1. A computer-implemented method, comprising: performing, by one or more computers: for each location on each surface in a scene for which an irradiance value is to be computed: predetermining a number of samples to be taken along a polar angle of a hemisphere above the surface at the respective location, wherein the hemisphere corresponds to and is centered on the respective location, and wherein the number of samples at the respective location is determined according to one or more perceptual metrics at the respective location; and storing a value indicating the predetermined number of samples to be taken along the polar angle of the hemisphere corresponding to the respective location; subsequent to said predetermining and said storing, computing irradiance for the scene, wherein said computing irradiance comprises, for each said location: determining a total number of samples to take for the respective location from the stored value indicating the predetermined number of samples to be taken along the polar angle of the hemisphere corresponding to the respective location; and calculating an irradiance value for the respective location according to the determined total number of samples for the respective location; wherein the determined total number of samples used in calculating the irradiance values for the locations varies according to differences in the one or more perceptual metrics at the locations used in said predetermining the number of samples at the locations such that the total number of samples used at the locations in said computing irradiance for the scene varies for different locations in the scene; and rendering the scene in accordance with the calculated irradiance values.