1. A computer-implemented rendering method for rendering an image of a scene, the rendering method performed in a rendering system that includes one or more processors, the rendering method comprising: receiving electronically readable representative data corresponding to one or more objects in a computer-generated imagery (“CGI”) scene of a virtual space; determine a first partition of cells of the one or more objects; for each of a plurality of progressive iterations: performing a photon simulation of a lighting distribution of the one or more objects using a respective set of photons; temporarily storing the respective set of photons in a memory; for each of a plurality of cells of the first partition: calculating statistics of the photons in the cell for the current iteration and any previous iterations using the respective set of photons and the calculated statistics from a previous iteration; fitting an initial mixture of density functions to the calculated statistics of the current iteration; and evicting from the memory the respective set of photons, wherein the iterations are performed until the initial mixture of density functions satisfy one or more criteria for fitting the calculated statistics; and rendering an image of the CGI scene using the initial mixture of density functions.