Non-Photorealistic Computer Graphics Library

An algorithm for shading of regions on vector display devices

Author(s): Kurt E. Brassel, Robin Fegeas.
Proceedings: Proceedings of the 6th annual conference on Computer graphics and interactive techniques, Siggraph 79, pp. 126--133, 1979.
[BibTeX]

Abstract:
The display of shaded polygons by line, cross-hatch, and dot patterns on vector devices is a task frequently used in computer graphics and computer cartography. In applications such as the production of shaded maps polygon shading turns out to be critical with respect to time requirements, and the development of efficient algorithms is of importance. Given an arbitrary polygon in the plane without self-crossing edges (simply-connected polygon), the task at hand is to shade this polygon with one or two sets of parallel lines where for each set a shading angle and a line distance are given. The basic concept of this new algorithm is to decompose the polygon into a set of mutually exclusive trapezoids (in special cases triangles) where the parallel edges of the trapezoids are parallel to the desired shading lines. These trapezoids and triangles are then shaded in a fast procedure. In its present form the algorithm handles regions with up to 300 islands. Possible extensions include the construction of dash and cross patterns.