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Specular energy reflected in the opposite direction shouldn't be simulated using photon mapping. Remember, a large number of photons are required to reflect a mirror image. It's therefore better to use ray tracing to create mirror reflections. This is done at the time of the rendering process when a new ray is traced in the mirror direction using backward ray tracing. The mirror reflection code used in ray tracing is added to the photon contribution method to combine the lighting effects (ray tracing and photon mapping effects) into one solution. Figure 14.8 shows an example of diffuse interaction done with photon mapping and mirror reflection done with ray tracing.
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