Am 29.08.2014 09:16, schrieb scott:
>>> Here is the difference between two renders of the scene a few minutes
>>> apart. It seems some blocks always get given the exact same seed for the
>>> RNG.
>>
>> Did you use the Windows version? If so, did you start the renders from
>> the same instance of the GUI?
>
> Yes and yes.
>
>> I suspect that I'm only computing the base seed when POV-Ray is started,
>> erroneously using it over and over again during the entire session.
>
> It seems like it.
>
>> The difference between many of the blocks would then be solely due to
>> different assignment of render blocks to threads (because of different
>> thread scheduling), and using "+WT1" you should indeed see no
>> differences at all.
>
> The new version you just uploaded works as expected now, thanks! I find
> that after a long render if I'm still not happy with the level of noise
> I can run it again and average the results - which will work now :-)
>
> BTW I had the same issue when I first started using multi-instances of
> mcpov, it uses the number of seconds as the seed by default, so if you
> start 12 instances simultaneously they all render exactly the same result.

IIRC mcpov also had some command-line or SDL option to choose a custom seed.

>  From a programming point of view, wouldn't it better to use
> milli/micro/nano seconds or ticks to give more chance of avoiding
> creating repeatable RNG streams?

Unfortunately, functions that provide this information tend to (A) not 
be portable, or (B) have an unknown granularity, possibly only 
increasing the value every second as well.

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