Andrew Coppin <orp### [at] btinternetcom> wrote:
> Does POV-Ray do *real* radiosity?

  What is "real radiosity"?

  Firstly, the exact meaning of "radiosity" is a specific algorithm where
lightmaps are calculated projecting (usually with scanline rendering) the
scene onto each surface of the scene. This algorithm has nothing to do
with raytracing nor with light sources per se.

  I suppose that you are using the term "radiosity" as meaning any
global illumination algorithm. Even so, there's no such a thing as
a "real" algorithm. There are several different algorithms, but we
can't say any of them is more "real" than the other. They are all
approximations, different ways of calculating a similar thing.
  One algorithm can be "better" than another (by whatever criteria
you happen to choose), but quality does not make it the "real" algorithm
(and the other ones "fake").

> According to my understanding (which may or may not be correct) "radiosity"
> algorithms start from the light source(s) and follow the paths of the rays
> forwards into the scene, eventually reaching the camera.

  That's just *one* way of doing it. In no way it's the only way, nor
necessarily the "best" way. And as I already said, there's nothing in
this algorithm which would make it "real".

> However, POV-Ray is
> "radiosity mode" seems to still follow light backwards, only this time
> attempting to take diffuse interreflections into account.

  Right. There are good advantages (and perhaps some disadvantages) of
doing it that way.

> Does this really count as radiosity?

  This is not the algorithm called "radiosity", which is a scanline-rendering
algorithm.
  There's nothing in this algorithm which would make it "less" global
illumination algorithm than any other.

> Could the
> photon mapping algorithm be changed to include diffuse elements also? Would
> this be worth doing?

  This is a FAQ.
  Nathan Kopp tried to implement global illumination using photon mapping,
but AFAIK the results were not very promising.
  If you think about the amount of samples needed to calculate global
illumination for a huge scene, you will understand why this is so. It's
very difficult to calculate more samples where they are needed and less
samples where they aren't. This is an advantage of the stochastic algorithm
used by POV-Ray.

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

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