> While I'm here - slightly different question.
>
> They recommend you don't try to render radiosity images in parallel.
> Would it be possible to run the pretrace step, save the rad data, and
> then start a full render going on 2 instances of POV-Ray, both loading
> back the same rad data? (I'd have to turn off additional sampling,
> probably.) Would that work OK? Anyone ever tried it?
>
> (My dad has a faster computer than me - and it's dual CPU.)
>
> Andrew.

Yup, as long as both machines use the same irradiance cache to render their
subimages, and you turn off always_sample, you wont get any tiling
artifacts (*).

*If your shared irradiance cache is not sufficiently filled, however,
POV-Ray will still take samples if it can't find any to reuse, even if
always_sample is set to off. This would have the effect of not using the
same irradiance data for each tile, and may lead to the tiling artifacts
depending on how often POV-Ray is forced to sample during the final pass.

Ideally you want the caches to be the same exactly, but even having the
caches close but not matched suppresses the tiling artifacts to varying
degrees (sometimes greatly).

If you render in parallel simply by controlling POV-Ray with an external
script (using +SC/+EC/+SR/+ER), it will do a radiosity pretrace for each
tile, using only the image dimensions that tile occupies. This would make
the irradiance data used per tile drastically different, leading to the
dreaded tile artifacts. It would also by default throw away all the rad
data used after every tile.

In the parallel software I've written (which involves a patch to POV-Ray), I
do the pretrace only once per machine using the full image dimensions. (I
preserve the rad data internally after the pretrace and between individual
tiles). Then I crank out each tile (subimage) without doing any partial
image pretraces for the tiles. This greatly suppressed those tile
artifacts! I'm pleased and encouraged by this, and I want to take it
further.

As it stands now, each machine is doing duplicate work in the pretrace. The
next thing I want to try is to have each machine render a slightly
different pretrace (using jittered, or stratified sampling), then have all
the machines upload their results to the Client, combine them, and send the
full cache to each machine in preperation for the real rendering (the
tiles). Assuming
sending the rad data does not take too much time, this would be a
significant speedup over the current approach!

George

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