Regions where the surface normal changes abruptly, C 1 discontinuities or creases, are also important in defining the shape of an object. For a polygonal model, a crease is determined by two front facing polygons whose dihedral angle is above some threshold. In an interactive system, the user should be allowed to control this parameter based on the model and the intent of the image. Sometimes surface boundaries also need to be drawn, but only in the case where there is not a surface connecting another surface or where the joint between surfaces changes abruptly. The image on the right looks as if it has a deeper pit because the isolines go thru the maximum curvature point on the surface.
Illustrators use lines to separate parts of objects and define important shape characteristics.