HI, all.  I've been away from the forums for awhile (life got in the way.)

I've been working with radiosity lately ("without light sources"), trying to
understand how best to produce a scene using a single, small ambient 1
"light" as the sole light source. I'm using a heavily modified and
simplified version of the "cornell.pov" file in the RADIOSITY examples
folder included with POV-Ray. Just a simple room with simple shapes in it
(and one wall removed.) But I'm just not happy with the results,
particularly the too-muted "color spill" and color-mixing from the
intensely-colored walls and sphere. And it seems that there should be
better-defined shadows. (HOWEVER, these could be just my own flawed,
subjective opinions; other opinions are welcome.)

I've been tweaking radiosity values for weeks(!), and each new permutation
produces visual differences that are far from subtle. The hints and tips in
the POV docs are only marginally helpful in this case. It's difficult to
tell what the scene SHOULD look like, when each new value produces
something visually different (particularly under the purple sphere.) Since
there are twelve (relevant) values in the global radiosity block that can
be adjusted, I could be at this for months!! (And I've noticed that the
smaller the single light becomes--smaller in on-screen pixel size--the more
exacting the values become...to the point of almost not working at all.)

Could anyone give me some *direction* as to how to improve this scene, what
the most important radiosity values might be? Has anyone come up with an
"overall formula" for adjusting values for scenes like this? I'm hoping to
find an approach that isn't so subjective (not to mention time-consuming.)
If I can get this simple(?) scene to be as realistic as possible, I plan to
use the knowledge gained for more complex scenes (media-sphere torches in an
isosurface cave, for example.)

All objects in the scene have finish{ambient 0 diffuse .75}. The ceiling is
pigment{rgb 1}, the floor and boxes are various shades of gray. The single
light-sphere is pigment {rgb 1} finish {ambient 1 diffuse 0} and no_shadow.

Here are the global/radiosity values I finally settled on for this image
(mostly guesswork, having all but given up on making sense of the process):

assumed_gamma 1.0

brightness 23
pretrace_start .08 // default value
pretrace_end .01
count 500
max_sample 1.0
low_error_factor .025 // half the default
recursion_limit 5
minimum_reuse .0075 // half the default
nearest_count 12.5 // midpoint between 5 (default) and 20 (max)
error_bound .9 // half the default
gray_threshold 0 // default
always_sample off
media off // default


If I've left out any necessary info, let me know and I'll post it.

Ken Walker

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