Feature Edges: Silhouettes, Boundaries, and Creases
For uniformity throughout this book, we assume a standard definition of a polygon normal, such that polygon normals point outward from surfaces. This assumption yields the following:
if TV • V < 0 then the polygon is front-facing;
if TV • V > 0 then the polygon is back-facing;
if TV • V = 0 then the polygon is perpendicular to the view direction.