Fig. 2.

C₁, C₂, and C₃ are direction curves on the unit sphere centered at the point x. These curves indicate the directions from which x are illuminated. $A_j^s$ and $A_j^e$ are the start and end positions of C_j, respectively, j ∈ {1,2,3}. The arrows are from the start to the end positions. The dashed curves are on the back surface of the sphere, which would be invisible if the sphere is opaque. The dotted lines are through the center of the sphere O. (a) A 3-D view, and (b) the top view of the unit sphere.