1. A lithography simulation system, comprising: a three-dimensional mask model of a type of photolithography mask, comprising one or more of: a correction factor configured to modify a mathematical transform of a mask transmission function of a mask; a correction factor configured to modify a mathematical transform of a horizontal mask-edge function of the mask; a correction factor configured to modify a mathematical transform of a vertical mask-edge function of the mask; and a correction factor configured to modify a mathematical transform of a mask-corner function of the mask, wherein the correction factors represent one or more effects of the topography of the type of photolithography mask on light passing through a mask of that type; and a software tool adapted to use the model to simulate an image produced by a lithographic tool using the mask.