Vincent Le Chevalier <gal### [at] libertyallsurfspamfr> wrote:
> First, I don't understand completely what you are trying to do. If you 
> implement an unbiased method, of course the pictures will be radically 
> different.

  Unless I have understood something completely incorrectly, I think that
"unbiased rendering" simply means that light is not assumed to be coming
from a specific direction, but the entire space is sampled for possible
incoming light.

  Surface properties are described as BRDFs. (The advantage of unbiased
rendering is that it allows, due to its unbiased sampling nature, much
richer and complex BRDFs to be defined than traditional rendering methods.)

  Well, the phong lighting model is a perfectly valid BRDF, so I see
absolutely no reason why unbiased rendering would exclude the possibility
of using it. I don't believe that unbiased rendering would somehow limit
what kind of BRDFs you can use.

  If the phong lighting model is used as the BRDF for all surfaces the
end result should be pretty much the same using unbiased rendering than
using the traditional raytracing method (except for the possible graininess).

  The advantage, in this case, is that much more complex scenes,
traiditionally requiring enormous amounts of rays (because of ray
bifurcation and things like area lights) may render faster using single
ray paths which do not bifurcate. (This requires *much* higher antialiasing
settings, but in certain situations the overall amount of rays traced may
in fact be smaller than with the traditional raytracing method.)

> There is no way around that. If the pictures, with the exact 
> same parameters, were only more grainy, but eventually converged to the 
> exact same image on average, then the method would be biased...

  I'm more interested in the single-path-tracing than in the unbiasing.

  (Besides, I think the only difference pure unbiased rendering would do
is to make the image have global illumination, ie. what povray calls
"radiosity", especially if big area lights are used instead of point
lights.)

> The other problem is indeed that many material models within POV-Ray are 
> physically inaccurate

  I bet all BRDFs are physically inaccurate to some extent, and only
*approximate* the real thing. The phong lighting model is one approximation
among others (it might not be the best one, but it's a simple and fast one,
and often gives good results).

  I also have hard time believing that unbiased rendering would somehow
exclude "physically inaccurate" BRDFs.

> Simply follow the light through the surface. 100% of the light should be 
> transmitted, and 100% reflected? It's clearly not conservative.

  It doesn't have to be 100% physically accurate. The only thing that
matters is that it can be described as a BRDF.

> I suspect the case of your black reflective object is a bit similar.

  No, that phenomenon happens in real life. Even surfaces which do not
reflect diffusely light almost at all can have strong specular reflection
properties. That's the reason why pitch-black plastic can have bright
highlights. It has something to do with quantum mechanics or something
similar, don't remember any details.

> And of course you cannot use max_trace_level because it introduces a bias...

  Using a high-enough max_trace_level will probably not have too much
influence in the resulting image (except in cases where it would have
a significant influence in the traditional raytracing method as well).

-- 
                                                          - Warp

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