143 Shading in Two Dimensions

Shading is a fundamental technique for conveying three dimensional form in two dimensions . Shading can also be used, however, to emphasize the relative importance of different elements in a scene, to establish the visible priority of overlapping regions, and to indicate three dimensionality symbolically by gradations of tone. In animation, shading adds a graphic dimension to twodimensional objects and characters which is very expensive to achieve by conventional means, and may be of stylistic importance when merging animated figures with live-action scenes and situations. This paper reviews some of the typical computer techniques employed for two dimensional shading functions , and attempts to apply them to generate three-dimensional objects which can be shaded as such. This addresses a problem of persistent interest in the machine vision community: the estimate of three-dimensional shape from silhouettes.