Shadowing

  Illustrators only include shadows when they do not occlude detail in other parts of the object [20,21,23]. In 3D interactive illustrations, adding only a drop shadow on a ground plane, not the shadows that an object may cast onto itself, provide helpful visual clues without occluding important details on the object. It is probably not important that these shadows be highly accurate to provide valuable information about three-dimensional structure, especially the spatial layout of a scene [17,29]. In Section 10 of the course notes, we will discuss the implementation details for adding drop shadows to an interactive illustration system.