The first two dimensions of scrambled, shuffled Sobol and shuffled PMJ02 are equivalent, so this makes no real difference for the first two dimensions. But Sobol allows us to naturally extend to more dimensions.
This in turn allows the following two things (also implemented):
- Use proper 3D samples for combined lens + motion blur sampling. This notably reduces the noise on objects that are both out-of-focus objects are also motion blurred.
- Use proper 3D samples for combined light selection + light sampling. Cycles was already doing something clever here with 2D samples, but this is probably(?) more robust, as per the issue reported in T101356.
In the future this will also allow for proper sampling of e.g. volumetric light sources and other things that may need three or four dimensions.