On 5 Oct 1998 11:14:19 -0500, Nieminen Mika <war### [at] assaricctutfi> wrote:
>  Note that the coincident surfaces problem (this is a so used term it should
>be abbreviated CSP :) ) can also occur with objects not in the same CSG.

In this case, treat it as if their common ancestor were a union, since 
adding a union around the whole frame should have no net effect.

>  Also note that a very special case of CSP occurs when a light source is
>exactly on a surface: Povray can't calculate accurately if the light source
>is inside or outside the surface, so black spots will appear in every
>place which is illuminated by this light.

Well, one could see that the light source and one of the intersections
coincide within the same tolerance while looking for shadow rays.  In
this case, one would always discard the intersection.

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