Normals of Low Polygon Mesh with hard/soft edge

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Hello,

We are looking at producing assets without subdivisions (subdivisionScheme = “none”).
The assets are produced in Maya using a low polygon approach where the modeler choose the edge “hardness” manually.

In maya, the notion of “smooth” or “hard” edge controls the normals of the vertices of that edge.
If the edge is “hard” the normals are split per face, so the vertex has as many normals as adjacent faces.
If the edge is “smooth” the vertex has a single normal, being the mean of the normal of adjacent faces.
The look of the mesh is generated from this information.

From my investigation, it seems that USD cannot store this king of data.
Normals are either implicite (when using a subdivisionScheme which “define their own normals”) or explicit per vertex per face.

I know from experience that Alembic stores normals like USD, per vertex per face or nothing.

From my understanding, it looks like both Alembic and USD have been created to handle subdivision mesh and thus per edge normal has not been implemented.

Do you know the reason for this implementation ?
Would it make sens to allow the declaration of edge hardness ? (since a crease value per edge already exists).
Or maybe using the crease value to compute the normal even without subdivisionScheme.

Having the edge hardness stored rather than the vertex normal would reduce a lot the data size since normals would not be written to disk.

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UsdGeomMesh can absolutely accommodate this. There may be limitations or work to be done on the Maya Mesh exporter, but you can use SetNormalsInterpolation() on a Mesh object to set the normals’ interpolation to faceVarying (follow the link on the page I referenced above), which allows specification of a normal per face-vertex. Or you can not author the builtin normals property and instead create a UsdGeomPrimvar called normals and use all the primvar API to set interpolation to faceVarying and author the normals themselves.

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Sorry, I realize you are asking for per-edge normals… but shouldn’t per-face-vertex normals give you strictly more expressibility? Though I realize it is not as good an encoding for interchange of “edge hardness”, but edge hardness as a concept is, as you say, something we only define for subdivision surfaces, since without a well-defined algorithm for specifying what hardness means, it becomes tricky.