CShake <cshake+pov### [at] gmailcom> wrote:
> Unfortunately, it's not something that seems to have been fixed with the
> beta. Here is a simpler test image, it renders identically in all the
> version I have to test in: Windows: Offical 3.6.1 & 3.7b31 and MegaPOV
> 1.2.1; Mac OS X: official 3.6.1, MegaPov 1.2.1 (PowerPC)
>
> I didn't try clipka's idea of media sampling quality yet, but here's the
> code for this test image so you can verify:

This is indeed a sampling quality issue: As the ray travels through the media,
it encounters alternating intervals of bright / dark light caused by the
"patterned" spotlight from the right.

The default number of samples is not enough to sample each single of these light
cones reliably. Note that you have something like 5 light cones in sequence to
be traversed by a ray; the default of 10 samples is barely enough to place a
single sample in each bright and each dark interval, and leaves no room for any
precision.

So with default settings you inevitably get an aliasing effect in the media
sampling.

With just that single light source, it isn't much of a problem: The aliasing
errors average out to a good deal, and the result looks convincing enough to
the human eye.

The problem surfaces, however, with the additional spotlight, as it causes POV
to modify the distribution of the media samples in an attempt to optimize it
for the standard case (whic is that the spotlight's cone makes up one single
illuminated interval), resulting in modulations to the aliasing effects, and
eventually moiree patterns.

Cranking up the quality helps indeed, e.g. using intervals 10 and samples 10,10
significantly reduces the amount of moiree (though it still remains visible; at
the same time the light pattern becomes less smooth, but close inspection shows
that this is actually more realistic.)

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