Guess what I’ve been doing! No no no you’re all wrong!

I’ve been playing with the Mental Ray Misss_physical shader, combined with Daniel Rind’s Diffraction shader to introduce refraction blur and chromatic aberration. You should have been able to guess that.

I’ve been having a lot of trouble with final gathering (the rendering technique, not the Highlander sequel) and black spots. The web-lore on these spots is that they are breakouts caused by bacteria. They are *not* caused by eating too much chocolate or greasy foods.

Exempli Gratia

In this scene, the number and size of the spots varies directly with the number of refraction rays allotted to the raytracer. They also appear around poorly-built geometry, particularly as it affects transparency, such as a single object with intersecting surfaces. The modeler I’m using, Teddy, allows bad geometry to be created very easily.

But then sometimes the spots show up for no apparent reason, and the only way to fix it is to reduce the number of refraction rays, to the point where all refraction (and its attendant benefits) are lost.

The 3D forums are littered with dead-end threads about this problem, none of them particularly illuminating. If only there were some sort of online centralized repository of relevant information contributed by knowledgeable users in an easily-editable format.

[Update: the consensus is that the black pixels are the mental ray equivalent of “Nan” and are generally avoidable by better-written shaders. The MR production shaders do a much better job than your average possibly-buggy third-party plugin.]

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