length = rootNodes.length;

        var nodeStack = scratchNodeStack;
        var computedModelMatrix = model._computedModelMatrix;

        if ((model._mode !== SceneMode.SCENE3D) && !model._ignoreCommands) {
            var translation = Matrix4.getColumn(computedModelMatrix, 3, scratchComputedTranslation);
            if (!Cartesian4.equals(translation, Cartesian4.UNIT_W)) {
                computedModelMatrix = Transforms.basisTo2D(projection, computedModelMatrix, scratchComputedMatrixIn2D);
                model._rtcCenter = model._rtcCenter3D;
            } else {
                var center = model.boundingSphere.center;
                var to2D = Transforms.wgs84To2DModelMatrix(projection, center, scratchComputedMatrixIn2D);
                computedModelMatrix = Matrix4.multiply(to2D, computedModelMatrix, scratchComputedMatrixIn2D);

                if (defined(model._rtcCenter)) {
                    Matrix4.setTranslation(computedModelMatrix, Cartesian4.UNIT_W, computedModelMatrix);
                    model._rtcCenter = model._rtcCenter2D;
                }
            }
        }

        for (var i = 0; i < length; ++i) {
            var n = rootNodes[i];

            getNodeMatrix(n, n.transformToRoot);
            nodeStack.push(n);

            while (nodeStack.length > 0) {
                n = nodeStack.pop();
                var transformToRoot = n.transformToRoot;
                var commands = n.commands;

                if ((n.dirtyNumber === maxDirtyNumber) || modelTransformChanged || justLoaded) {
                    var nodeMatrix = Matrix4.multiplyTransformation(computedModelMatrix, transformToRoot, n.computedMatrix);
                    var commandsLength = commands.length;
                    if (commandsLength > 0) {
                        // Node has meshes, which has primitives.  Update their commands.
                        for (var j = 0; j < commandsLength; ++j) {
                            var primitiveCommand = commands[j];
                            var command = primitiveCommand.command;
                            Matrix4.clone(nodeMatrix, command.modelMatrix);

                            // PERFORMANCE_IDEA: Can use transformWithoutScale if no node up to the root has scale (including animation)
                            BoundingSphere.transform(primitiveCommand.boundingSphere, command.modelMatrix, command.boundingVolume);

                            if (defined(model._rtcCenter)) {
                                Cartesian3.add(model._rtcCenter, command.boundingVolume.center, command.boundingVolume.center);
                            }

                            // If the model crosses the IDL in 2D, it will be drawn in one viewport, but part of it
                            // will be clipped by the viewport. We create a second command that translates the model
                            // model matrix to the opposite side of the map so the part that was clipped in one viewport
                            // is drawn in the other.
                            command = primitiveCommand.command2D;
                            if (defined(command) && model._mode === SceneMode.SCENE2D) {
                                Matrix4.clone(nodeMatrix, command.modelMatrix);
                                command.modelMatrix[13] -= CesiumMath.sign(command.modelMatrix[13]) * 2.0 * CesiumMath.PI * projection.ellipsoid.maximumRadius;
                                BoundingSphere.transform(primitiveCommand.boundingSphere, command.modelMatrix, command.boundingVolume);
                            }
                        }
                    }
                }

                var children = n.children;
                if (defined(children)) {
                    var childrenLength = children.length;
                    for (var k = 0; k < childrenLength; ++k) {
                        var child = children[k];

                        // A node's transform needs to be updated if
                        // - It was targeted for animation this frame, or
                        // - Any of its ancestors were targeted for animation this frame

                        // PERFORMANCE_IDEA: if a child has multiple parents and only one of the parents
                        // is dirty, all the subtrees for each child instance will be dirty; we probably
                        // won't see this in the wild often.
                        child.dirtyNumber = Math.max(child.dirtyNumber, n.dirtyNumber);

                        if ((child.dirtyNumber === maxDirtyNumber) || justLoaded) {
                            // Don't check for modelTransformChanged since if only the model's model matrix changed,
                            // we do not need to rebuild the local transform-to-root, only the final
                            // [model's-model-matrix][transform-to-root] above.
                            getNodeMatrix(child, child.transformToRoot);
                            Matrix4.multiplyTransformation(transformToRoot, child.transformToRoot, child.transformToRoot);
                        }

                        nodeStack.push(child);
                    }
                }
            }
        }

        ++model._maxDirtyNumber;
    }

    var scratchObjectSpace = new Matrix4();

    function applySkins(model) {
        var skinnedNodes = model._runtime.skinnedNodes;
        var length = skinnedNodes.length;

        for (var i = 0; i < length; ++i) {
            var node = skinnedNodes[i];

            scratchObjectSpace = Matrix4.inverseTransformation(node.transformToRoot, scratchObjectSpace);

            var computedJointMatrices = node.computedJointMatrices;
            var joints = node.joints;
            var bindShapeMatrix = node.bindShapeMatrix;
            var inverseBindMatrices = node.inverseBindMatrices;
            var inverseBindMatricesLength = inverseBindMatrices.length;

            for (var m = 0; m < inverseBindMatricesLength; ++m) {
                // [joint-matrix] = [node-to-root^-1][joint-to-root][inverse-bind][bind-shape]
                if (!defined(computedJointMatrices[m])) {
                    computedJointMatrices[m] = new Matrix4();
                }
                computedJointMatrices[m] = Matrix4.multiplyTransformation(scratchObjectSpace, joints[m].transformToRoot, computedJointMatrices[m]);
                computedJointMatrices[m] = Matrix4.multiplyTransformation(computedJointMatrices[m], inverseBindMatrices[m], computedJointMatrices[m]);
                if (defined(bindShapeMatrix)) {
                    // Optimization for when bind shape matrix is the identity.
                    computedJointMatrices[m] = Matrix4.multiplyTransformation(computedJointMatrices[m], bindShapeMatrix, computedJointMatrices[m]);
                }
            }
        }
    }

    function updatePerNodeShow(model) {
        // Totally not worth it, but we could optimize this:
        // http://blogs.agi.com/insight3d/index.php/2008/02/13/deletion-in-bounding-volume-hierarchies/

        var rootNodes = model._runtime.rootNodes;
        var length = rootNodes.length;

        var nodeStack = scratchNodeStack;

        for (var i = 0; i < length; ++i) {
            var n = rootNodes[i];
            n.computedShow = n.publicNode.show;
            nodeStack.push(n);

