>> 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.

 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?

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