There has been great interest in expressive visualization over the last few years. This kind of
visualization is used in 2D animation, illustration, and schematic drawing. The flat shading
and shape lines, which we call external and internal sketches, are all visual charasteristics
of this type of visualization. We present an algorithm for obtaining the sketches from a
polygonal model. The algorithm is based on the concept of virtual lights, which allows us to
control the visualization of internal sketches in a flexible way.