Szirmay-Kalos, LászlóTóth, BalázsMagdics, MilánEduard Groeller and Holly Rushmeier2015-02-272015-02-2720111467-8659https://doi.org/10.1111/j.1467-8659.2010.01831.xThis paper presents efficient algorithms for free path sampling in heterogeneous participating media defined either by high-resolution voxel arrays or generated procedurally. The method is based on the concept of mixing ‘virtual’ material or particles to the medium, augmenting the extinction coefficient to a function for which the free path can be sampled in a straightforward way. The virtual material is selected such that it modifies the volume density but does not alter the radiance. We define the total extinction coefficient of the real and virtual particles by a low-resolution grid of super-voxels that are much larger than the real voxels defining the medium. The computational complexity of the proposed method depends just on the resolution of the super-voxel grid and does not grow with the resolution above the scale of super-voxels. The method is particularly efficient to render large, low-density, heterogeneous volumes, which should otherwise be defined by enormously high resolution voxel grids and where the average free path length would cross many voxels.