1. A photolithography simulation system implemented by a computer, the system comprising: a three-dimensional mask model of a photolithography mask having certain characteristics, the three-dimensional mask model 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 photolithography mask on light passing through a mask having the certain characteristics, and wherein existing symmetry properties of one or more correction factors of the three-dimensional mask model are used to improve computational efficiency; and a software tool executed by the computer adapted to use the three-dimensional mask model to simulate a near-field image expected to be produced by a photolithographic tool using the mask.