Reading poly meshes with missing indexed UVs

[Andrew Harvey]

Hi there,

I'm currently trying to read indexed UV data from an Alembic file that
was exported using the reference Maya plugin. There are no problems if
every face-vertex has an associated UV. I get into trouble though when
UVs are missing from the Maya geometry. That is, the number of UV
indices is less than the number of face indices.

If we look at MayaMeshWriter::getUVs() in the abcexport plugin:

http://code.google.com/p/alembic/source/browse/maya/AbcExport/MayaMeshWriter.cpp#94

we can see the following:

MIntArray uvCounts, uvIds;
status = lMesh.getAssignedUVs(uvCounts, uvIds, &uvSetName);

The information in uvCounts is not written out anywhere... The
assumption therefore seems to be that the UV counts should match the
polygon face counts (the number of vertices per face.) If we are
missing UVs though, this assumption no longer holds.

Is this an oversight in the abcexport plugin or am I missing
something? (probably the latter! :)

Many thanks,

- Andrew

[Kevin Sallée]

We had the same issue: all the faces/vertices must have an associated UV if not it doesn't manage to read them when you import the .abc. For the moment we asked the texturing team to make UV's for all faces/vertices.

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[Lucas Miller]

The documentation on this is a little confusing:

http://download.autodesk.com/us/maya/2010help/api/class_m_fn_mesh.html#902345bbf950ca4b82079c2af3b890a6

It does say:

This method finds all texture coordinates for the mesh that have been mapped, and returns them in the same format as the assignUVs.

But then it goes on to say:

The uvCounts array that is returned will contain the number of uv's per polygon. Since uv's are mapped per-polygon per-vertex, the entries in this array will match the vertex counts for each polygon in the mesh. For example, the array for a cube would be { 4, 4, 4, 4, 4, 4 } since there are 6 polygons each with 4 vertices.

So when there are unmapped texture coordinates, is this reflected in uvCounts or uvIds, or possibly both?

An example would be nice.

Lucas

[Andrew Harvey]

Hi there,

For polygons without UVs, I would imagine their entry in the uvCounts
array will be 0. The sum of all the counts in the uvCounts array
should then equal the length of the uvIds array.

If this is indeed the case there is a quick solution: Write out a
padded uvIndices array to the Alembic cache, thereby ensuring all the
lengths match. For every 0 entry in uvCounts, a suitable number of
"dummy" indices are inserted (one for each of the polygon vertices.)
The dummy index points to a (0,0) UV value, and is written to the
cache alongside all the other UV values. The advantage of this
approach is that only the Maya exporter needs to be modified and
nothing else.

- Andrew

[Lucas Miller]

While reviewing issues that have been addressed for an upcoming release, I may have fixed this in my own repo a while ago:

http://code.google.com/r/millerlucas-dev/source/detail?r=3678ea403483033363f3077874525f4cfde127c8

Here is the issue for it:

http://code.google.com/p/alembic/issues/detail?id=263

Lucas

[Kevin Sallée]

sweet! thank you! we'll have less hassles with UV's thanks to this, at least until I find a way to bypass UV's when connecting an abc to a mesh.

Kevin

[Andrew Harvey]

Hi Lucas - that's great to hear. Thank you very much!

- Andrew