Calvin wrote:
> 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?
Other than what?
I tried it on a W2k machine with POV 3.6.
I also get your artifacts. Artifacts are not in the same place
as in your picture. That is probably OK, as every time I render
it, the artifacts are in different places. If I change the
1*<-1,-1,0> to 1+<-1,-1,0> the scene seems to be OK.

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

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