In order to compute volume caustics and global illumination in participating media, the media need volume photon shaders. A volume photon shader will usually have the same name as the corresponding volume shader, but with " _photon" appended. Details on photon tracing in volumes can be found in the mental ray manual.
parti_volume_photon is the volume photon shader that matches parti_volume. It has the same parameters.
color "parti_volume_photon" (
integer "mode",
color "scatter",
scalar "extinction",
scalar "r",
scalar "g1",
scalar "g2",
scalar "height",
scalar "nonuniform",
scalar "min_step_len",
scalar "max_step_len",
scalar "light_dist",
integer "min_level",
boolean "no_globil_where_direct",
array light "lights")
All other parameters, especially extinction, need to be identical to the parameters of parti_volume, this to preserve consistency in the algorithm.
transmat_photon is the material photon shader that matches transmat. It simply continues to trace the incoming photon in the same direction without any interaction with the current object. Typically, helper objects are used to enclose volume effects such as participating media implemented as volume shaders. Such a volume container should be invisible to photons itself. This shader is only required when no mental ray hull object is used as volume container.
color "transmat_photon" ()
There are no parameters.
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