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KHR_texture_transform

Contributors

Status

Complete

Dependencies

Written against the glTF 2.0 spec.

Overview

Many techniques can be used to optimize resource usage for a 3d scene. Chief among them is the ability to minimize the number of textures the GPU must load. To achieve this, many engines encourage packing many objects' low-resolution textures into a single large texture atlas. The region of the resulting atlas that corresponds with each object is then defined by vertical and horizontal offsets, and the width and height of the region.

To support this use case, this extension adds offset, rotation, and scale properties to textureInfo structures. These properties would typically be implemented as an affine transform on the UV coordinates. In GLSL:

varying in vec2 Uv;

uniform vec2 Offset, Scale;
uniform float Rotation;

mat3 translation = mat3(1,0,0, 0,1,0, Offset.x, Offset.y, 1);
mat3 rotation = mat3(
    cos(Rotation), sin(Rotation), 0,
   -sin(Rotation), cos(Rotation), 0,
                0,             0, 1
);
mat3 scale = mat3(Scale.x,0,0, 0,Scale.y,0, 0,0,1);

mat3 matrix = translation * rotation * scale;
vec2 uvTransformed = ( matrix * vec3(Uv.xy, 1) ).xy;

This is equivalent to Unity's Material#SetTextureOffset and Material#SetTextureScale, or Three.js's Texture#offset and Texture#repeat. UV rotation is not widely supported as of today, but is included here for forward compatibility.

glTF Schema Updates

The KHR_texture_transform extension may be defined on textureInfo structures. It may contain the following properties:

Name Type Default Description
offset array[2] [0.0, 0.0] The offset of the UV coordinate origin as a factor of the texture dimensions.
rotation number 0.0 Rotate the UVs by this many radians counter-clockwise around the origin. This is equivalent to a similar rotation of the image clockwise.
scale array[2] [1.0, 1.0] The scale factor applied to the components of the UV coordinates.
texCoord integer Overrides the textureInfo texCoord value if supplied, and if this extension is supported.

Though this extension's values are unbounded, they will only produce sane results if the texture sampler's wrap mode is REPEAT, or if the result of the final UV transformation is within the range [0, 1] (i.e. negative scale settings and correspondingly positive offsets).

Implementation Note: For maximum compatibility, it is recommended that exporters generate UV coordinate sets both with and without transforms applied, use the post-transform set in the texture texCoord field, then the pre-transform set with this extension. This way, if the extension is not supported by the consuming engine, the model still renders correctly. Including both will increase the size of the model, so if including the fallback UV set is too burdensome, either add this extension to extensionsRequired or use the same texCoord value in both places.

Implementation Note: From the glTF core specification, the origin of the UV coordinates (0, 0) corresponds to the upper left corner of a texture image.

JSON Schema

KHR_texture_transform.textureInfo.schema.json

Example JSON

This example utilizes only the lower left quadrant of the source image, rotated clockwise 90°.

{
  "materials": [{
    "emissiveTexture": {
      "index": 0,
      "extensions": {
        "KHR_texture_transform": {
          "offset": [0, 1],
          "rotation": 1.57079632679,
          "scale": [0.5, 0.5]
        }
      }
    }
  }]
}

This example inverts the T axis, effectively defining a bottom-left origin.

{
  "materials": [{
    "emissiveTexture": {
      "index": 0,
      "extensions": {
        "KHR_texture_transform": {
          "offset": [0, 1],
          "scale": [1, -1]
        }
      }
    }
  }]
}

Known Implementations