            while (nodeStack.length > 0) {
                n = nodeStack.pop();
                var show = n.computedShow;

                var nodeCommands = n.commands;
                var nodeCommandsLength = nodeCommands.length;
                for (var j = 0; j < nodeCommandsLength; ++j) {
                    nodeCommands[j].show = show;
                }
                // if commandsLength is zero, the node has a light or camera

                var children = n.children;
                if (defined(children)) {
                    var childrenLength = children.length;
                    for (var k = 0; k < childrenLength; ++k) {
                        var child = children[k];
                        // Parent needs to be shown for child to be shown.
                        child.computedShow = show && child.publicNode.show;
                        nodeStack.push(child);
                    }
                }
            }
        }
    }

    function updatePickIds(model, context) {
        var id = model.id;
        if (model._id !== id) {
            model._id = id;

            var pickIds = model._pickIds;
            var length = pickIds.length;
            for (var i = 0; i < length; ++i) {
                pickIds[i].object.id = id;
            }
        }
    }

    function updateWireframe(model) {
        if (model._debugWireframe !== model.debugWireframe) {
            model._debugWireframe = model.debugWireframe;

            // This assumes the original primitive was TRIANGLES and that the triangles
            // are connected for the wireframe to look perfect.
            var primitiveType = model.debugWireframe ? PrimitiveType.LINES : PrimitiveType.TRIANGLES;
            var nodeCommands = model._nodeCommands;
            var length = nodeCommands.length;

            for (var i = 0; i < length; ++i) {
                nodeCommands[i].command.primitiveType = primitiveType;
            }
        }
    }

    function updateShowBoundingVolume(model) {
        if (model.debugShowBoundingVolume !== model._debugShowBoundingVolume) {
            model._debugShowBoundingVolume = model.debugShowBoundingVolume;

            var debugShowBoundingVolume = model.debugShowBoundingVolume;
            var nodeCommands = model._nodeCommands;
            var length = nodeCommands.length;

            for (var i = 0; i < length; ++i) {
                nodeCommands[i].command.debugShowBoundingVolume = debugShowBoundingVolume;
            }
        }
    }

    function updateShadows(model) {
        if (model.shadows !== model._shadows) {
            model._shadows = model.shadows;

            var castShadows = ShadowMode.castShadows(model.shadows);
            var receiveShadows = ShadowMode.receiveShadows(model.shadows);
            var nodeCommands = model._nodeCommands;
            var length = nodeCommands.length;

            for (var i = 0; i < length; i++) {
                var nodeCommand = nodeCommands[i];
                nodeCommand.command.castShadows = castShadows;
                nodeCommand.command.receiveShadows = receiveShadows;
            }
        }
    }

    function getTranslucentRenderState(renderState) {
        var rs = clone(renderState, true);
        rs.cull.enabled = false;
        rs.depthTest.enabled = true;
        rs.depthMask = false;
        rs.blending = BlendingState.ALPHA_BLEND;

        return RenderState.fromCache(rs);
    }

    function deriveTranslucentCommand(command) {
        var translucentCommand = DrawCommand.shallowClone(command);
        translucentCommand.pass = Pass.TRANSLUCENT;
        translucentCommand.renderState = getTranslucentRenderState(command.renderState);
        return translucentCommand;
    }

    function updateColor(model, frameState, forceDerive) {
        // Generate translucent commands when the blend color has an alpha in the range (0.0, 1.0) exclusive
        var scene3DOnly = frameState.scene3DOnly;
        var alpha = model.color.alpha;
        if ((alpha > 0.0) && (alpha < 1.0)) {
            var nodeCommands = model._nodeCommands;
            var length = nodeCommands.length;
            if (!defined(nodeCommands[0].translucentCommand) || forceDerive) {
                for (var i = 0; i < length; ++i) {
                    var nodeCommand = nodeCommands[i];
                    var command = nodeCommand.command;
                    nodeCommand.translucentCommand = deriveTranslucentCommand(command);
                    if (!scene3DOnly) {
                        var command2D = nodeCommand.command2D;
                        nodeCommand.translucentCommand2D = deriveTranslucentCommand(command2D);
                    }
                }
            }
        }
    }

    function getProgramId(model, program) {
        var programs = model._rendererResources.programs;
        for (var id in programs) {
            if (programs.hasOwnProperty(id)) {
                if (programs[id] === program) {
                    return id;
                }
            }
        }
    }

    function createSilhouetteProgram(model, program, frameState) {
        var vs = program.vertexShaderSource.sources[0];
        var attributeLocations = program._attributeLocations;
        var normalAttributeName = model._normalAttributeName;

        // Modified from http://forum.unity3d.com/threads/toon-outline-but-with-diffuse-surface.24668/
        vs = ShaderSource.replaceMain(vs, 'gltf_silhouette_main');
        vs +=
            'uniform float gltf_silhouetteSize; \n' +
            'void main() \n' +
            '{ \n' +
            '    gltf_silhouette_main(); \n' +
            '    vec3 n = normalize(czm_normal3D * ' + normalAttributeName + '); \n' +
            '    n.x *= czm_projection[0][0]; \n' +
            '    n.y *= czm_projection[1][1]; \n' +
            '    vec4 clip = gl_Position; \n' +
            '    clip.xy += n.xy * clip.w * gltf_silhouetteSize / czm_viewport.z; \n' +
            '    gl_Position = clip; \n' +
            '}';

        var fs =
            'uniform vec4 gltf_silhouetteColor; \n' +
            'void main() \n' +
            '{ \n' +
            '    gl_FragColor = czm_gammaCorrect(gltf_silhouetteColor); \n' +
            '}';

        return ShaderProgram.fromCache({
            context : frameState.context,
            vertexShaderSource : vs,
            fragmentShaderSource : fs,
            attributeLocations : attributeLocations
        });
    }

    function hasSilhouette(model, frameState) {
        return silhouetteSupported(frameState.context) && (model.silhouetteSize > 0.0) && (model.silhouetteColor.alpha > 0.0) && defined(model._normalAttributeName);
    }

    function hasTranslucentCommands(model) {
        var nodeCommands = model._nodeCommands;
        var length = nodeCommands.length;
        for (var i = 0; i < length; ++i) {
            var nodeCommand = nodeCommands[i];
            var command = nodeCommand.command;
            if (command.pass === Pass.TRANSLUCENT) {
                return true;
            }
        }
        return false;
    }

    function isTranslucent(model) {
        return (model.color.alpha > 0.0) && (model.color.alpha < 1.0);
    }

    function isInvisible(model) {
        return (model.color.alpha === 0.0);
    }

    function alphaDirty(currAlpha, prevAlpha) {
        // Returns whether the alpha state has changed between invisible, translucent, or opaque
        return (Math.floor(currAlpha) !== Math.floor(prevAlpha)) || (Math.ceil(currAlpha) !== Math.ceil(prevAlpha));
    }

    var silhouettesLength = 0;

    function createSilhouetteCommands(model, frameState) {
        // Wrap around after exceeding the 8-bit stencil limit.
        // The reference is unique to each model until this point.
        var stencilReference = (++silhouettesLength) % 255;

