A directional light is thought as one that is infinitely far away, similar to the sun. Thus, its "rays" are parallel and, in turn, we give the direction of the rays in defining the light. We also use the position property in doing this. However, we set the fourth component of the "position" to 0.0, which makes the light directional versus positional. In this case, the first three values define the direction as the vector (-x, -y, -z). Therefore, the easy approach is pick a point in world coordinates and then think of the direction as going from that point to the origin. For our example, the direction is parallel to the vector from the point (5.0, 1.0, 1.0) to the origin.

Color is the second property we define for our directional light. For our example, the color is set to a medium gray. As noted in the description of the omni light, it does not matter what we be pick for the alpha value. Further, we set this color to be used in the diffuse reflection calculations only. Setting a specular color property for a directional light is also an option.