restir_denoise

Averages the resampled light across neighbouring cells and composites it onto the frame, one thread per pixel.

One source per cell is correct on average and different from its neighbour, which is the grain a single sample always carries. The cells around a pixel stand on the same surface and estimate the same quantity, so averaging over them keeps the value and drops the variance.

Taps are a cell apart, so the same tap count reaches over as many independent estimates at half resolution as at full. Each is weighed by how nearly its surface is this one — a tap across a depth step, or facing elsewhere, is estimating something else.