A system performs mask rule checks (MRC) for curvilinear shapes. The width of a curvilinear shape is different along different parts of the shape. A medial axis for a curvilinear shape is determined. The medial axis is trimmed to exclude portions that are within a threshold distance from corners or too far from edges. The trimmed medial axis is used to perform width checks for mask rules. The system generates medial axis between geometric shapes and uses it to determine whether two geometric shapes are at least a threshold distance apart. The system performs acute angle checks for sharp corners. The system determines angles using lines drawn from vertices to end points on the boundary of the shape that are at a threshold distance. These angles are used for checking acute angle mask rule violations.