Cycles has a sample offset feature allowing users to render X samples
in a single frame on one device, then the remaining Y samples later or
on a different device and combine them back together at the end.
However in most situations the result from using this method was
different, and usually lower quality than rendering all the samples in
one go.
This was because Cycles tunes it's random number sequence for the
number of samples being rendered. And the random number sequence was
being tuned for the wrong number of samples in the case that a user
was using the sample offset.
This commit fixes this issue by adding a "sample subset" feature.
The user specifies the total sample count being rendered across all
devices in the existing `Max Samples` parameter, then specifies per
device which subset of samples will be rendered (E.g. Render samples
0-1024 out of a 0-2048 range).
This commit also contains some additional clean up work
inside Cycles related to the area being changed.
Co-authored-by: Brecht Van Lommel <brecht@blender.org>
Pull Request: https://projects.blender.org/blender/blender/pulls/132961
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