export with compress on cause uv error

[zhou1993yu]

Describe the bug
export with compress on cause uv error.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'file->export->gltf 2.0'
2. with compress on and press export
3. reimport the exported model
4. See error below

Expected behavior

Screenshots

.blend file/ .gltf (mandatory)
untitled.zip

Version

• OS: windows
• Blender Version 4.0 ~4.2

Additional context
Add any other context about the problem here.

[julienduroure]

I can confirm.
Bug is visible in online viewer like https://gltf-viewer.donmccurdy.com/, so problem comes from exporter

[donmccurdy]

If you zoom out considerably in the UV Editor view, on the uncompressed mesh, you'll see a small outlying UV island —

This will break quantization, which both Draco and Meshopt compression rely on — most of the available precision in the quantization grid is spent on empty space. I'm not sure how UVs end up there to begin with, but it's been a recurring issue in glTF Transform's compression as well:

• Draco compression issue donmccurdy/glTF-Transform#1215
• Draco compression breaks UV coordinates of mesh (?) donmccurdy/glTF-Transform#936
• Draco breaking UV's donmccurdy/glTF-Transform#976

I've been considering adding a step to compute a histogram for the UVs, and to show a warning if there are major outliers, or perhaps repacking the UVs, but these are potentially expensive operations for an exporter to spend time on...

[julienduroure]

Thanks @donmccurdy for the investigation!