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.
Volume Photon Shader: parti_volume_photon
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")
min_level is the minimum refraction level at which photons are stored. (The maximum level is set with the global option photon_refraction_depth.)
light_dist, no_globil_where_direct, and lights are ignored since there is no direct illumination in the photon shader.
All other parameters are identical to the parameters of parti_volume.
Photon Shader: transmat_photon
transmat_photon is the material photon shader that matches transmat. It has the same parameters. It simply traces a photon further in the direction it came from.
color "transmat_photon" ()
There are no parameters.