How to access material:binding relation from GeomSubset object?

[xahon]

I didn't find an API to get material binding relation like from GeomMesh object with materialBinding field

[syoyo]

https://github.com/syoyo/tinyusdz/blob/41970d1aa8dd94e754c95e78ac159afe93c74337/examples/tydra_api/tydra-api-main.cc#L361

[syoyo]

If you are looking for materialBind info from GeomSubset, not GeomMesh, currently handy API is not provided. You may find it through generic Property field GeomSubset::props https://github.com/syoyo/tinyusdz/blob/41970d1aa8dd94e754c95e78ac159afe93c74337/src/usdGeom.hh#L388

[xahon]

@syoyo props field is empty in my case. Looks like a bug then

[syoyo]

You can contribute implementing similar API as done in GeomMesh to GeomSubset!

FYI, there are two places you need to look

• USDA: https://github.com/syoyo/tinyusdz/blob/41970d1aa8dd94e754c95e78ac159afe93c74337/src/usda-reader.cc#L1454
• USDC: Oh I didn't implemented GeomSubset in USDC reader... https://github.com/syoyo/tinyusdz/blob/41970d1aa8dd94e754c95e78ac159afe93c74337/src/usdc-reader.cc#L1498 https://github.com/syoyo/tinyusdz/blob/41970d1aa8dd94e754c95e78ac159afe93c74337/src/usdc-reader.cc#L357

[xahon]

I'd like to contribute but I still can't understand how GeomSubset was intended to work. Pixar's API reference for that type is kind of vague and I found no clear description of Mesh and GeomSubset. I have an ascii model definition with that structure:

-  type Mesh
   points, normals, indices
   -  type GeomSubset
      indices
   -  type GeomSubset
      indices

And some indices from both GeomSubsets are out of bounds if they are used as indices in Mesh's points array. I think GeomSubset is for defining a submesh but then why Mesh contains indices at all.

[syoyo]

I'd like to contribute

👍

I still can't understand how GeomSubset was intended to work

Yeah! Even me :-)

It'd be first look at this:

PixarAnimationStudios/OpenUSD#542
https://github.com/PixarAnimationStudios/USD/tree/release/pxr/usdImaging/usdImagingGL/testenv/testUsdImagingGLGeomSubsets

In basic GeomSuset configuration, GeomSubset::indices are indices of face id in its parent's GeomMesh::faceVertexIndices, not array index of GeomMesh::points, since GeomSubset is usually for representing per-face materials of GeomMesh.

[xahon]

I found out that indices in GeomSubset are not indices at all. It's a face offset in main Mesh indices array. I mean that if you have (Pseudo USD)

Mesh:
    faceVertexCounts = [4, 4, 4, 4]
    faceVertexIndices = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
   
    GeomSubset:
        indices = [3]

Then you want to access indices at faceVertexIndices[sum(faceVertexCounts[0], faceVertexCounts[3])] with subarray length of faceVertexCounts[3]

In other words to access Nth GeomSubset's face you want to calculate:

size_t offset = 0;
for (size_t i = 0; i < N; i++) offset += faceVertexIndices[i];
uint32_t *faceIndices = faceVertexIndices[offset];
size_t faceIndicesCount = faceVertexCounts[N];

I made a helper structure that firstly builds an array of polygon data in O(N) in my user code, then I can easily triangulate them into my custom mesh format

If you first build a faces array then GeomSubset indices will be indices into that array

[syoyo]

MaterialBinding API is finally supported in dev branch.

You can look into https://github.com/syoyo/tinyusdz/blob/dev/src/tydra/render-data.cc for how to use it.
It also contains how to process indices in GeomSubset(e.g. rebuild indices in GeomSubset in trinangulation)