Abstract: A technique for selecting a subset of determined defects in a mask pattern is described. In this technique, defects in the mask pattern may be determined based on differences between a pattern produced at an image plane in a photolithographic process, when the mask pattern, illuminated by an associated source pattern, is at an object plane in the photolithographic process, and a target pattern that excludes the defects. These defects may be classified by associating them with types of geometric features in the target pattern and/or the mask pattern. Moreover, the subset may be selected by filtering the defects associated with the types of geometric features. For example, the subset may defects corresponding to the differences that exceed filtering values that are associated with the types of geometric features.