Zero-length vertex normals on closed shell volumes

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What steps will reproduce the problem?
1. Export a plane and then a sphere from Maya
2. Test the vertex normals (we imported into Houdini and Katana and then shaded 
the model displaying the vertex normals to see which geo is "bad")
3. The closed surfaces have bad vertext normals

What is the expected output? What do you see instead?
Valid vertex normals.

What version of the product are you using? On what operating system?
CentOS 5.4, Maya 2012.5sp1, Alembic 1.0.5

Please provide any additional information below.

Here's the export options that were used:

AbcExport options={'jobArg': u'-root sphere -frameRange 1 1 -attr identifier 
-attr sharedSourceFile -attr userSourceFile -attr uvs -attr vn -uvWrite 
-stripNamespaces -file /tmp/sphere.abc', 'verbose': True} 

Original issue reported on code.google.com by shappysf...@gmail.com on 14 Jul 2012 at 9:19

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Since the normals haven't changed from the Maya default, no normals are written 
to the polymesh.  It's not that they are written out as zero-length, they 
aren't written out at all, the normals property isn't valid., i.e.  
IPolyMeshSchema::getNormalsParam().isValid() returns false.

Original comment by miller.lucas on 15 Jul 2012 at 12:02

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Original comment by miller.lucas on 16 Jul 2012 at 7:53

• Changed state: Resolved

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One thing to note if you are comparing normals being written on a plane vs not 
being written for spheres:

We needed to write out normals if any of the edges turned out to be a hard edge.

http://code.google.com/p/alembic/issues/detail?id=211

Original comment by miller.lucas on 16 Jul 2012 at 8:08