        // If the model is translucent the silhouette needs to be in the translucent pass.
        // Otherwise the silhouette would be rendered before the model.
        var silhouetteTranslucent = hasTranslucentCommands(model) || isTranslucent(model) || (model.silhouetteColor.alpha < 1.0);
        var silhouettePrograms = model._rendererResources.silhouettePrograms;
        var scene3DOnly = frameState.scene3DOnly;
        var nodeCommands = model._nodeCommands;
        var length = nodeCommands.length;
        for (var i = 0; i < length; ++i) {
            var nodeCommand = nodeCommands[i];
            var command = nodeCommand.command;

            // Create model command
            var modelCommand = isTranslucent(model) ? nodeCommand.translucentCommand : command;
            var silhouetteModelCommand = DrawCommand.shallowClone(modelCommand);
            var renderState = clone(modelCommand.renderState);

            // Write the reference value into the stencil buffer.
            renderState.stencilTest = {
                enabled : true,
                frontFunction : WebGLConstants.ALWAYS,
                backFunction : WebGLConstants.ALWAYS,
                reference : stencilReference,
                mask : ~0,
                frontOperation : {
                    fail : WebGLConstants.KEEP,
                    zFail : WebGLConstants.KEEP,
                    zPass : WebGLConstants.REPLACE
                },
                backOperation : {
                    fail : WebGLConstants.KEEP,
                    zFail : WebGLConstants.KEEP,
                    zPass : WebGLConstants.REPLACE
                }
            };

            if (isInvisible(model)) {
                // When the model is invisible disable color and depth writes but still write into the stencil buffer
                renderState.colorMask = {
                    red : false,
                    green : false,
                    blue : false,
                    alpha : false
                };
                renderState.depthMask = false;
            }
            renderState = RenderState.fromCache(renderState);
            silhouetteModelCommand.renderState = renderState;
            nodeCommand.silhouetteModelCommand = silhouetteModelCommand;

            // Create color command
            var silhouetteColorCommand = DrawCommand.shallowClone(command);
            renderState = clone(command.renderState, true);
            renderState.depthTest.enabled = true;
            renderState.cull.enabled = false;
            if (silhouetteTranslucent) {
                silhouetteColorCommand.pass = Pass.TRANSLUCENT;
                renderState.depthMask = false;
                renderState.blending = BlendingState.ALPHA_BLEND;
            }

            // Only render silhouette if the value in the stencil buffer equals the reference
            renderState.stencilTest = {
                enabled : true,
                frontFunction : WebGLConstants.NOTEQUAL,
                backFunction : WebGLConstants.NOTEQUAL,
                reference : stencilReference,
                mask : ~0,
                frontOperation : {
                    fail : WebGLConstants.KEEP,
                    zFail : WebGLConstants.KEEP,
                    zPass : WebGLConstants.KEEP
                },
                backOperation : {
                    fail : WebGLConstants.KEEP,
                    zFail : WebGLConstants.KEEP,
                    zPass : WebGLConstants.KEEP
                }
            };
            renderState = RenderState.fromCache(renderState);

            // If the silhouette program has already been cached use it
            var program = command.shaderProgram;
            var id = getProgramId(model, program);
            var silhouetteProgram = silhouettePrograms[id];
            if (!defined(silhouetteProgram)) {
                silhouetteProgram = createSilhouetteProgram(model, program, frameState);
                silhouettePrograms[id] = silhouetteProgram;
            }

            var silhouetteUniformMap = combine(command.uniformMap, {
                gltf_silhouetteColor : createSilhouetteColorFunction(model),
                gltf_silhouetteSize : createSilhouetteSizeFunction(model)
            });

            silhouetteColorCommand.renderState = renderState;
            silhouetteColorCommand.shaderProgram = silhouetteProgram;
            silhouetteColorCommand.uniformMap = silhouetteUniformMap;
            silhouetteColorCommand.castShadows = false;
            silhouetteColorCommand.receiveShadows = false;
            nodeCommand.silhouetteColorCommand = silhouetteColorCommand;

            if (!scene3DOnly) {
                var command2D = nodeCommand.command2D;
                var silhouetteModelCommand2D = DrawCommand.shallowClone(silhouetteModelCommand);
                silhouetteModelCommand2D.boundingVolume = command2D.boundingVolume;
                silhouetteModelCommand2D.modelMatrix = command2D.modelMatrix;
                nodeCommand.silhouetteModelCommand2D = silhouetteModelCommand2D;

                var silhouetteColorCommand2D = DrawCommand.shallowClone(silhouetteColorCommand);
                silhouetteModelCommand2D.boundingVolume = command2D.boundingVolume;
                silhouetteModelCommand2D.modelMatrix = command2D.modelMatrix;
                nodeCommand.silhouetteColorCommand2D = silhouetteColorCommand2D;
            }
        }
    }

    function modifyShaderForClippingPlanes(shader, clippingPlaneCollection, context) {
        shader = ShaderSource.replaceMain(shader, 'gltf_clip_main');
        shader += Model._getClippingFunction(clippingPlaneCollection, context) + '\n';
        shader +=
            'uniform sampler2D gltf_clippingPlanes; \n' +
            'uniform mat4 gltf_clippingPlanesMatrix; \n' +
            'uniform vec4 gltf_clippingPlanesEdgeStyle; \n' +
            'void main() \n' +
            '{ \n' +
            '    gltf_clip_main(); \n' +
            getClipAndStyleCode('gltf_clippingPlanes', 'gltf_clippingPlanesMatrix', 'gltf_clippingPlanesEdgeStyle') +
            '} \n';
        return shader;
    }

