Figure 15.1 shows a polygon. It is the outline of a shape, drawn with straight line segments. Since such shapes are all a printer or plotter can draw, just about every computer-generated drawing consists of polygons. If we add an “eye” to the bird-shaped polygon from Figure 15.1, and if we apply a sequence of rotations and translations to it, then we arrive at Figure 15.2 — it turns out copies of our special bird polygon can cover the whole plane! This technique is also present in the Escher illustration, Figure 6.8. Polygon: straight line segments forming a bird shape. Mixing maps: a pattern is created by composing rotations and translations.