Scene Requirements

In order to work correctly with the misss_physical shader, it is necessary to set up the scene following these requirements:

Version: this shader will not work with version of mental ray prior to 3.3.
Link/include: link the subsurface shader library and include the subsurface.mi shader declaration file in the .rayrc file, or in the scene file itself, if this is not handled automatically by the modeling package that mental ray is built into.
Raytracing must be activated.
Caustics and/or global illumination must be enabled. (Caustics only is preferred to global illumination only.)
A light, preferably focused on the subsurface-scattering media object must specify an energy value as well as emit caustic and/or globillum photons.
The misss_physical shader must be used both as a material and a photon shader, or the material Phenomenon must be used. It must have a light that emits photons in its lightlist.
Subsurface-scattering objects or their instances must be flagged as receiving and casting caustics and/or global illumination.

The following .mi scene fragments ensure that the conditions are met:

At the beginning of the scene file:

    min version "3.3.0.7"
    link "subsurface.so"
    $include "subsurface.mi"

In the options block:

    trace on
    caustic on
    globillum on

In the light definition (example):

    energy 10000 10000 10000
    caustic photons 250000
    globillum photons 250000

The shader and material definition:

    shader "myshader" "misss_physical" ("lights" [ "lightinstancename" ])
    material options
    material "materialinstancename"
        = "myshader"
        photon = "myshader"
    end material

In the object or its instance definition:
```
    caustic 3
    globillum 3
```

The shader provides useful feedback when executed, using info messages. In any case verbosity level 5 is suggested to debug your scene. The shader statistics reports are especially valuable to estimate correct values for the following parameters:

depth: look for information in "channel 0 1 2" and "photon statistics".
max_radius and max_photons. Look for information in "multi-scatter photon per samples statistics".