An example of the lines an illustrator would use to convey the shape of this airplane foot pedal. A wireframe display is common because it can give a lot of information which is occluded by shading. However, a wireframe display of a complex model can be confusing due to the number of lines being displayed. Changing which isolines are displayed can change the perception of the surface. 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.
An example of these three different line types is provided in Figure 3.4 . Regions where the surface normal changes abruptly, C 1 discontinuities, are also important in defining the shape of an object. 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.