    function updateSilhouette(model, frameState, force) {
        // Generate silhouette commands when the silhouette size is greater than 0.0 and the alpha is greater than 0.0
        // There are two silhouette commands:
        //     1. silhouetteModelCommand : render model normally while enabling stencil mask
        //     2. silhouetteColorCommand : render enlarged model with a solid color while enabling stencil tests
        if (!hasSilhouette(model, frameState)) {
            return;
        }

        var nodeCommands = model._nodeCommands;
        var dirty = alphaDirty(model.color.alpha, model._colorPreviousAlpha) ||
                    alphaDirty(model.silhouetteColor.alpha, model._silhouetteColorPreviousAlpha) ||
                    !defined(nodeCommands[0].silhouetteModelCommand);

        model._colorPreviousAlpha = model.color.alpha;
        model._silhouetteColorPreviousAlpha = model.silhouetteColor.alpha;

        if (dirty || force) {
            createSilhouetteCommands(model, frameState);
        }
    }

    function updateClippingPlanes(model, frameState) {
        var clippingPlanes = model._clippingPlanes;
        if (defined(clippingPlanes) && clippingPlanes.owner === model) {
            if (clippingPlanes.enabled) {
                clippingPlanes.update(frameState);
            }
        }
    }

    var scratchBoundingSphere = new BoundingSphere();

    function scaleInPixels(positionWC, radius, frameState) {
        scratchBoundingSphere.center = positionWC;
        scratchBoundingSphere.radius = radius;
        return frameState.camera.getPixelSize(scratchBoundingSphere, frameState.context.drawingBufferWidth, frameState.context.drawingBufferHeight);
    }

    var scratchPosition = new Cartesian3();
    var scratchCartographic = new Cartographic();

    function getScale(model, frameState) {
        var scale = model.scale;

        if (model.minimumPixelSize !== 0.0) {
            // Compute size of bounding sphere in pixels
            var context = frameState.context;
            var maxPixelSize = Math.max(context.drawingBufferWidth, context.drawingBufferHeight);
            var m = defined(model._clampedModelMatrix) ? model._clampedModelMatrix : model.modelMatrix;
            scratchPosition.x = m[12];
            scratchPosition.y = m[13];
            scratchPosition.z = m[14];

            if (defined(model._rtcCenter)) {
                Cartesian3.add(model._rtcCenter, scratchPosition, scratchPosition);
            }

            if (model._mode !== SceneMode.SCENE3D) {
                var projection = frameState.mapProjection;
                var cartographic = projection.ellipsoid.cartesianToCartographic(scratchPosition, scratchCartographic);
                projection.project(cartographic, scratchPosition);
                Cartesian3.fromElements(scratchPosition.z, scratchPosition.x, scratchPosition.y, scratchPosition);
            }

            var radius = model.boundingSphere.radius;
            var metersPerPixel = scaleInPixels(scratchPosition, radius, frameState);

            // metersPerPixel is always > 0.0
            var pixelsPerMeter = 1.0 / metersPerPixel;
            var diameterInPixels = Math.min(pixelsPerMeter * (2.0 * radius), maxPixelSize);

            // Maintain model's minimum pixel size
            if (diameterInPixels < model.minimumPixelSize) {
                scale = (model.minimumPixelSize * metersPerPixel) / (2.0 * model._initialRadius);
            }
        }

        return defined(model.maximumScale) ? Math.min(model.maximumScale, scale) : scale;
    }

    function releaseCachedGltf(model) {
        if (defined(model._cacheKey) && defined(model._cachedGltf) && (--model._cachedGltf.count === 0)) {
            delete gltfCache[model._cacheKey];
        }
        model._cachedGltf = undefined;
    }

    ///////////////////////////////////////////////////////////////////////////

    function CachedRendererResources(context, cacheKey) {
        this.buffers = undefined;
        this.vertexArrays = undefined;
        this.programs = undefined;
        this.sourceShaders = undefined;
        this.silhouettePrograms = undefined;
        this.textures = undefined;
        this.samplers = undefined;
        this.renderStates = undefined;
        this.ready = false;

        this.context = context;
        this.cacheKey = cacheKey;
        this.count = 0;
    }

    function destroy(property) {
        for (var name in property) {
            if (property.hasOwnProperty(name)) {
                property[name].destroy();
            }
        }
    }

    function destroyCachedRendererResources(resources) {
        destroy(resources.buffers);
        destroy(resources.vertexArrays);
        destroy(resources.programs);
        destroy(resources.silhouettePrograms);
        destroy(resources.textures);
    }

    CachedRendererResources.prototype.release = function() {
        if (--this.count === 0) {
            if (defined(this.cacheKey)) {
                // Remove if this was cached
                delete this.context.cache.modelRendererResourceCache[this.cacheKey];
            }
            destroyCachedRendererResources(this);
            return destroyObject(this);
        }

        return undefined;
    };

    ///////////////////////////////////////////////////////////////////////////

    function getUpdateHeightCallback(model, ellipsoid, cartoPosition) {
        return function(clampedPosition) {
            if (model.heightReference === HeightReference.RELATIVE_TO_GROUND) {
                var clampedCart = ellipsoid.cartesianToCartographic(clampedPosition, scratchCartographic);
                clampedCart.height += cartoPosition.height;
                ellipsoid.cartographicToCartesian(clampedCart, clampedPosition);
            }

            var clampedModelMatrix = model._clampedModelMatrix;

            // Modify clamped model matrix to use new height
            Matrix4.clone(model.modelMatrix, clampedModelMatrix);
            clampedModelMatrix[12] = clampedPosition.x;
            clampedModelMatrix[13] = clampedPosition.y;
            clampedModelMatrix[14] = clampedPosition.z;

            model._heightChanged = true;
        };
    }

    function updateClamping(model) {
        if (defined(model._removeUpdateHeightCallback)) {
            model._removeUpdateHeightCallback();
            model._removeUpdateHeightCallback = undefined;
        }

        var scene = model._scene;
        if (!defined(scene) || !defined(scene.globe) || (model.heightReference === HeightReference.NONE)) {
            //>>includeStart('debug', pragmas.debug);
            if (model.heightReference !== HeightReference.NONE) {
                throw new DeveloperError('Height reference is not supported without a scene and globe.');
            }
            //>>includeEnd('debug');
            model._clampedModelMatrix = undefined;
            return;
        }

        var globe = scene.globe;
        var ellipsoid = globe.ellipsoid;

