When ambient light is reflected and reradiated in an image, finding the final distribution of light in the image requires the solution of a very large linear algebra problem. The total light *R_i *radiating from a face *i* is given by R_i = E_i + Σ F_ij R_j, where _{i} is the light emitted by face *i* and the *F*_{ij} are “view factors” describing the relative geometry of faces *i *and *j*. In the scene on the left, there are 64 polyhedra, each with 32 faces. The resulting system of 2048 linear equations in 2048 variables is solved in several tenths of a second using an iterative method implemented in Mathematica, but would take much longer with a standard solver. In a real application, like a scene from an animated movie, there would be a few million polygons in the scene and the resulting solution would require solving a system *A**x* = *b* where the matrix *A* was (say) 2,000,000 by 2,000,000. Even storing such a matrix would take on the order of 4 terabytes of memory! Luckily, such matrices are very sparse, so they are (barely) tractable with good computing hardware.