Le 18-10-29 à 23:58, Robert Munyer a écrit :
> The documentation explains the rendering problem that happens when one
> of the surfaces of the minuend in a difference operation is coincident
> with one of the surfaces of the subtrahend.
> 
> I've been encountering a similar-looking rendering problem even when
> there is no such coincidence.  In the scene attached below, I expected
> the center of the image to be a green solid surface, not a speckled
> "window" into the object's interior.
> 
> Question 1:  Is this a bug?
>

Coincident surfaces problems can happen whenever you have two or more 
surfaces that are at the same location.
It can come from two objects that touch each other.
It can show up in unions and merges.

It show more often in differences and intersections because, in the 
other cases, the affected surfaces are hidden unless you have at least 
one transparent object.

Solution : Always make sure that you don't have surfaces that just 
touch. If it happen, move one of the surfaces by a tiny amount.
This will solve your problem :
#declare Bar =
   union {
     // No more surface coincidences _within_ this union.
     box { <-.75, -.75, -2>, <.25, .25, 0.000001> }
     box { <-.25, -.25, -2>, <.75, .75, 0> }
     pigment { Green }
   }

as well as this :
#declare Bar =
   union {
     // No more surface coincidences _within_ this union.
     box { <-.75, -.75, -2>, <.25, .25, -0.000001> }
     box { <-.25, -.25, -2>, <.75, .75, 0> }
     pigment { Green }
   }

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