Return the right-handed normal (not necessarily unit length) for an
edge in the direction of the positive v-axis at the given u-value on
the given face. (This vector is perpendicular to the plane through
the sphere origin that contains the given edge.)
Return the right-handed normal (not necessarily unit length) for an edge in the direction of the positive v-axis at the given u-value on the given face. (This vector is perpendicular to the plane through the sphere origin that contains the given edge.)