Darren New wrote:
> Warp wrote:
>> jhu wrote:
>>> Can the radiosity data be collected in parallel
>>   No.
> 
> Out of curiousity, is this a technical limitation or a theoretical one? 

   I understood the question as "is it possible with pov3.6 to collect
radiosity in parallel?" and the answer is a clear "no".

   (Btw, even though povray calls it "radiosity", this shouldn't be
confused with the algorithm (used mostly in scanline rendering) called
radiosity which is completely different. What povray calculates is a
stochastic inter-reflection between surfaces, but I'll call it
global illumination (which is the most generic term) for short.)

   In theory it is possible to collect global illumination data in
parallel, but either there needs to be a data container with
multithreading support shared between all the nodes (possible but
not trivial, and increases drastically the amount of communication
between the nodes) or they could be calculated separately and then
merged (which would cause considerable overlapping, ie. needless
calculations).

   The problem in distributing the stochastic algorithm used by
povray is that values already stored in the spatial data container
are used to speed up further calculations. This is naturally done for
speed, but it has the side-effect that changing rendering parameters
even a bit can cause visible differences in the end result (eg. just
changing the starting line of the rendering can change the
illumination). This is why separate threads/processes/computers cannot
calculate global illumination of different parts of the scene
separately without visible differences in illumination appearing.
All the calculations would need to be synchronized in order to get
an uniform result.

   I actually don't know how the pov-team intends to correct this
problem because it is a rather difficult one. (I actually don't know
how the current pov3.7 beta does it, but it looks to me as if it used
just one thread when radiosity is used.)

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