        // Compute cartographic position so we don't recompute every update
        var modelMatrix = model.modelMatrix;
        scratchPosition.x = modelMatrix[12];
        scratchPosition.y = modelMatrix[13];
        scratchPosition.z = modelMatrix[14];
        var cartoPosition = ellipsoid.cartesianToCartographic(scratchPosition);

        if (!defined(model._clampedModelMatrix)) {
            model._clampedModelMatrix = Matrix4.clone(modelMatrix, new Matrix4());
        }

        // Install callback to handle updating of terrain tiles
        var surface = globe._surface;
        model._removeUpdateHeightCallback = surface.updateHeight(cartoPosition, getUpdateHeightCallback(model, ellipsoid, cartoPosition));

        // Set the correct height now
        var height = globe.getHeight(cartoPosition);
        if (defined(height)) {
            // Get callback with cartoPosition being the non-clamped position
            var cb = getUpdateHeightCallback(model, ellipsoid, cartoPosition);

            // Compute the clamped cartesian and call updateHeight callback
            Cartographic.clone(cartoPosition, scratchCartographic);
            scratchCartographic.height = height;
            ellipsoid.cartographicToCartesian(scratchCartographic, scratchPosition);
            cb(scratchPosition);
        }
    }

    var scratchDisplayConditionCartesian = new Cartesian3();
    var scratchDistanceDisplayConditionCartographic = new Cartographic();

    function distanceDisplayConditionVisible(model, frameState) {
        var distance2;
        var ddc = model.distanceDisplayCondition;
        var nearSquared = ddc.near * ddc.near;
        var farSquared = ddc.far * ddc.far;

        if (frameState.mode === SceneMode.SCENE2D) {
            var frustum2DWidth = frameState.camera.frustum.right - frameState.camera.frustum.left;
            distance2 = frustum2DWidth * 0.5;
            distance2 = distance2 * distance2;
        } else {
            // Distance to center of primitive's reference frame
            var position = Matrix4.getTranslation(model.modelMatrix, scratchDisplayConditionCartesian);
            if (frameState.mode === SceneMode.COLUMBUS_VIEW) {
                var projection = frameState.mapProjection;
                var ellipsoid = projection.ellipsoid;
                var cartographic = ellipsoid.cartesianToCartographic(position, scratchDistanceDisplayConditionCartographic);
                position = projection.project(cartographic, position);
                Cartesian3.fromElements(position.z, position.x, position.y, position);
            }
            distance2 = Cartesian3.distanceSquared(position, frameState.camera.positionWC);
        }

        return (distance2 >= nearSquared) && (distance2 <= farSquared);
    }

    /**
     * Called when {@link Viewer} or {@link CesiumWidget} render the scene to
     * get the draw commands needed to render this primitive.
     * <p>
     * Do not call this function directly.  This is documented just to
     * list the exceptions that may be propagated when the scene is rendered:
     * </p>
     *
     * @exception {RuntimeError} Failed to load external reference.
     */
    Model.prototype.update = function(frameState) {
        if (frameState.mode === SceneMode.MORPHING) {
            return;
        }

        if (!FeatureDetection.supportsWebP.initialized) {
            FeatureDetection.supportsWebP.initialize();
            return;
        }
        var supportsWebP = FeatureDetection.supportsWebP();

        var context = frameState.context;
        this._defaultTexture = context.defaultTexture;

        if ((this._state === ModelState.NEEDS_LOAD) && defined(this.gltf)) {
            // Use renderer resources from cache instead of loading/creating them?
            var cachedRendererResources;
            var cacheKey = this.cacheKey;
            if (defined(cacheKey)) { // cache key given? this model will pull from or contribute to context level cache
                context.cache.modelRendererResourceCache = defaultValue(context.cache.modelRendererResourceCache, {});
                var modelCaches = context.cache.modelRendererResourceCache;

                cachedRendererResources = modelCaches[this.cacheKey];
                if (defined(cachedRendererResources)) {
                    if (!cachedRendererResources.ready) {
                        // Cached resources for the model are not loaded yet.  We'll
                        // try again every frame until they are.
                        return;
                    }

                    ++cachedRendererResources.count;
                    this._loadRendererResourcesFromCache = true;
                } else {
                    cachedRendererResources = new CachedRendererResources(context, cacheKey);
                    cachedRendererResources.count = 1;
                    modelCaches[this.cacheKey] = cachedRendererResources;
                }
                this._cachedRendererResources = cachedRendererResources;
            } else { // cache key not given? this model doesn't care about context level cache at all. Cache is here to simplify freeing on destroy.
                cachedRendererResources = new CachedRendererResources(context);
                cachedRendererResources.count = 1;
                this._cachedRendererResources = cachedRendererResources;
            }

            this._state = ModelState.LOADING;
            if (this._state !== ModelState.FAILED) {
                var extensions = this.gltf.extensions;
                if (defined(extensions) && defined(extensions.CESIUM_RTC)) {
                    var center = Cartesian3.fromArray(extensions.CESIUM_RTC.center);
                    if (!Cartesian3.equals(center, Cartesian3.ZERO)) {
                        this._rtcCenter3D = center;

                        var projection = frameState.mapProjection;
                        var ellipsoid = projection.ellipsoid;
                        var cartographic = ellipsoid.cartesianToCartographic(this._rtcCenter3D);
                        var projectedCart = projection.project(cartographic);
                        Cartesian3.fromElements(projectedCart.z, projectedCart.x, projectedCart.y, projectedCart);
                        this._rtcCenter2D = projectedCart;

                        this._rtcCenterEye = new Cartesian3();
                        this._rtcCenter = this._rtcCenter3D;
                    }
                }

                addPipelineExtras(this.gltf);

                this._loadResources = new ModelLoadResources();
                if (!this._loadRendererResourcesFromCache) {
                    // Buffers are required to updateVersion
                    ModelUtility.parseBuffers(this, bufferLoad);
                }
            }
        }

        var loadResources = this._loadResources;
        var incrementallyLoadTextures = this._incrementallyLoadTextures;
        var justLoaded = false;

        if (this._state === ModelState.LOADING) {
            // Transition from LOADING -> LOADED once resources are downloaded and created.
            // Textures may continue to stream in while in the LOADED state.
            if (loadResources.pendingBufferLoads === 0) {
                if (!loadResources.initialized) {
                    frameState.brdfLutGenerator.update(frameState);

                    ModelUtility.checkSupportedExtensions(this.extensionsRequired, supportsWebP);
                    ModelUtility.updateForwardAxis(this);

