Communications From Readers

The peak-pit-pass polygon line-shading procedure partitions any concave polygon into a series of triangles and trapezoids that are shaded independently. These convex subpolygons are identified by processing each vertex of the polygon outline in descending rank order. Alternative actions are initiated whenever a local maximum (peak), a local minimum (pit), or neither (pass) is encountered. Being relatively insensitive to the topology of the polygon and to the density of shade lines while minimizing storage and time requirements, this algorithm achieves a balance between efficiency and flexibility.