Scene Requirements

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

The subsurface shader package needs to be loaded into mental ray.
Ray tracing needs to be enabled.
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/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 should be linked to lights which emit photons.
Subsurface-scattering objects or their instances must be flagged as casting and receiving caustics/global illumination.

The following .mi scene fragments show an example setup:

At the beginning of the scene file:

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" ("mode" 4) # use instance lights
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".