3.0 Directional Light Sources

Calculating the shadow caster volume for a directional light source is very similar to the calculations for point light sources. Diagram 6 shows the shadow caster volume for a directional light source.

Diagram 6: Shadow volume for an infinite directional light source

Notice that the shadow volume is not a true polyhedron, as it has an infinite volume. Depending on your needs, you may wish to add an additional plane to make the volume finite.

When generating the shadow volume for a directional light, we keep view frustum planes if their normal points towards the light source (plane normal dotted with the light's direction is negative), and discard them otherwise. Silhouette edges are again the edges between kept and discarded planes.

New planes are generated using the two silhouette edge endpoints, and a third point obtained by adding the light source direction vector to one of the end points. Again the direction of the plane's normal should be checked to determine if it needs to be flipped.

4.0 Semi-Transparent Objects