Ken <tyl### [at] pacbellnet> wrote:
> How can a second instance of POV-Ray possibly know exactly which frames had
> been rendered by the first instance.

  The two processes must communicate in order for them to know where to stop.
That is, when one process has rendered a frame and is ready to start the
next frame, it asks the other process if that frame is free. If the other
process answers "yes", then it proceeds, else it stops.

  And yes, communication between two processes suffer from all the
mutual exclusion problems typical to parallel processing, and if not
well done, the two processes might start rendering the same frame. This
shouldn't be a big problem in this case, though. And besides, there are
algorithms which handle the mutual exclusion problem.

  For those who don't know, the mutual exclusion problem means that
if the access to a common variable is not locked between the "ask if
the frame is free" and "set the frame as non-free" commands, then the
other process might ask for the variable after the first process asked,
but before it could set it as non-free. Then both will get a "it's free"
value which is of course not desired.
  How to lock other processes from accessing the variable while one process
is reading and setting it is an interesting problem with interesting
algorithmical solutions.

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

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