                    // glTF pipeline updates, not needed if loading from cache
                    if (!defined(this.gltf.extras.sourceVersion)) {
                        var gltf = this.gltf;
                        // Add the original version so it remains cached
                        gltf.extras.sourceVersion = ModelUtility.getAssetVersion(gltf);
                        gltf.extras.sourceKHRTechniquesWebGL = defined(ModelUtility.getUsedExtensions(gltf).KHR_techniques_webgl);

                        this._sourceVersion = gltf.extras.sourceVersion;
                        this._sourceKHRTechniquesWebGL = gltf.extras.sourceKHRTechniquesWebGL;

                        updateVersion(gltf);
                        addDefaults(gltf);

                        var options = {
                            addBatchIdToGeneratedShaders: this._addBatchIdToGeneratedShaders
                        };

                        processModelMaterialsCommon(gltf, options);
                        processPbrMaterials(gltf, options);
                    }

                    this._sourceVersion = this.gltf.extras.sourceVersion;
                    this._sourceKHRTechniquesWebGL = this.gltf.extras.sourceKHRTechniquesWebGL;

                    // Skip dequantizing in the shader if not encoded
                    this._dequantizeInShader = this._dequantizeInShader && DracoLoader.hasExtension(this);

                    // We do this after to make sure that the ids don't change
                    addBuffersToLoadResources(this);
                    parseArticulations(this);
                    parseTechniques(this);
                    if (!this._loadRendererResourcesFromCache) {
                        parseBufferViews(this);
                        parseShaders(this);
                        parsePrograms(this);
                        parseTextures(this, context, supportsWebP);
                    }
                    parseMaterials(this);
                    parseMeshes(this);
                    parseNodes(this);

                    // Start draco decoding
                    DracoLoader.parse(this, context);

                    loadResources.initialized = true;
                }

                if (!loadResources.finishedDecoding()) {
                    DracoLoader.decodeModel(this, context)
                        .otherwise(ModelUtility.getFailedLoadFunction(this, 'model', this.basePath));
                }

                if (loadResources.finishedDecoding() && !loadResources.resourcesParsed) {
                    this._boundingSphere = ModelUtility.computeBoundingSphere(this);
                    this._initialRadius = this._boundingSphere.radius;

                    DracoLoader.cacheDataForModel(this);

                    loadResources.resourcesParsed = true;
                }

                if (loadResources.resourcesParsed &&
                    loadResources.pendingShaderLoads === 0) {
                    createResources(this, frameState);
                }
            }

            if (loadResources.finished() ||
                (incrementallyLoadTextures && loadResources.finishedEverythingButTextureCreation())) {
                this._state = ModelState.LOADED;
                justLoaded = true;
            }
        }

        // Incrementally stream textures.
        if (defined(loadResources) && (this._state === ModelState.LOADED)) {
            if (incrementallyLoadTextures && !justLoaded) {
                createResources(this, frameState);
            }

            if (loadResources.finished()) {
                this._loadResources = undefined;  // Clear CPU memory since WebGL resources were created.

                var resources = this._rendererResources;
                var cachedResources = this._cachedRendererResources;

                cachedResources.buffers = resources.buffers;
                cachedResources.vertexArrays = resources.vertexArrays;
                cachedResources.programs = resources.programs;
                cachedResources.sourceShaders = resources.sourceShaders;
                cachedResources.silhouettePrograms = resources.silhouettePrograms;
                cachedResources.textures = resources.textures;
                cachedResources.samplers = resources.samplers;
                cachedResources.renderStates = resources.renderStates;
                cachedResources.ready = true;

                // The normal attribute name is required for silhouettes, so get it before the gltf JSON is released
                this._normalAttributeName = ModelUtility.getAttributeOrUniformBySemantic(this.gltf, 'NORMAL');

                // Vertex arrays are unique to this model, do not store in cache.
                if (defined(this._precreatedAttributes)) {
                    cachedResources.vertexArrays = {};
                }

                if (this.releaseGltfJson) {
                    releaseCachedGltf(this);
                }
            }
        }

        var iblSupported = OctahedralProjectedCubeMap.isSupported(context);
        if (this._shouldUpdateSpecularMapAtlas && iblSupported) {
            this._shouldUpdateSpecularMapAtlas = false;
            this._specularEnvironmentMapAtlas = this._specularEnvironmentMapAtlas && this._specularEnvironmentMapAtlas.destroy();
            this._specularEnvironmentMapAtlas = undefined;
            if (defined(this._specularEnvironmentMaps)) {
                this._specularEnvironmentMapAtlas = new OctahedralProjectedCubeMap(this._specularEnvironmentMaps);
                var that = this;
                this._specularEnvironmentMapAtlas.readyPromise.then(function() {
                    that._shouldRegenerateShaders = true;
                });
            }

            // Regenerate shaders to not use an environment map. Will be set to true again if there was a new environment map and it is ready.
            this._shouldRegenerateShaders = true;
        }

        if (defined(this._specularEnvironmentMapAtlas)) {
            this._specularEnvironmentMapAtlas.update(frameState);
        }

        var recompileWithDefaultAtlas = !defined(this._specularEnvironmentMapAtlas) && defined(frameState.specularEnvironmentMaps) && !this._useDefaultSpecularMaps;
        var recompileWithoutDefaultAtlas = !defined(frameState.specularEnvironmentMaps) && this._useDefaultSpecularMaps;

        var recompileWithDefaultSHCoeffs = !defined(this._sphericalHarmonicCoefficients) && defined(frameState.sphericalHarmonicCoefficients) && !this._useDefaultSphericalHarmonics;
        var recompileWithoutDefaultSHCoeffs = !defined(frameState.sphericalHarmonicCoefficients) && this._useDefaultSphericalHarmonics;

        this._shouldRegenerateShaders = this._shouldRegenerateShaders || recompileWithDefaultAtlas || recompileWithoutDefaultAtlas || recompileWithDefaultSHCoeffs || recompileWithoutDefaultSHCoeffs;

        this._useDefaultSpecularMaps = !defined(this._specularEnvironmentMapAtlas) && defined(frameState.specularEnvironmentMaps);
        this._useDefaultSphericalHarmonics = !defined(this._sphericalHarmonicCoefficients) && defined(frameState.sphericalHarmonicCoefficients);

        var silhouette = hasSilhouette(this, frameState);
        var translucent = isTranslucent(this);
        var invisible = isInvisible(this);
        var displayConditionPassed = defined(this.distanceDisplayCondition) ? distanceDisplayConditionVisible(this, frameState) : true;
        var show = this.show && displayConditionPassed && (this.scale !== 0.0) && (!invisible || silhouette);

        if ((show && this._state === ModelState.LOADED) || justLoaded) {
            var animated = this.activeAnimations.update(frameState) || this._cesiumAnimationsDirty;
            this._cesiumAnimationsDirty = false;
            this._dirty = false;
            var modelMatrix = this.modelMatrix;

            var modeChanged = frameState.mode !== this._mode;
            this._mode = frameState.mode;

