Creating shadows using shadow volumes and the geometry shader
As we discovered in the previous recipes, one of the main problems with shadow maps is aliasing. The problem essentially boils down to the fact that we are sampling the shadow map(s) at a different frequency (resolution) than we are using when rendering the scene. To minimize the aliasing we can blur the shadow edges (as in the previous recipes), or try to sample the shadow map at a frequency that is closer to the corresponding resolution in projected screen space. There are many techniques that help with the latter; for more details, I recommend the book Real-Time Shadows.
An alternate technique for shadow generation is called shadow volumes. The shadow volume method completely avoids the aliasing problem that plagues shadow maps. With shadow volumes, you get pixel-perfect hard shadows, without the aliasing artifacts of shadow maps. The following figure shows a scene with shadows that are produced using the shadow volume technique...
