mental ray comes with material shaders that support caustics and global illumination, like dgs_material and dielectric_material. Their photon shader equivalents are dgs_material_photon and dielectric_material_photon. When defining a material it is necessary to specify both the regular material shader and the photon shader. Most often, however, the photon shader can inherit the parameter setting from the regular material shader. In addition to these shaders, users or applications can provide new material and photon material shaders.
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