Support the KHR_texture_transform GLTF extension

[janhohenheim]

What problem does this solve or what need does it fill?

In Blender, the default way of making sure a texture tiles n times is to add a UV Map and a Mapping node, like this;

The above example tiles the texture exactly 5 times.

This transforms the UV. The Blender GLTF exporter supports this by writing it to the KHR_texture_transform extension. So, setting the nodes up as follows, we get a nicely tiled GLTF:

I have a simple greyboxing grid texture applied to a cube in this case. Viewing it through a GLTF viewer, I get this:

As you can see, the text in the grid is repeated 5 times per face. Opening this up in Bevy however... (ignore the poor lighting)

Oh no! Bevy does not support GLTF extensions yet, so it just stretches the texture instead of tiling it!

What solution would you like?

Ideally, Bevy should apply the transforms to the UV automatically when encountering this extension. As the Khronos group link specifies, most implementations ignore the rotation value, so I think it's fine if we do too. Most important are the offset for texture atlases and scale for tiling.

What alternative(s) have you considered?

Leave it to the user. #11138 allows us to read the extension manually, so a user could write a custom shader or a shader extension for this.

Additional context

• Related issue: Tiled/repeating textures #399
• Fixes: glTF not being rendered correctly. UV texture issue?  #6335
• Implements a part of Add support for Khronos Material Extensions for glTF 2.0 #5178 and Robust GLTF Extension Support #11350

[janhohenheim]

I'd be ready to implement this myself if anyone gave me some pointers on how/where to change the UVs :)
Edit: looks like there is already a PR for this: #8266. Is there anything I can do to help it go along?

[alice-i-cecile]

It looks like the PR has been abandoned by the author: adopt it by forking the work and submitting a new PR, then ping the original reviewers for a re-review :)

[janhohenheim]

@alice-i-cecile, alright, I'll try to get it done later, hopefully quick enough for 0.13 💪