Determining back-facing curved model surfaces by analysis at the boundary

An examination is made of the problem of predicting when a model surface patch on a three-dimensional object is totally back-facing, and hence need not be searched for during object recognition. Examining every point on the surface patch is inelegant and impractical, yet difficulties arise with curved surface patches. The authors conclude that visibility can be determined from an analysis of the surface orientation at the patch boundary for a wide class of model surface patch types (i.e. those having constant principal curvature signs), under orthographic projection