They are, in order of increasing realism, absorption only, emission only, emission and absorption combined, single scattering of external illumination without shadows, single scattering with shadows, and multiple scattering. For each model I give the physical assumptions, describe the applications for which it is appropriate, derive the differential or integral equations for light transport, present calculation methods for solving them, and show output images for a data set representing a cloud.
Special attention is given to calculation methods for the multiple scattering model.
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