Global illumination effects can be rendered with path ray tracing. On contrary to the distributed ray tracing, for each ray hit, at most one ray is reflected or refracted when path ray tracing is used. With sufficiently large trace depth limits, most of the indirect illumination effects are computed correctly. However, the results are very noisy and an extremely high number of samples is required for obtaining high quality images.
An implementation of the path raytracing is provided by the path_material shader which, for convenience, uses the same parameters as dgs_material.
Note that the mental ray final gathering and global illumination settings have no effect on the surfaces with path_material: the diffuse indirect illumination component is computed by reflecting at most one ray in a direction driven by with Lambertian distribution. As a consequence, there is no path_material_photon shader. For rendering speedup, final gathering and photon emission should be disabled. Comparing with the scenes using photon maps, the trace depth has to be increased.
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