Am 31.07.2011 15:25, schrieb Le_Forgeron:

> Looks like some code is using a global resource, unprotected (like a
> random generator ?)

Possibly the radiosity sample cache; for pretrace efficiency, this is 
shared among all threads, with the effect that a sample at a certain 
point may or may not be taken depending on whether another thread has 
already taken samples nearby or not.

You may try using +HR (or "High_Reproducibility=on"); this will 
essentially prevent re-use of radiosity samples taken for a different 
render block in the same pretrace pass (with certain well-controlled 
exceptions), so that the radiosity pretrace always produces the very 
same set of samples regardless of thread scheduling and number of render 
threads. It is slower though.

At present this option affects only radiosity sampling; however, the 
idea is for the option to eliminate any non-determinisms in render 
artifacts (there still are some, but I never found the time to hunt them 
down yet) by throwing a moderate deal of extra computing effort at them 
if they can't be avoided otherwise.

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