9.5.2

880 • CHAPTER 18
THE SHADERS • 881
 ChamLum lets light “creep around corners” when using this offset. As the illustration above shows,
even though the top-most surface point lies on the shadowed (left) side, it still receives light because
the sample point ((S) see previous illus.) still does not lie in the shade. This is the basic concept behind
ChamLum.
 That‘s why ChanLum also samples to be “spread out”. This can be achieved in two ways: The rst
method (“Along Normal”) is illustrated above. The sample is actually an average sample of the area (S)
(the rectangular space). In other words, ChanLum simply takes a set of fog samples along the surface
normal, not further away from the original (offset) sample point than radius (R). This smoothes the
transition from bright to dark, and allows the light to creep along the surface even further.
 When combining this type of lighting with a fresnel effect (above), a fuzzy, velvet-like effect will
result, that correctly attenuates for shadows.