> Vincent Le Chevalier wrote:
>> Well Scott did, and it's not all that difficult to perfect it to 
>> obtain the result you seek, i.e. to have different colors for 
>> diffusion and reflection.
> 
> As long as we're simplifying, can anyone describe what the benefit of
>  this technique is, compared to biased ray tracing? What does biased
>  ray-tracing miss that this one catches?
> 

Having had a look in the books again...

Basically, all the methods creating images by firing rays into the scene
can be thought of as attempts to evaluate a very hairy integral (the
rendering equation) using a monte-carlo approach.

To do this, paths of light inside the scene must be sampled, in a way
that makes the monte-carlo method converge when the number of samples rises.

Classical raytracing, where no diffuse rays are fired, is obviously
biased, which means that whatever the number of rays you will fire,
you'll never reach the physically correct image. That's because there is
an entire category of paths that never gets sampled, so we have no idea
of what their contribution is. For example, no path such as Light
source<->diffuse surface<->diffuse surface<->camera are ever created.

On the other hand, the picture is not noisy (statistically speaking, the
estimator has a low variance).

There are other ways for a bias to creep in. For example, if you limit
systematically the length of sampled paths to a fixed value. Russian
roulette is one of the methods to avoid such problems.

Methods such as irradiance caching, in my limited understanding, also
introduces a bias. Simply speaking, every time you try to minimize the
variance (the high frequency noise in the picture), you are in danger of
introducing a bias unless great care is taken.

The bias is not in itself something that prevents beautiful pictures. It
prevents accurate pictures. The benefit would be more of a practical
nature: since you know that the picture will eventually be physically
correct, you can use real-life parameters and trust the physics, instead
of having to adjust the algorithm themselves and being forced to include
non-physical "hacks" everywhere in your scene. In way, you can be more
straightforward and coherent. But it's slower...

That being said, I never used an unbiased renderer, so who am I to talk :-)

-- 
Vincent

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