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EXT_texture_webp

Contributors

Status

Complete, Ratified by the Khronos Group

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 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.

glTF Schema Updates

The EXT_texture_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_texture_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_texture_webp": {
                "source": 1
            }
        }
    }
],
"images": [
    {
        "mimeType": "image/png",
        "bufferView": 1
    },
    {
        "mimeType": "image/webp",
        "bufferView": 2
    }
]

JSON Schema

glTF.EXT_texture_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 not be present in extensionsRequired. 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.

WebP images using extended file format should not contain animations or color profile information.

Using Without a Fallback

To use WebP images without a fallback, define EXT_texture_webp in both extensionsUsed and extensionsRequired. The texture node will then have only an extensions property as shown below.

"textures": [
    {
        "extensions": {
            "EXT_texture_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 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).

Resources

Google's WebP developer page provides information about the format as well as pre-compiled and source code versions of an encoder and a decoder. The WebP Compression Study is a detailed comparison between JPEG and WebP.

For browsers that do not natively support WebP, webp_js decodes WebP images in JavaScript. Sharp is a NodeJS library for fast encode/decode of WebP using native modules (built on top of Google's implementation above).