            // Model's model matrix needs to be updated
            var modelTransformChanged = !Matrix4.equals(this._modelMatrix, modelMatrix) ||
                (this._scale !== this.scale) ||
                (this._minimumPixelSize !== this.minimumPixelSize) || (this.minimumPixelSize !== 0.0) || // Minimum pixel size changed or is enabled
                (this._maximumScale !== this.maximumScale) ||
                (this._heightReference !== this.heightReference) || this._heightChanged ||
                modeChanged;

            if (modelTransformChanged || justLoaded) {
                Matrix4.clone(modelMatrix, this._modelMatrix);

                updateClamping(this);

                if (defined(this._clampedModelMatrix)) {
                    modelMatrix = this._clampedModelMatrix;
                }

                this._scale = this.scale;
                this._minimumPixelSize = this.minimumPixelSize;
                this._maximumScale = this.maximumScale;
                this._heightReference = this.heightReference;
                this._heightChanged = false;

                var scale = getScale(this, frameState);
                var computedModelMatrix = this._computedModelMatrix;
                Matrix4.multiplyByUniformScale(modelMatrix, scale, computedModelMatrix);
                if (this._upAxis === Axis.Y) {
                    Matrix4.multiplyTransformation(computedModelMatrix, Axis.Y_UP_TO_Z_UP, computedModelMatrix);
                } else if (this._upAxis === Axis.X) {
                    Matrix4.multiplyTransformation(computedModelMatrix, Axis.X_UP_TO_Z_UP, computedModelMatrix);
                }
                if (this.forwardAxis === Axis.Z) {
                    // glTF 2.0 has a Z-forward convention that must be adapted here to X-forward.
                    Matrix4.multiplyTransformation(computedModelMatrix, Axis.Z_UP_TO_X_UP, computedModelMatrix);
                }
            }

            // Update modelMatrix throughout the graph as needed
            if (animated || modelTransformChanged || justLoaded) {
                updateNodeHierarchyModelMatrix(this, modelTransformChanged, justLoaded, frameState.mapProjection);
                this._dirty = true;

                if (animated || justLoaded) {
                    // Apply skins if animation changed any node transforms
                    applySkins(this);
                }
            }

            if (this._perNodeShowDirty) {
                this._perNodeShowDirty = false;
                updatePerNodeShow(this);
            }
            updatePickIds(this, context);
            updateWireframe(this);
            updateShowBoundingVolume(this);
            updateShadows(this);
            updateClippingPlanes(this, frameState);

            // Regenerate shaders if ClippingPlaneCollection state changed or it was removed
            var clippingPlanes = this._clippingPlanes;
            var currentClippingPlanesState = 0;
            var useClippingPlanes = defined(clippingPlanes) && clippingPlanes.enabled && clippingPlanes.length > 0;
            var usesSH = defined(this._sphericalHarmonicCoefficients) || this._useDefaultSphericalHarmonics;
            var usesSM = (defined(this._specularEnvironmentMapAtlas) && this._specularEnvironmentMapAtlas.ready) || this._useDefaultSpecularMaps;
            if (useClippingPlanes || usesSH || usesSM) {
                var clippingPlanesOriginMatrix = defaultValue(this.clippingPlanesOriginMatrix, modelMatrix);
                Matrix4.multiply(context.uniformState.view3D, clippingPlanesOriginMatrix, this._clippingPlaneModelViewMatrix);
            }

            if (useClippingPlanes) {
                currentClippingPlanesState = clippingPlanes.clippingPlanesState;
            }

            var shouldRegenerateShaders = this._shouldRegenerateShaders;
            shouldRegenerateShaders = shouldRegenerateShaders || this._clippingPlanesState !== currentClippingPlanesState;
            this._clippingPlanesState = currentClippingPlanesState;

            // Regenerate shaders if color shading changed from last update
            var currentlyColorShadingEnabled = isColorShadingEnabled(this);
            if (currentlyColorShadingEnabled !== this._colorShadingEnabled) {
                this._colorShadingEnabled = currentlyColorShadingEnabled;
                shouldRegenerateShaders = true;
            }

            if (shouldRegenerateShaders) {
                regenerateShaders(this, frameState);
            } else {
                updateColor(this, frameState, false);
                updateSilhouette(this, frameState, false);
            }
        }

        if (justLoaded) {
            // Called after modelMatrix update.
            var model = this;
            frameState.afterRender.push(function() {
                model._ready = true;
                model._readyPromise.resolve(model);
            });
            return;
        }

        // We don't check show at the top of the function since we
        // want to be able to progressively load models when they are not shown,
        // and then have them visible immediately when show is set to true.
        if (show && !this._ignoreCommands) {
            // PERFORMANCE_IDEA: This is terrible
            var commandList = frameState.commandList;
            var passes = frameState.passes;
            var nodeCommands = this._nodeCommands;
            var length = nodeCommands.length;
            var i;
            var nc;

            var idl2D = frameState.mapProjection.ellipsoid.maximumRadius * CesiumMath.PI;
            var boundingVolume;

            if (passes.render || (passes.pick && this.allowPicking)) {
                for (i = 0; i < length; ++i) {
                    nc = nodeCommands[i];
                    if (nc.show) {
                        var command = translucent ? nc.translucentCommand : nc.command;
                        command = silhouette ? nc.silhouetteModelCommand : command;
                        commandList.push(command);
                        boundingVolume = nc.command.boundingVolume;
                        if (frameState.mode === SceneMode.SCENE2D &&
                            (boundingVolume.center.y + boundingVolume.radius > idl2D || boundingVolume.center.y - boundingVolume.radius < idl2D)) {
                            var command2D = translucent ? nc.translucentCommand2D : nc.command2D;
                            command2D = silhouette ? nc.silhouetteModelCommand2D : command2D;
                            commandList.push(command2D);
                        }
                    }
                }

                if (silhouette && !passes.pick) {
                    // Render second silhouette pass
                    for (i = 0; i < length; ++i) {
                        nc = nodeCommands[i];
                        if (nc.show) {
                            commandList.push(nc.silhouetteColorCommand);
                            boundingVolume = nc.command.boundingVolume;
                            if (frameState.mode === SceneMode.SCENE2D &&
                                (boundingVolume.center.y + boundingVolume.radius > idl2D || boundingVolume.center.y - boundingVolume.radius < idl2D)) {
                                commandList.push(nc.silhouetteColorCommand2D);
                            }
                        }
                    }
                }
            }
        }
    };

    function destroyIfNotCached(rendererResources, cachedRendererResources) {
        if (rendererResources.programs !== cachedRendererResources.programs) {
            destroy(rendererResources.programs);
        }
        if (rendererResources.silhouettePrograms !== cachedRendererResources.silhouettePrograms) {
            destroy(rendererResources.silhouettePrograms);
        }
    }

