I originally posted about this in povray.general, and no one was able to
give an explanation. Gilles Tran was very helpful in finding a simplified
scene that still produces the artifact in question. He also pointed out to
me that using the -UL flag causes the problem to go away. Here is the
simplified scene description:

#include "colors.inc"
camera {location  <4.5, 5.5, -7> look_at <0.5, 2.5,  0>}
light_source {<10, 50, -16>  color 2}
box {<-2, 0, 5>,<4, 6, 5.01> texture {finish {reflection 1}}}
#declare D =difference {
    box {<-1, 0, -1>,<1, 0.6, 1>}
    box {<-2, 0, 0>,<2, 1, 1> rotate x*-45 translate -z}
}
union {
    cylinder {<-0.7, -2, 0.7>,<-0.7, 2, 0.7>,0.3 pigment {Clear}}
    box {<-2, 0, 0>,<2, 1, 1> rotate x*-45 translate -z translate y*0.6
pigment {Clear}}
    difference {
        cylinder {<0.7, 0.3,-0.7>,<-0.7, 0.3,-0.7>,0.1}
        intersection {
            union {
                cylinder {<0.7, -3, -0.7>,<0.7, 1, -0.7>,0.3}
                cylinder {<-0.7, -3, 0.7>,<-0.7, 1, 0.7>,0.3}
            }
            union {
                object {D translate y*-0.001}
                object {D translate y*0.001}
            }
        }
        translate y*0.6
    }
    sphere {<0, 0.3, -0.4>, 0.35}
    sphere {<-0.4, 0.3, 0.4>, 0.35}
    texture {pigment {Yellow}}
    translate y*5
}

I have tried to simplify the scene even more, but very small changes in the
description change the way the bug is manifested. For example, simply
moving the spheres outside the union makes the problem go away.

The problem can be seen in the shadow that the cylinder projects onto the
sphere.

Another interesting factor, if only the affected part of the scene is
rendered:

+sc0.388106 +sr0.091858 +ec0.491393 +er0.217119

then the bug is not visible, even with light buffers enabled. It only shows
up if the entire scene is rendered. Here are a couple examples of how this
bug is manifesting itself in some real scenes I have rendered.

http://www.hentschels.com:8000/~pictures/cgi-bin/album/index.cgi?file=/2004/08August/DansBlocks/block.jpg

The shadows in question are on the blue and yellow balloons in the top
block. The same thing can also be seen in this other image from a different
perspective:

http://www.hentschels.com:8000/~pictures/cgi-bin/album/index.cgi?file=/2004/04April/Blocks/block_04-03-04.jpg

I have run this on various versions of POVWin 3.5-3.6 on several computers
including an AMD 1700+ 768MB, an 800 P3 512MB, a 2.6 Celeron 384MB, and a
3.2 P4 512MB. All had Windows XP Home or Pro installed on them. If you need
more information, please let me know.

page-dan( **at!** )hentschels.com

Post a reply to this message