EXT_texture_webp extension proposal

extensions/2.0/Vendor/EXT_image_webp/README.md

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+# EXT_image_webp
+
+## Contributors
+
+* Omar Shehata, Cesium, [@OmarShehata](https://github.com/OmarShehata)
+* TBA, Google
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+This extension allows glTF models to use WebP as a valid image format. A client that does not implement this extension can ignore the provided WebP image and continue to rely on the PNG and JPG textures available in the base specification. Defining a fallback texture is optional. The [best practices](#best-practices) section describes the intended use case of this extension and the expected behavior when using it without a fallback texture.
+
+WebP is an image format developed and maintained by Google that provides superior lossless and lossy compression rates for images on the web. It is typically 26% smaller in size compared to PNGs and 25-34% smaller than comparable JPEG images - [source](https://developers.google.com/speed/webp/).  
+
+## glTF Schema Updates
+
+The `EXT_image_webp` extension is added to the `textures` node and specifies a `source` property that points to the index of the `images` node which in turn points to the WebP image.
+
+The following glTF will load `image.webp` in clients that support this extension, and otherwise fall back to `image.png`.
+
+```
+"textures": [
+    {
+        "source": 0,
+        "extensions": {
+            "EXT_image_webp": {
+                "source": 1
+            }
+        }
+    }
+],
+"images": [
+    {
+        "uri": "image.png"
+    },
+    {
+        "uri": "image.webp"
+    }
+]
+```
+
+When used in the glTF Binary (.glb) format the `images` node that points to the WebP image uses the `mimeType` value of `image/webp`. 
+
+```
+"textures": [
+    {
+        "source": 0,
+        "extensions": {
+            "EXT_image_webp": {
+                "source": 1
+            }
+        }
+    }
+],
+"images": [
+    {
+        "mimeType": "image/png",
+        "bufferView": 1
+    },
+    {
+        "mimeType": "image/webp",
+        "bufferView": 2
+    }
+]
+```
+
+### JSON Schema
+
+[glTF.EXT_image_webp.schema.json](schema/glTF.EXT_image_webp.schema.json)
+
+## Best Practices
+
+Since WebP is not as widely supported as JPEG and PNG, it is recommended to use this extension only when transmission size is a significant bottleneck. For example, in geospatial applications the total texture payload can range from gigabytes to terabytes of data. In those situations, WebP textures can be used without a fallback to improve storage and transmission.
+
+When a fallback image is defined, this extension should be defined in `extensionsUsed`. This will allow all clients to render the glTF correctly, and those that support this extension can request the optimized WebP version of the textures.
+
+### Using Without a Fallback
+
+To use WebP images without a fallback, define `EXT_image_webp` in `extensionsRequired`. The `texture` node will then have only an `extensions` property as shown below.
+
+```
+"textures": [
+    {
+        "extensions": {
+            "EXT_image_webp": {
+                "source": 1
+            }
+        }
+    }
+]
+```
+
+If a glTF contains a WebP with no fallback texture and the browser does not support WebP, the client should either display an error or decode the WebP image at runtime (see [resource](#resources) for decoding libraries).
+
+## Known Implementations
+
+CesiumJS uses it to significantly cut load times for massive models that contain gigabytes of texture data (see [implementation and sample model](https://github.com/AnalyticalGraphicsInc/cesium/pull/7486)). 
+
+## Resources
+
+Google's [WebP developer page](https://developers.google.com/speed/webp/) provides information about the format as well as [pre-compiled and source code versions](https://developers.google.com/speed/webp/download) of an encoder and a decoder.
+
+The library [libwebpjs](http://libwebpjs.appspot.com) decodes WebP images in JavaScript for browsers that do not natively support it. [Sharp](http://sharp.pixelplumbing.com/en/stable/) is a NodeJS library for fast encode/decode of WebP (built on top of Google's implementation above).

extensions/2.0/Vendor/EXT_image_webp/schema/glTF.EXT_image_webp.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "EXT_image_webp glTF extension",
+    "type": "object",
+    "description": "glTF extension to specify textures using the WebP image format.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "source": {
+          "allOf": [ { "$ref": "glTFid.schema.json" } ],
+          "description": "The index of the images node which points to a WebP image."
+        },
+        "extensions": {},
+        "extras": {}
+      }
+}