    // Run from update iff:
    // - everything is loaded
    // - clipping planes state change OR color state set
    // Run this from destructor after removing color state and clipping plane state
    function regenerateShaders(model, frameState) {
        // In regards to _cachedRendererResources:
        // Fair to assume that this is data that should just never get modified due to clipping planes or model color.
        // So if clipping planes or model color active:
        // - delink _rendererResources.*programs and create new dictionaries.
        // - do NOT destroy any programs - might be used by copies of the model or by might be needed in the future if clipping planes/model color is deactivated

        // If clipping planes and model color inactive:
        // - destroy _rendererResources.*programs
        // - relink _rendererResources.*programs to _cachedRendererResources

        // In both cases, need to mark commands as dirty, re-run derived commands (elsewhere)

        var rendererResources = model._rendererResources;
        var cachedRendererResources = model._cachedRendererResources;
        destroyIfNotCached(rendererResources, cachedRendererResources);

        var programId;
        if (isClippingEnabled(model) || isColorShadingEnabled(model) || model._shouldRegenerateShaders) {
            model._shouldRegenerateShaders = false;

            rendererResources.programs = {};
            rendererResources.silhouettePrograms = {};

            var visitedPrograms = {};
            var techniques = model._sourceTechniques;
            var technique;

            for (var techniqueId in techniques) {
                if (techniques.hasOwnProperty(techniqueId)) {
                    technique = techniques[techniqueId];
                    programId = technique.program;
                    if (!visitedPrograms[programId]) {
                        visitedPrograms[programId] = true;
                        recreateProgram({
                            programId: programId,
                            techniqueId: techniqueId
                        }, model, frameState.context);
                    }
                }
            }
        } else {
            rendererResources.programs = cachedRendererResources.programs;
            rendererResources.silhouettePrograms = cachedRendererResources.silhouettePrograms;
        }

        // Fix all the commands, marking them as dirty so everything that derives will re-derive
        var rendererPrograms = rendererResources.programs;

        var nodeCommands = model._nodeCommands;
        var commandCount = nodeCommands.length;
        for (var i = 0; i < commandCount; ++i) {
            var nodeCommand = nodeCommands[i];
            programId = nodeCommand.programId;

            var renderProgram = rendererPrograms[programId];
            nodeCommand.command.shaderProgram = renderProgram;
            if (defined(nodeCommand.command2D)) {
                nodeCommand.command2D.shaderProgram = renderProgram;
            }
        }

        // Force update silhouette commands/shaders
        updateColor(model, frameState, true);
        updateSilhouette(model, frameState, true);
    }

    /**
     * Returns true if this object was destroyed; otherwise, false.
     * <br /><br />
     * If this object was destroyed, it should not be used; calling any function other than
     * <code>isDestroyed</code> will result in a {@link DeveloperError} exception.
     *
     * @returns {Boolean} <code>true</code> if this object was destroyed; otherwise, <code>false</code>.
     *
     * @see Model#destroy
     */
    Model.prototype.isDestroyed = function() {
        return false;
    };

    /**
     * Destroys the WebGL resources held by this object.  Destroying an object allows for deterministic
     * release of WebGL resources, instead of relying on the garbage collector to destroy this object.
     * <br /><br />
     * Once an object is destroyed, it should not be used; calling any function other than
     * <code>isDestroyed</code> will result in a {@link DeveloperError} exception.  Therefore,
     * assign the return value (<code>undefined</code>) to the object as done in the example.
     *
     * @exception {DeveloperError} This object was destroyed, i.e., destroy() was called.
     *
     *
     * @example
     * model = model && model.destroy();
     *
     * @see Model#isDestroyed
     */
    Model.prototype.destroy = function() {
        // Vertex arrays are unique to this model, destroy here.
        if (defined(this._precreatedAttributes)) {
            destroy(this._rendererResources.vertexArrays);
        }

        if (defined(this._removeUpdateHeightCallback)) {
            this._removeUpdateHeightCallback();
            this._removeUpdateHeightCallback = undefined;
        }

        if (defined(this._terrainProviderChangedCallback)) {
            this._terrainProviderChangedCallback();
            this._terrainProviderChangedCallback = undefined;
        }

        // Shaders modified for clipping and for color don't get cached, so destroy these manually
        if (defined(this._cachedRendererResources)) {
            destroyIfNotCached(this._rendererResources, this._cachedRendererResources);
        }

        this._rendererResources = undefined;
        this._cachedRendererResources = this._cachedRendererResources && this._cachedRendererResources.release();
        DracoLoader.destroyCachedDataForModel(this);

        var pickIds = this._pickIds;
        var length = pickIds.length;
        for (var i = 0; i < length; ++i) {
            pickIds[i].destroy();
        }

        releaseCachedGltf(this);
        this._quantizedVertexShaders = undefined;

        // Only destroy the ClippingPlaneCollection if this is the owner - if this model is part of a Cesium3DTileset,
        // _clippingPlanes references a ClippingPlaneCollection that this model does not own.
        var clippingPlaneCollection = this._clippingPlanes;
        if (defined(clippingPlaneCollection) && !clippingPlaneCollection.isDestroyed() && clippingPlaneCollection.owner === this) {
            clippingPlaneCollection.destroy();
        }
        this._clippingPlanes = undefined;

        this._specularEnvironmentMapAtlas = this._specularEnvironmentMapAtlas && this._specularEnvironmentMapAtlas.destroy();

        return destroyObject(this);
    };

    // exposed for testing
    Model._getClippingFunction = getClippingFunction;
    Model._modifyShaderForColor = modifyShaderForColor;

    return Model;
});