The Lafortune representation is a BRDF model  (as Phong, Blinn, etc...) fitted to experimental values. It allows to create realistic specular with color fluctuation depending on the viewing / lighting angles. The T2S_Lafortune shader includes this model, as well as Oren-Nayar and Wrap Lighting for the diffuse component.

The BRDF (Bidirectional Reflectance Distribution Function ) of a material represents the amount of light of a given wavelength perceived for each couple of  "Viewing / Lighting" angles.

This data determines completely the appearance of a material, since all the (complex) interactions between the object and  light are included in it. However, this data can hardly be used by itself and a mathematical model is needed to interpolate between all the measured values.

In 1997, Eric Lafortune et al.  proposed a mathematical model, which can represent the whole BRDF of an isotropic material by a set of 27 parameters (3 coefs * 3 colors * 3 lobes). This scheme has been successfully used with different materials.

Since the parameters are determined independently for R, G and B, highly realistic illumination can be obtained, which is impossible  with MentalRay simplified BRDF shaders (Blinn, Phong, DGS, etc....).  The T2S_Lafortune shader is based on this model.

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

Download the  t2s_lafortune 10/07/2006,16:45 37.19 Kb for Win32.
 t2s_materials_menu 27/06/2006,15:49 1.61 Kb allows you to open the T2S Shaders directly in Get->Materials.

For the diffuse component an Oren Nayar model is used. This model accounts for complex phenomena such as masking, shadowing and interreflections between the microscopic Lambertian facets on the surface.  

Wrap Lighting is also implemented in the shader, which can add a global illumination look in simple scenes by "wrapping" the light around the objects.

The Specular Lafortune presets contain the parameters provided by Lafortune et al.  for  different 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). Anisotropic materials and highly complex BRDF
( > 3 lobes) can also be used.  For more details concerning the T2S File Format click here.