How do I force gltfpack to keep texture coords?

[bennlich]

Hiya! I'm wondering if there's a flag I can pass to tell gltfpack not to throw away my UVs, even if my model has no textures.

Here's a simple cube exported from blender: https://drive.google.com/file/d/1SoBpm1-9eHL7PkB6ejYhQxT2RrbB-Pp0/view?usp=sharing

gltf-transform inspect simple-cube.glb shows that it has 3 attributes:

NORMAL:float32, POSITION:float32, TEXCOORD_0:float32

Then I pack it:

gltfpack -i simple-cube.glb -o packed-cube.glb

Now my NORMAL and POSITION attributes are quantized (WOO!), but the packed-cube no longer has a TEXCOORD attribute :(

NORMAL:int8, POSITION:uint16

Is there a flag I can pass so that the TEXCOORDs don't get tossed during packing?

[Rennzie]

I have a similar issue but using a custom attribute field to store featureIds on a per vertex basis. Wondering if there is a solution to keep the additional attributes around? @bennlich did you have any luck getting this working?

For context, I'm not doing any simplification with gltfpack. The warning that appears is:

Warning: ignoring unknown attribute _FEATURE_ID_0 in primitive i of mesh n

which comes from parseMeshesGltf

meshoptimizer/gltf/parsegltf.cpp

Line 139 in ad3d7e1

static void parseMeshesGltf(cgltf_data* data, std::vector<Mesh>& meshes, std::vector<std::pair<size_t, size_t> >& mesh_remap)

I'm interested in resolving this because we'd like to make use of the upcoming gltf extension EXT_mesh_features. I'm testing it without the extension for now i.e only using the attribute semantic.

Would the behaviour change if there was an extension embedded in the gltf?

[zeux]

By itself it wouldn't; since gltfpack repacks the geometry, it will discard attributes that it doesn't know the format for. I was assuming that gltfpack can implement EXT_mesh_features as an output extension (that is, instead of preserving feature ids it could generate them), but maybe some simpler version of feature preservation would make sense as well depending on how complex the input data can be. This would need to be supported in cgltf first which gltfpack uses to parse glTF files.