black regions at glancing angles
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#363 black regions at glancing angles
This can be seen in a test from the aqsis RTS: RIBs/Primitives/NURBS/complexnurbs1.rib.
All of the regions on the NURBS surface which are viewed at very shallow angles (where the surface is tangent to the viewing direction) are outlined in black. This problem doesn't seem specific to NURBS; I've also observed something similar with a plain displaced bilinear patch.
I was just poking about with the sampling code and noticed that our point-in-polygon test seems to assume convex polygons; this might be related...
Problems with non-convex micropolys are looking more likely. Attached is a test case which has a non-flat bilinear patch, generating some nice nonconvex "bow tie" micropolygons at the center of the image. The two sides of the patch should presumably meet at a point (this happens when the shading rate is small).
However, when the shading rate is really large, black regions appear at the center corresponding to entire micropolygons worth of missed samples.