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The photon map is so cleverly designed that photons that are reflected can be homed in dark areas in the model contributing very little power. Ray tracing was poor at simulating the penumbras in shadows, but with photon mapping it's an automatic and simple effect. This is because photons sit within dark areas that contribute to a shadow's penumbra . Light in the real world is everywhere and no shadow is completely black. You used the ambient factor to fix this problem with shadows in ray tracing, but this is a forced-looking fix. Ray tracing is poor at calculating shadows because it can't determine if the light is gathered in some small parts of a shadow. This isn't the case with photon mapping because photons contribute to more detail in some lighting effects. See Figure 14.11.
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