Gail Shaw <initialsurname@sentech sa dot com> wrote:
> Since always. Area lights are implemented as arrays of point lights

  I wouldn't say there's too much difference between having, technically
speaking, an array of (perhaps jittered) point lights and randomly shooting
samples towards an area light. They are extremely similar ways of doing
the same thing, although the minor differences introduce advantages and
disadvantages to both methods.

  A randomly-sampled "illuminating object" can have any shape, form,
brightness variation and surface coloration whatsoever, without any limit.
You can simulate this to some extent with the point lights idea, but with
very large free-form shapes it can become quite inefficient because you
would need hundreds if not thousands of point lights, and freedom of form
makes it more difficult to implement adaptive sub-sampling.

  OTOH a grid of point lights becomes much more efficient if the physical
size of the area light is small. The smaller the area light, the more
efficient it becomes compared to random sampling (because if the light
source is very small, the vast majority of random samples will miss it).

  Both methods suffer from speed/graininess tradeoffs. The less grainy you
want the result to be, the more it takes to render (even though the light
grid method becomes the faster the smaller the area light is).

  Perhaps the best "true area light" lighting method is radiosity (the
algorithm called "radiosity", which is related to calculating lightmaps,
not the stochastic sampling method used by POV-Ray), even though in this
case it's also a discrete sampling method because you are basically
calculating light maps which have a certain pixel resolution. The end
result are completely grain-free regardless of this resolution, though.
(The only artifact caused by low sampling is unrealistic smoothing).
This also suffers somewhat if the area light is very small, although not
nearly as badly as random sampling. (Another problem with radiosity is
that it cannot be used to calculate specular reflections caused by area
lights.)

-- 
                                                          - Warp

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