Shadows

Abstract

For any realistic computer-generated image, light sources have to be considered as key elements in the scene. However, unless the light source is unique and located at an eyepoint or the illumination is very diffuse, as with an overcast sky, images are not complete without shadows. As noted by Crow (1977a), algorithms for shadows require considerable computation time. Moreover, the main problem is that most algorithms are unable to produce shadows for any object, virtual camera, and light condition, except ray-tracing (Whitted 1980; Kay and Greenberg 1979; Kajiya 1983), which is very expensive in terms of CPU time to be used for computer-generated films. In a computer animation scene, where the lights and/or the cameras are moving, an algorithm which gives perfect results for 100 frames may happen give a bad result for frame 101.