The purpose of this demo is to create shadows using a technique called stencil

shadow volumes in a 3d scene. To do this we will not need the shadow matrix from

previous demo. In fact in order to perform this all we need is to render the

silhouette edges of an object (which are the outer edges of a object) extended in

a direction (which depends on the light position) into the stencil buffer twice. After

that we draw the scene twice, once for the dark parts of the scene (areas that are

shadowed) and once for the lit parts of the scene (areas that are not in a shadow).

With that you can create extremely realistic shadows in your scenes. At first glance

this sounds a lot more complicated than it really is. When it comes to the silhouette

edges we don t need to worry about that because the object casting a shadow in this

demo is a simple quad. So its outer edges are the same as every point that makes

it up.