I figured I would post this in General first, to test the waters.

Povray appears to have a problem with radiosity and negative surface colors.
I have a material which has a solid pigment of {rgb<-1,-1,0>}, and usually
it works quite nicely and produces nice deep blue colors in its
interreflections with other objects. Occasionally, though, I get mysterious
artifacts:
http://idt.gatech.edu/~cashmore/pov/debug01a.jpg

That black shadow that looks like it's being produced by a sphere is in
fact, not the shadow of any object that exists in the scene.

This is exacerbated by the use of reflections. In the following I am
alternating the black spheres with reflective ones. The artifacts should
clearly be incorrect, since the two images have different error_bounds
http://idt.gatech.edu/~cashmore/pov/debug01_05.jpg error_bound 0.5
http://idt.gatech.edu/~cashmore/pov/debug01_02.jpg error_bound 0.2

If I take away the negative color, the problem goes away, but I'd prefer to
not need to do that. I suspect the problem lies with the implementation of
adc_bailout, in that pov sees the net magnitude of the reflected color as
being less than zero, and short circuts to make the resulting color darker
than it would be even otherwise.

Any thoughts or comments?
Can people reproduce this on other architectures?

Here is my source:
http://idt.gatech.edu/~cashmore/pov/debug01.pov

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