I was exploring some strange artifacts in a render of a hair prop which appeared to show sharp corners and straight edge lines despite smoothing and one level of subdivision being applied. I wondered if the subdivision was somehow affecting the interpolation of the UV mapping inaccurately, so I decided to add the Cycles Wireframe node to see what the subdivided vertices were doing.

Lo, and behold, that node appears to triangulate the mesh in a way that neither the original mesh or subdivision shows.

The original render shows some wavyness in the striations which I can't attribute to the texture map, since it's just a lot of vertical, straight lines to my eye. Maybe there's something strange happening with the UV interpolation for subdivision?

Here's the Wireframe preview (the turquoise is due to the ambient colour which is where the wireframe node plugs in.

All of the subdivided facets are still quads, not triangles.

And here's the wireframe area render over the original:

All those diagonals have been (arbitrarily) generated by the wireframe node, since they don't represent actual facet edges in the mesh (whether subdivided or not). If they are actually arbitrary, even if just a product of the wireframe algorithm, surely there's a point where the algorithm decides to add one or the other diagonal to a quad facet. It would be really useful if it just didn't add a virtual edge at all there.

Has anyone else seen or remarked on this?

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