Wrong compression to ktx if occlusion Texture have more 1 channels

[Yason83]

Describe the bug
This image was created in RGBA mode and is used in the occlusionTexture.
During the optimization of the GLB file, it compresses it incorrectly. As far as I understand, it expects the image to be one channel.

Please advise if there is a way to bypass this issue.
Or is it possible to preprocess all occlusionTexture images into grayscale beforehand?

To Reproduce

1. Blender add this image to occlusion_1 in model
2. Export to glb
3. Compress gltf-transform uastc input.glb output.glb --level 4 --rdo 4 --zstd 24 --verbose
4. Extract images gltf-transform copy output.glb output.gltf

Versions:

• cli.js 3.4.5 — Command-line interface (CLI) for the glTF Transform SDK.

[donmccurdy]

Thanks @Yason83! Unless I've misunderstood the KTX-Software CLI's options, I believe the bug may be upstream. I've filed a bug here:

• Unexpected texture tiling with --target_type=R (v4.2 stable) KhronosGroup/KTX-Software#727

The new alpha release of KTX-Software doesn't appear to have this problem so the issue may get resolved on its own soon enough too. glTF Transform will need to be updated to use the next KTX Software version, when that is released.

[Yason83]

Could you give an approximate timeline on when we can expect this issue to be fixed?

[donmccurdy]

This will depend on when the issue is fixed upstream in KTX-Software and released. I have the impression they are working on it, but I don't know when that will be complete.

[donmccurdy]

KTX Software v4.3 is now stable — moving this to the glTF Transform v4 milestone.

https://github.com/KhronosGroup/KTX-Software/releases