A third distance d2 is computed between a point of intersection p03 on triangle T1 on a point of intersection
p13 on the polygon. This distance d2 is also relatively small, generating a dark shadow. To compute the third distance
d2, the distance from the light source to the intersection point p13 is held in the secondary depth buffer and the distance
from the light source to the intersection point p03 is held in the primary depth buffer