Manuel Kasten <kas### [at] gmxde> wrote:
> As you can see, the room is artifact-free except the one marked
> with the arrow.

  The reason for that artifact is a rather simple one, but I can't think
of a good solution in the general case. In this specific case there's
probably a working solution.

  The problem is that at that specific point (the center of the circle)
the radiosity samples are taken from a point which is almost exactly at
the corner of the room, ie. at the junction of the two walls.
  That is, that point is practically in the same place as the wall on
the right.

  Now, think how the radiosity sampling works: POV-Ray sends semi-random
rays from that point to all directions (well, in all directions of the
hemisphere pointing out of the wall surface).
  Since the sampling point is located just at the junction of the two
walls, some of these sample rays miss the right wall and are actually
going outside the room. Since it's dark outside, it affects the lighting
of that point.

  I really can't think how this problem could be fixed in the POV-Ray
source code.
  However, you could try translating your camera by a minuscule amount
(like 0.001) to some direction (but not vertically, because the walls
are vertical at the problematic place), and it might correct the problem.
It's enough for the sampling point to be taken just slightly from a different
place.
  If you are out of luck, you might get a similar artifact somewhere else.
If this is the case, try translating the camera by a different amount.

  Another possible solution (which is not a generic one but which probably
works here) is to make the right wall shadowless and put a copy of it
behind it (very close to it, but not too close). This way the sample
rays going outside the room will hit this second wall (which will be
illuminated by the light source because the original wall is now
shadowless).

  By the way, can I use this image (well, a section of it) to make a new
entry in the http://tag.povray.org/povQandT/ site explaining this problem?

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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