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If an object is opaque , it absorbs most of the incident light. This means that you must simulate this concept using photon mapping of light being absorbed. Light is absorbed mostly on diffuse surfaces. Sometimes when light hits an object, it is randomly absorbed. The random absorption of light in an object can become a very complex task because this is a factor of an object's molecular structure. In nature, light scatters in an object and reflects many times before it comes back out from some arbitrary direction. See Figure 11.10, which shows two photons . P1 depicts a photon released from the light source and sitting at the point of intersection. P2 shows the same photon randomly positioned because of the object's molecular structure.
The light that is reflected outside the object can end up in some mysterious places you would never expect, and deriving an efficient rendering algorithm poses a series of problems. For this reason light is absorbed at the point of intersection. Recall that when a photon hits an opaque object and somehow gets randomly absorbed, the photon is recorded at the point of intersection and doesn't travel any longer. The photon's life is ended. How opaque the object appears must be taken into consideration when absorbing a photon. This means that when storing photons for absorption in the photon map, an object's opacity must be incorporated.
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