1. A computer-implemented method for determining a reconstructed mask pattern, comprising: receiving a target mask pattern; receiving an image of at least a portion of a photo-mask corresponding to a mask pattern, wherein the image is determined using inspection optics having an optical path; and reconstructing, using a computer, the mask pattern from the image based on a characteristic of the optical path and the target mask pattern, wherein the mask pattern is reconstructed using a constrained inverse optical calculation in which there are a finite number of discrete feature widths allowed in the reconstructed mask pattern, and wherein a given feature has a constant feature width.