Andrew Coppin <orp### [at] btinternetcom> wrote:
>>   If you think about the amount of samples needed to calculate global
>> illumination for a huge scene, you will understand why this is so.

> Yeah, actually... caustics are heigh-detail, but they don't take up such
> surface area, so the photon maps don't cover very much. Global lighting
> would take too much memory I suppose.

  Photon mapping for caustics works well because they can be concentrated on
very specific areas of the scene (you tell which specific objects need to
be shot photons at).
  Global illumination would require shooting photons at *everything*. And
lots of them.
  Even caustics can sometimes show odd artifacts when the lights spreads
too much, in which case you need to increase the photon mapping parameters
to shoot more photons. This slows down, but not unacceptably because the
total amount is still quite limited. However, having to do this for the
entire scene is just enormous (I'm not saying it's not possible for simple
scenes, but just imagine a huge one).

  The advantage of the global illumination method used in POV-Ray is that
it can concentrate on the parts which really need it. It only calculates
the illumination of the parts of the scene which are visible on the final
image or have some indirect effect on visible areas. Areas which are not
visible nor have any effect are simply skipped. It also calculates more
samples where they are needed.
  The algorithm shows often some artifacts. It's still quite unclear to me
whether this is a problem with the algorithm itself or whether its
implemention in POV-Ray has some problems (bugs, misunderstandings or bad
designs). AFAIK the same algorithm is used in the Radiance renderer, which
is used professionally, and its results are great (at least based on the
few images I have seen).

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