
4.4. Planar Receivers of Shadows 111
Fi gure 4.10. Dierent regions in the penumbra (le). Same conguration but with the
discontinuity events indicated by a color mesh on the oor (right). Image courtesy of James
Stewart.
extremal stabbing lines [584] dened as VV, VEE, and EEEE to incrementally com-
pute a visibility skeleton. is structure corresponds to a graph where a node is an
extremal stabbing line, and a link between two nodes indicates that a visibility event
(a surface dened by a line swath between two extremal stabbing lines) occurs be-
tween them. With a careful implementation and catalogs of events, Durand et al.
areabletohandledisco