restir_sample

Draws candidate sources and merges the cell's own history, one thread per grid cell.

Each cell picks a few sources at random and resamples them down to one, weighted by how much each would put on the surface the cell stands on. Drawing uniformly costs nothing and knows nothing; the resampling is what makes the survivor a sample of the sources that matter, without ever looking at the ones it did not draw.

The cell then merges the reservoir it held last frame, read and written at its own index. That merge costs one evaluation and buys the cell every candidate every previous frame drew, which is why the candidate count can stay small while quality keeps climbing.