Render time is reduced for overexposed scenes, by taking into account absolute
light intensity for adaptive sampling.
This can negatively affect some scenes where compositing or color management
are used to make the scene much darker or lighter. For best results adjust the
Film > Exposure setting to bring the intensity into a good range, and then do
further compositing and color management on top of that. Note that this setting
is different than color management exposure.
Previously Cycles' adaptive sampling used sqrt(I) to normalize noise level to
conform to a viewer's eye sensitivity. It is great for darker regions of the
image, but also requests too much samples in bright regions, sometimes several
times more than needed. Highlights can tolerate more noise because in most
examples it is still less noticeable then the noise in darker areas in the same
render.
Differential Revision: https://developer.blender.org/D16392
857bb1b5ec