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Indirectly reflecting light from one surface to another is easily simulated using photon mapping. The photon map was constructed by direct and indirect incoming flux. The photon map is very good at telling the indirect energy in a given area. For this reason, some programmers use photon mapping only for indirect illumination and use ray tracing for all direct illumination effects. The only energy that is recorded in the photon map is the energy that is reflected. This means the photon map is constructed only by reflected energy. Figure 14.10 shows an example of direct illumination done with ray tracing and indirect illumination done with photon mapping.
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