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The conventional procedure for generating shaded images of curved surfaces is to approximate each surface element by a mosaic of polygons and to then apply one of several established polygon display algorithms. The method described here produces an excellent approximation of bi-cubic parametric surfaces in scan line order. Each surface patch is described in terms of edge curves which are intersected by successive scanning planes to form endpoints of scan line segments. Visibility is calculated for each segment by a hybrid priority/zbuffer scheme. Shading is computed using Phongs illumination model with interpolated surface normal values. CR Categories: 8.2.
Turner Whitted (1978) studied this question.