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From: erich@eye.com (Eric Haines)
Newsgroups: comp.graphics
Subject: Re: good radiosity question
Keywords: radiosity
Message-ID: <1991May03.174940.9299@eye.com>
Date: 3 May 91 17:49:40 GMT
References: <20917@ogicse.ogi.edu>
Sender: Eric Haines
Reply-To: erich@eye.com (Eric Haines)
Organization: 3D/Eye Inc., Ithaca, NY
Lines: 42

In article <20917@ogicse.ogi.edu> aburke@ogicse.cse.ogi.edu (Andrew Burke) writes:
>a few days ago wuly@vax5.cit.cornell.edu queried the net:
>>> 10x10, with a light 3x3 pointing at one of the walls.  Now with 90% diffuse
>>> surfaces, the relative energy out of the wall *behind* the light (i.e. light
>>> left the lightsource, hit the wall in front, and was re-radiated to the back
>>> wall, and re-radiated again) is about 1/1000 the energy leaving the light.
>>> This seems wrong. ...
>
>I have basically the same problem. I've read all the papers, scratched a 
>bald spot on my head, but still am a bit puzzled. My code also just doesn't
>seem quite correct. My solution has been to either increase the intensity
>of the source light, or to make the surface area of the light source larger.

I just finished up my SIGGRAPH course notes on this and other problems in
radiosity algorithms.  Here's a relevant extract:


VI. Scaling

Once the radiosity solution has been computed, we often must scale the
results so that a meaningful image can be output.  The problem is that we
do not necessarily know what the best mapping of radiosity values is for
our solution.  That is, we do not know the best way to display the set of
generated radiosity values.

[...]

[On our solution:]
On output the user can specify a scale factor, a gamma curve for intensity
[not the same as gamma correction:  this one scales the RGBs uniformly, based
on intensity], and a minimum output level.  Optionally, the user can obtain
the level of intensity for the brightest reflector (i.e. not including light
sources) in the environment and use this for scaling.  By changing these and
seeing its effect on the output, the user can fine tune the appearance of the
illumination to create the desired effect.  This gives control to the user
similar to what is given to a photographer:  exposure time and contrast can be
adjusted for different effects.  These can be adjusted and the scene output
again, giving the user near-real-time feedback and control.

----

Eric