Support decompressing KTX2 textures

[elalish]

Describe the bug KTX2 textures are not converted when demanded using the CLI.

To Reproduce Steps to reproduce the behavior:

1. gltf-transform mozjpeg input.glb output.glb (input.glb must have KTX2 textures)
2. File size is identical and still fails the validator with "unknown mimetype image/ktx2".

Expected behavior All textures would now be jpegs.

Versions:

• Version: 2.1.5
• Environment: OSX CLI

Additional context webp had the same result. Does it not read KTX textures? Also, would be nice if you could set multiple flags; like if my input is draco compressed, it gets uncompressed if I change the textures and then I have to recompress it. Might be nice to put gltf-transform -draco -webp input.glb output.glb.

[donmccurdy]

I'd love to be able to decompress KTX2 textures in glTF-Transform, but I don't see that landing until https://github.com/KhronosGroup/Basis-Universal-Transcoders supports at least converting UASTC and ETC1S to RGBA32. If https://github.com/GoogleChromeLabs/squoosh/pull/1017 landed, that might also fix the issue. In the meantime, yes, .ktx2 files will be ignored by the squoosh-based compression.

For now, the library can convert anything @squoosh/lib can read. That does include WebP, but you'll have to opt-in to changing formats, since the mozjpeg and oxipng commands affect only JPEG and PNG textures respectively by default:

gltf-transform mozjpeg input.glb output.glb --formats "*"

... it gets uncompressed if I change the textures and then I have to recompress it...

So far I've made this explicit because re-applying Draco compression is (I expect?) going to compound the compression artifacts. Seems better to be really explicit about that danger, and the same applies to Meshopt compression.

[fire]

See also https://github.com/BinomialLLC/basis_universal/issues/318 and from basisu to png.

[donmccurdy]

Unfortunately I'm blocked on this, as no cross-platform packages appear to support decoding UASTC and ETC1S to RGBA32, without bringing in GL-related dependencies. Such dependencies, like headless-gl, are unfortunately too heavy and complex for this library to rely on. If the situation changes I'd be glad to reopen this.

[donmccurdy]

Just filed a request, feel free to upvote. :)

• https://github.com/KhronosGroup/Basis-Universal-Transcoders/issues/20

I'm hoping ETC1S → RGBA8 could be prioritized on the KhronosGroup/Basis-Universal-Transcoders list, which would unblock us using the lightweight transcoders for applications that just need to decode to RGBA8 from UASTC or ETC1S.