P = normalize(Plane);
L = Light;
d = dot(P, L)
P.a * L.x + d P.a * L.y P.a * L.z P.a * L.w
P.b * L.x P.b * L.y + d P.b * L.z P.b * L.w
P.c * L.x P.c * L.y P.c * L.z + d P.c * L.w
P.d * L.x P.d * L.y P.d * L.z P.d * L.w + d
If the light's w-component is 0, the ray from the origin to the light represents a directional light. If it is 1, the light is a point light.