bagginsbill said:

Quote - In fact, we can get VERY smooth lighting gradients by choosing enormous sample size. Think about it - if the whole card was sampled at only the corners, then by definition, the entire card would be smooth and splotch-free because it would all be interpolated.

Exactly! And that is precisely what we should be trying to do - go for the largest Irradiance Sample Size we can manage. (In the SIGGRAPH course Stewer pointed to, this seems to be called the Neighborhood Clamp Radius.) This scene is so dominated by really smooth gradients that large ISS should work very well indeed. I'm trying a larger ISS now and it is certainly going faster; quality yet to be determined. Ah, who needs sleep.

Whichway