1. A photolithography simulation system implemented by a computer, the system comprising: a three-dimensional mask model of a type of photolithography mask, 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 type of photolithography mask on light passing through a mask of that type; and simulating a near-field image expected to be produced by a photolithographic tool using the mask, using the computer executing a software tool using the three dimensional mask model.