Christoph Hormann <chr### [at] gmxde> wrote:
> This is a very comprehensive description of radiosity workings in
> POV-Ray but i think you are mistaken about one central point:  The
> radiosity pretrace will not be able to (and was never designed to) take
> all samples necessary during pretrace in a real life scene.

It may not have been *designed* to do so, but for good quality it *must* do so
(at least to some good degree). Otherwise you get lots of artifacts everywhere
near new samples taken during final render, because some nearby pixels were
computed earlier, but would have been influenced by the new sample if it had
been there already.

There are a few things to note out here:

- When talking about samples *needed*, I mean needed by *algorithm*, not by
*theoretical* considerations.

- The algorithm in POV is designed so that it will enforce a certain minimum
effective radius of samples, so in this sense it will never *need* an infinite
number of samples.

- I'm advocating nothing more than *good* coverage, not a *perfect* one.

There's a point here to be made though, and it just gives me an idea how to
automatically get an exhaustive enough but not too slow pretrace:

Samples taken during final render that affect only a single pixel probably don't
hurt anyone. Artifacts appear only where samples are taken that should by
algorithm affect other pixels already calculated.

So there must be a way to use this fact to come up with a criterion when to stop
pretracing even though sample coverage doesn't seem to be rather incomplete yet.

Note however that just stopping pretrace at the final render's pixel size
doesn't suffice: on areas seen from a very shallow angle, a spot that still
needs samples may be "thin" enough on screen in one direction to slip through a
pixel-sized pretrace, but still "wide" enough in the other direction to do harm.

So the radiosity tracing code should report the "apparent" size of the largest
sample gathered for a pretrace ray (or some value directly enough related to
it), and if the pretrace finds that it needs to gather no more "oversized"
samples then it is (with a certain probability) safe to leave sampling in that
area to the final trace.

(I *love* having inspiring arguments with you, guys!)

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