
178 5. Other Treatments of Shadows
Walter et al. [614] construct a hierarchical representation for VPLs (emitted
and reemitted), where one VPL (or a few, to allow for stochastic selection) gets
the intensity of all its children VPLs. At the point to be shaded, they test in a top-
down fashion (a cut in the tree of VPLs) when the combined VPL contribution
is large enough to deserve sending shadow rays. Bounds on each shading term
allow ecient evaluations of both local VPLs and global, more intense VPLs. e
method is conservative, but because tight bounds on shadowing are dicult to
eciently compute, all VPLs are simply assumed to be visible. is