In Computer Graphics, HighLights (or Specular term) correspond to the glossy reflection of light sources. Since the radiosity coming from a light  source is much higher than indirect illumination, even low-reflective materials may show a small highlight at mirroring angles in addition to the diffuse color. However, Mental Ray doesn't rely on radiosity, and has indeed to consider separate "artificial" highlight properties.

The amount of light reflected for each couple of viewing / ligthing angles is determined by the Bidirectional Reflectance Distribution Function (BRDF). This BRDF is responsible of the appearance of an object, and allows us to differenciate the different materials surrounding us. 

The T2S_illumination shader includes 4 BRDF models :

• Phong
• Blinn
• Ward Anisotropic
• Lafortune

These models can here be enabled and mixed with separate parameters and weights. Custom highlights can therefore be easily created.

“Gollum had a subsurface term mixed with a straight lambertian term and 4 specular terms, one of which was a LaFortune which is essentially 3 specular calls.“ 
                  -Kevin Smith 

The Lafortune model is extremely interesting since it is based on experimental BRDF measures . This model uses separate coefficients for Red, Green and Blue components, making the specular coloration change as a function of the viewing and lighting angles.

Comparison between Phong (left) and Lafortune (right) highlights

The Lafortune presets contain the parameters provided by the Cornell University Program of Computer Graphics for the following materials :

• Blue Paint
• Matte Steel
• Gray Primer
• Caucasian Male
• Indian Male
• Red Clay
  

In addition, we implement the possibility of using new material data (.t2s files), which contain the Lafortune coefs.  Anisotropic materials and highly complex BRDF ( > 3 lobes) can therefore be used. 
For more details concerning the Lafortune model and the t2s file format, please refer to T2S File Format
Roll Off affects the HighLight intensity.

If Enabled, the Ward Anisotropic parameters can be linked to the scattering settings, giving them the same color and shiny values. This is similar to Mental Ray "dgs_material" scheme.