KTX2Loader: Add support for ASTC 6x6 format

[donmccurdy]

Related:

• Fixes KTX2Loader: Support for ASTC and ETC1 formats #26569

To create samples with KTX-Software v4.2:

# no supercompression
toktx --t2 --encode astc output.ktx2 input.png

# zstd supercompression
toktx --t2 --encode astc --zcmp 18 output.ktx2 input.png

ASTC is a GPU texture format, and works only on GPUs that support it — primarily mobile devices. ASTC is not a transcoded format like Basis Universal's UASTC, which would work on any device.

[donmccurdy]

Aside – it would be helpful to have more test cases for KTX2 support, we are supporting a fair number of input types at this point:

• UASTC + THREE.CompressedTexture
• ETC1S + THREE.CompressedTexture
• DataTexture
• DataArrayTexture
• ASTC + THREE.CompressedTexture
• LDR cubemap + THREE.CompressedCubeTexture
• (future) HDR cubemap + THREE.CompressedCubeTexture

Do we have a preference about making one example with dropdown selection, vs. a number of KTX2 examples, vs. mixing KTX2 into existing examples where appropriate?

[CITIZENDOT]

ASTC supports various block sizes: 4x4, 5x4, 5x5, 6x5, 6x6, 8x5, 8x6, 10x5, 10x6, 8x8, 10x8, 10x10, 12x10, 12x12. I think it's good idea to cover all the block sizes.

Edit: My bad, We don't have a way to test other block sizes. You're right. We can think about other blocksizes, if we have a way to create them.

[Mugen87]

Do we have a preference about making one example with dropdown selection, vs. a number of KTX2 examples, vs. mixing KTX2 into existing examples where appropriate?

Since there are already many examples, I would prefer a single KTX2 demo with a dropdown selection. That said, I think it's also fine to update one or two of the existing (glTF) examples so they are using KTX2Loader.

[donmccurdy]

@CITIZENDOT Exactly — glad to support them, just not sure how to test it.

@Mugen87 I've added a dropdown with a number of small KTX2 2D texture samples for now. We're implementing KTX2 features that glTF files do not allow — ASTC, cubemaps, array textures, 3D textures, ... — so updating glTF examples won't help us here.

I have more samples to test (https://github.com/donmccurdy/KTX2-Samples), but have included only 2D textures in this PR.