Defining polygon sides with normals

So far in this book, we’ve calculated normals but not explored their many uses. One of these uses is to dictate which side of a polygon is visible. The same plane that you have been using up to this point has had the normals reversed in Autodesk Maya, a 3D modeling program, for the center polygons, as shown in Figure 11.4:

Figure 11.4: A plane with some normals reversed

The black section in the middle of the plane in Figure 11.4 when rendered would in fact appear as a hole when viewed from one direction and solid from the other. Even though it might look like a hole, that doesn’t mean there aren’t any polygons covering this area. When this plane is drawn with a cube behind it in Python and OpenGL, the hole is evident, as shown in Figure 11.5 (a):

Figure 11.5: A plane with normals reversed viewed from both sides

As can be seen in Figure 11.5, whichever polygons have normals on...