Here is the result of trying to visually match a Megapov 0.5 scene
with a CCD image of the Cornell box. The only assumption made, based
on observing the image on a calibrated monitor, was that the CCD image
gamma is 2.2. This gamma was applied to the final image and extracted
colors. Some red and green was subtracted from the extracted light
surface color to remove the effects of color bleeding. Extracted red
and green colors were only multiplied by a factor in linear space to
match intensity. Area light color is RGB white. The POV-Ray scene
geometry is reconstructed from the CCD image so it doesn't match
exactly with the physical box. I think it's good enough for artistic
purposes and considering that POV wasn't designed to produce realistic
simulations :)

The most difficult part was to get the darkening effect at the corner
of the back wall and ceiling and it's not even that good. This also
produced an anomaly at the small box/floor intersection. Notice also
the good red color bleed on the tall box and not so good green color
bleed on the small box/floor. The brightness of the lower front face
of the tall box could be caused by different scene geometry or the
incorrect light distribution from the area light (It's really a point
light in all but the shadow calculations. This is why large area lights
don't produce correct direct illumination. A patch to fix this would
be nice :). Spotlight was used to simulate the cosine intensity falloff
from planar light source.

I'm not sure whether the settings I used are generally valid for all kinds
of scenes or just this one. I will post the source with comments later
so that the settings can be tested on different scenes and conditions.

The most notable changes from the MP recommended values are:
  brightness 2.5 
  gray_threshold 0.5


First image: CCD image of the Cornell box.

Second image: Rendering time about 0.5h but reduces significantly
              without the (minor) corner effect. Requires more tweaking
              to reduce rendering time.

Third image: Megapov 0.5 high quality radiosity settings. 1.5min.

Last: No radiosity. 20sec


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Kari Kivisalo                                          www.kivisalo.net

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