KTX2Loader: Support RGBA 16-bit unsigned normalized formats

[donmccurdy]

Adds support for KTX2 files containing vkFormat = VK_FORMAT_R16G16B16A16_UNORM. Support for R, RG, RGB, and equivalent SNORM types is left open for PRs, as I don't have textures in these formats yet. The texture included here was produced with OpenImageIO and KTX-Software:

# convert 8-bit PNG to 16-bit PNG (for testing only)
oiiotool uint8.png --iscolorspace srgb --tocolorspace lin_rec709_srgb -d uint16 -o uint16.png

# convert 16-bit PNG to RGBA16 UNORM KTX2
ktx create --format R16G16B16A16_UNORM --assign-tf linear --assign-primaries bt709 --generate-mipmap uint16.png rgba16_unorm.ktx2

Related:

• Support for EXT_texture_norm16 #33193

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	360.01 85.46	360.07 85.47	+65 B +17 B
WebGPU	633.22 175.54	633.22 175.54	+0 B +0 B
WebGPU Nodes	631.34 175.25	631.34 175.25	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	492.21 120.03	492.28 120.05	+65 B +20 B
WebGPU	705.25 190.44	705.25 190.44	+0 B +0 B
WebGPU Nodes	654.47 177.68	654.47 177.68	+0 B +0 B

[Mugen87]

Do you mind adding this line in below module, too?

three.js/src/renderers/webgl-fallback/utils/WebGLTextureUtils.js

Lines 271 to 272 in a14d874

	if ( glType === gl.UNSIGNED_BYTE ) internalFormat = ( transfer === SRGBTransfer ) ? gl.SRGB8_ALPHA8 : gl.RGBA8;
	if ( glType === gl.UNSIGNED_SHORT_4_4_4_4 ) internalFormat = gl.RGBA4;

In this way, the WebGL 2 backend of WebGPURenderer support this feature as well.

The WebGPU backend should already cover this use case here:

three.js/src/renderers/webgpu/utils/WebGPUTextureUtils.js

Lines 1360 to 1362 in a14d874

	case UnsignedShortType:
	formatGPU = GPUTextureFormat.RGBA16Uint;
	break;

Edit: RGBA16Uint is obviously the wrong format. Alternative suggestion: #33250 (comment)