GLTFExporter: Add EXT_texture_webp support.

[BorisKourt]

Fixed #33116

In #23592 (r139), userData.mimeType was added so the exporter preserves
JPEG instead of inflating everything to PNG. WebP was deferred
to be handled via EXT_texture_webp: #23592 (comment)

We remove the image/webp → image/png guard in processTextureAsync
and register EXT_texture_webp in extensionsUsed when a WebP texture is
encountered.

[donmccurdy]

Thanks @BorisKourt! I agree it'd be good to support EXT_texture_webp, but it won't be enough to append EXT_texture_webp to the extensionsUsed list. We also need to extend the texture definition, as shown in the "Using without a fallback" section of the spec:

https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Vendor/EXT_texture_webp#using-without-a-fallback

I'd also recommend testing the output by exporting a file with WebP and then running it through https://github.khronos.org/glTF-Validator/ for validation.

[Mugen87]

@BorisKourt Updating processTextureAsync() like below should do the trick. Tested a round-rip with the following asset and it works as expected: https://github.khronos.org/glTF-Assets/model/SheenWoodLeatherSofa

async processTextureAsync( map ) {

	const writer = this;
	const options = writer.options;
	const cache = this.cache;
	const json = this.json;

	if ( cache.textures.has( map ) ) return cache.textures.get( map );

	if ( ! json.textures ) json.textures = [];

	// make non-readable textures (e.g. CompressedTexture) readable by blitting them into a new texture
	if ( map instanceof CompressedTexture ) {

		map = await this.decompressTextureAsync( map, options.maxTextureSize );

	}

	const mimeType = map.userData.mimeType;

	const imageIndex = this.processImage( map.image, map.format, map.flipY, mimeType );

	const textureDef = {
		sampler: this.processSampler( map )
	};

	if ( mimeType === 'image/webp' ) {

		textureDef.extensions = textureDef.extensions || {};
		textureDef.extensions[ 'EXT_texture_webp' ] = {
			source: imageIndex
		};

		this.extensionsUsed[ 'EXT_texture_webp' ] = true;
		this.extensionsRequired[ 'EXT_texture_webp' ] = true;

	} else {

		textureDef.source = imageIndex;

	}

	if ( map.name ) textureDef.name = map.name;

	await this._invokeAllAsync( async function ( ext ) {

		ext.writeTexture && await ext.writeTexture( map, textureDef );

	} );

	const index = json.textures.push( textureDef ) - 1;
	cache.textures.set( map, index );
	return index;

}

[BorisKourt]

Thanks for catching this @donmccurdy and thanks for the sample @Mugen87! I already started noodling so let me know if I properly adapted everything. Attached is an exported file. Should I add an example directly to examples for this update or is it too small?

webp_test_synthetic.glb.zip

---

{
    "uri": "webp_test_synthetic.glb",
    "mimeType": "model/gltf-binary",
    "validatorVersion": "2.0.0-dev.3.10",
    "validatedAt": "2026-03-05T08:14:16.292Z",
    "issues": {
        "numErrors": 0,
        "numWarnings": 0,
        "numInfos": 0,
        "numHints": 0,
        "messages": [],
        "truncated": false
    },
    "info": {
        "version": "2.0",
        "generator": "three.js test",
        "extensionsUsed": [
            "EXT_texture_webp"
        ],
        "extensionsRequired": [
            "EXT_texture_webp"
        ],
        "resources": [
            {
                "pointer": "/buffers/0",
                "mimeType": "application/gltf-buffer",
                "storage": "glb",
                "byteLength": 104
            },
            {
                "pointer": "/images/0",
                "mimeType": "image/webp",
                "storage": "buffer-view",
                "image": {
                    "width": 1,
                    "height": 1,
                    "format": "rgb",
                    "primaries": "srgb",
                    "transfer": "srgb",
                    "bits": 8
                }
            }
        ],
        "animationCount": 0,
        "materialCount": 1,
        "hasMorphTargets": false,
        "hasSkins": false,
        "hasTextures": true,
        "hasDefaultScene": true,
        "drawCallCount": 1,
        "totalVertexCount": 3,
        "totalTriangleCount": 1,
        "maxUVs": 1,
        "maxInfluences": 0,
        "maxAttributes": 2
    }
}

[Mugen87]

There is no need to add a new glTF asset to the repository. Instead, can you please add a box mesh with a WebP texture to misc_exporter_gltf? You can place it next to the Coffeemat mesh. In this way, we have a mesh that goes through the new code path.

The idea is to add a new button "Export WebP Model" so we can easily verify the EXT_texture_webp support.

[BorisKourt]

Will add what you suggest 👍 This was just an example export as per @donmccurdy's request, wasn't planning to add it to the repo!

[BorisKourt]

Added it to the file and tested the exports in the Khronos validator also.

[BorisKourt]

Looks like screenshots need to be updated, should I do that as part of this pr?

[Mugen87]

That would be good. You can use below screenshot and replace it with the one in examples/screenshots.

misc_exporter_gltf.jpg.zip

[BorisKourt]

Thanks, I have added it now.

[Mugen87]

Looking all good now. Thanks for your work on this!

[BorisKourt]

Glad I could help!