The authors present a shading logic approach to deal with non-convex geometries for an APL audience. Their algorithm not only creates a clipped object, but also makes reverse clipping possible. APL procedures greatly simplify the task. The algorithm has been successfully applied in IBM’s Draw Master program.

The purpose of this paper is a general tutorial. The cross product feature illustrates the relationship of points and lines. One APL statement generates the coefficients of a line given any two points on the line. The authors show the clipping logic with an example and additional APL functions. The paper concludes with remarks on the matrix M and its geometric implications.

In summary, the power of APL is utilized to solve the problem. Four references are provided. The paper could be improved if the 63 lines of APL code plus comments were also included.