Now here I am, brooding over various official and hacked versions of 
POV-Ray, figuring out whether my latest changes broke anything - only to 
find /this/:

Top left:
---------------------------------------------------------------
Persistence of Vision(tm) Ray Tracer Version 3.6.1 (g++ 3.4.1 @
  i686-pc-linux-gnu)
This is an official version prepared by the POV-Ray Team. See the
  documentation on how to contact the authors or visit us on the
  internet at http://www.povray.org/.
---------------------------------------------------------------

Top right:
---------------------------------------------------------------
Persistence of Vision(tm) Ray Tracer Version 3.6.1 (g++ 4.3.2 @
  x86_64-unknown-linux-gnu)
This is an unofficial version compiled by:
  Christoph Lipka
  The POV-Ray Team(tm) is not responsible for supporting this version.
---------------------------------------------------------------

(Bottom left: Difference; bottom right: averaged grayscale with 
differences highlighted)

Now this would not normally be too much of a surprise: You tamper with a 
version, it behaves differently...

The thing is, the /only/ thing I changed in that particular 3.6.1 
version - I swear! - was a constant that causes a slight shift in the 
checker, cells and brick patterns.

And recompile, of course. With the same brand of compiler.

Optimized for a different CPU type.

Now what would you think which of these might have the strongest impact 
on photon caustics?

The only conclusion I can draw from this is that POV-Ray is 
fundamentally incapable of producing identical results on different 
machines for some reason.


Well, maybe I should dig in the photons code to see whether it does some 
bit magics somewhere, in the photons cache handling maybe. Might be 
something that doesn't work as expected in a 64 bit environment - that 
would explain something. With a bit of luck, maybe it's even related to 
the segfaults I see with photons scenes when compiling POV-Ray with the 
Intel compiler.

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