Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!wuarchive!uunet!mcsun!corton!geocub!goofi!pelegrin
From: pelegrin@goofi.greco-prog.fr (Uncle Ben's)
Newsgroups: comp.graphics
Subject: Re: good radiosity question
Message-ID: <3315@geocub.UUCP>
Date: 29 May 91 07:04:20 GMT
References: <20917@ogicse.ogi.edu> <1991May26.020248.10869@milton.u.washington.edu>
Sender: news@geocub.UUCP
Organization: ENSERB, Bordeaux, France
Lines: 36

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:
>>> 	I have written a combination radiosity/raytracing rendering system
>>> (whew!) and I'm somewhat confused on one point: I have a scene of 5 walls,
>>> 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 see two possible answers:

-	If you say you have five walls, your enironment may not be fully
	closed. The loss of energy resulting of this is HUGE; for
	example, if the closed world is a cube, removing a wall will
	divide the overall energy by two.

-	You say your surfaces are 90% diffuse (I guess they are 10%
	specular 8^) ), but how many percents of their incoming energy
	do they radiate?
	If they reemit 10% of the incoming overall emergy, they will
	effectively radiate 9% of the energy (we only consider the
	diffuse contribution), then with 2 reflections you get
	0.09^2 ~~ 0.0001. Not much. Good reemission percentages are
	usually between 20 and 50%.

-------------------------------------------------------------------------------
|   Zeu<FP> : The craziest programmer in France  |         _________          |
|------------------------------------------------|     /   |        \   \     |
| Francois Pellegrini is :                       |    /    |__   ___/    \    |
|      goofi!pelegrin@geocub.greco-prog.fr       |    \    |     |       /    |
| mcsun!inria!geocub!goofi!pelegrin@uunet.uu.net |     \   |     |      /     |
|         goofi!pelegrin@geocub.UUCP             |                            |
-------------------------------------------------------------------------------