T2S_Illumination is a Softimage XSI shader dedicated to CG artists looking for a single Illumination node to be used as a basic building block for a great variety of materials.

This shader brings together different illumination models, as well as some of the last Mental Ray functions for optimized glossy scattering. New features, such as Coating surface effects, custom Lafortune settings, or Fresnel Mask are proposed here.

You can check our gallery to download free presets, or also get new T2S Files for custom materials.

Introduction

We separate the illumination components into 5 features :

• Diffuse
• HighLights
• Scattering
• Fresnel Mask
• Surface Effects

Two other Tabs are added for custom adjustments :

• Anisotropy
• Weights

The idea of this shader has arised from discussions with Christian Goetzinger and Harry Bardak, who also helped us as beta-testers.
The Anisotropy settings are based on Daniel Rind “Deriver” Shader.
The Lafortune parameters were given by the Cornell University Program of Computer Graphics.
The Fresnel Presets were obtained from experimental measures obtained by the Sopra NK-Database.

Tek2Shoot thanks all of the above for their valuable help.

Download the Zip file T2S Illumination - 193.4 KB for Windows (x86 and x64)
Note that for x64 version you will need Microsoft Visual C++ 2008 redistributable package.

You can also download Zip file T2S Material - 1.61 KB to open the T2S Shaders directly in Get->Materials

Unsupported Maya version for Win32 adapted by Pixero Jan SandStröm.

T2S files can store data for Lafortune model, IOR, etc. The goal is to be able to use real measurment data like Cornell university for BRDF to build preset files for our illumination shader.

Download the first pack T2S FilePack 01 - 11.15 KB. New packs coming soon…

Lafortune T2S files

The T2S file format is formated like an ini file, it has some sections that contain parameters that are used by the different shaders.

As a text file, t2s are made to be modified/created by hand or by tools that aim at helping build models.

For example here is an extract of such a file for a Lafortune model:

; Tek2Shoot physical parameters file format
;
; Lafortune model : Cornell's Dupont Cayman lacquer,
; as used by the Ford Motor Company
;

[Model]
Type=Lafortune

[LafortuneParameters]
DiffuseR=1.6975113561687E-09
DiffuseG=0.00528865064056913
DiffuseB=0.0282197430365825
NbLobes=3

[Lobe0]
R_Cx=-0.173325012813183
R_Cy=-0.173325012813183
R_Cz=-0.781461130884269
R_Power=3.49807873729296

G_Cx=0.454890871106113
G_Cy=0.454890871106113
G_Cz=-0.634748129093241
G_Power=99.2031830199456

B_Cx=-0.843858153317284
B_Cy=-0.843858153317284
B_Cz=0.855093112043271
B_Power=4.48149667757088

Everything can easily modified based on characteristics of the models.

The main goal is to provide such files based on real measurment data. In order to achieve this, Tek2Shoot has developped a BRDF to Lafortune model fitter. This is a solver that takes as input Cornell, Columbia Utrecht or any other  BRDF measurment data and tries to fit the measurement to a Lafortune model. The solver has different algorithms and error functions to find different fits.

T2S Solver

The extraction of Lafortune models has been performed  for Cornell and other sources. Some  associated models are included in the T2S FilePack 01 - 11.15 KB.

For details on the Lafortune Model, please refer to the original article.

A simple description of this model would be to consider a BRDF as a sum of L lobes, where the radiance caused by each of these lobes is, for each color channel :

Red_Radiance (L) = Cx(R,L) (Ux.Vx) + Cy(R,L) (Uy.Vy) + Cz(R,L) (Uz.Vz)
Blue_Radiance (L) = Cx(G,L) (Ux.Vx) + Cy(G,L) (Uy.Vy) + Cz(G,L) (Uz.Vz)
Green_Radiance (L) = Cx(B,L) (Ux.Vx) + Cy(B,L) (Uy.Vy) + Cz(B,L) (Uz.Vz)

where U and V are the incoming and viewing vectors, and xyz a basis with z corresponding to the surface normal.

The highlights may thus have slight colorations depending on the viewing / lighting angles.

Fresnel T2S files

The Fresnel T2S files are used to get colored Fresnel Masks. It allows to set an independent refractive index (ior) and absorbtion coefficient (k) for R,G and B, and creates fluctuations on the reflection colors (current version of T2S_Illumination only uses colored IOR for reflections).
Thus, the Fresnel Mask can be white for grazing angles, and become colored depending on the incidence.
Here is an example of a Fresnel T2S file

;   Gold Fresnel Preset
;
;
; IOR values adapted from SOPRA NK-Database measures
;

[Model]
Type=Fresnel

[FresnelParameters]

R_ior = 0.16
R_k   = 4.0

G_ior = 0.4
G_k   = 2.65

B_ior = 1.61
B_k   = 1.94

The values obtained for IOR presets have already been included in the Fresnel Presets of T2S_Illumination. Some of them are included in the T2S FilePack 01 - 11.15 KB in order to bring examples for those wanting to create their own Fresnel t2s files.

T2S_Coating adds a thin reflective coat to your materials, creating nice iridescent effects.

Thin Coats may be responsible of interferences in the reflected light, and let appear color fluctuations depending on light incidence.  Biological Iridescence can also be obtained by simulating mulitple layers, and calculating the ondulatory interference. These subtle effects can now be obtained in Mental Ray almost instantaneously.

Iridescence phenomena appear when a very thin coat (< 1µm) lies on a surface.
Since light is an electromagnetic field with a given wavelength and phase, when it hits a surface, the optical path difference between the wave reflected at  the top of the coat and the wave reflected at the end of the coat  is responsible of a  phase difference, and causes interferences.

Since these interferences depend on the incidence angle and on the wavelength of incoming light, iridescence phenomena appear, resulting in  impressive “rainbow-like” color fluctuations. These effects can be clearly seen in soap-bubbles, petroleum, sunglasses, etc…

Iridescence cannot be obtained in CG renders relying on usual geometric optics. Several complex render engines, such as Maxwell, treat  light as an electromagnetic field, and can thus create these subtle effects. However the rendering times are necessary longer since the physical information needed is much higher than in Mental Ray.

The T2S_Coating shader allows to create Iridescence in Mental Ray without needing a time-consuming rendering scheme. You just need to plug your previous shader in the Material Color port of the T2S_Coating node and an iridescent reflective layer will be added to your object.

Isotropic glossy reflections are used, sending a certain amount of Glossy Samples with a Shiny value for glossiness. A Fresnel Mask can be enabled to filter the reflections depending on incidence. A refractive index must be set for the Coat (Film IOR) and for ambient media (Media In). The Fresnel Mask may be calculated with Physical equations (see T2S_FresnelMask) or with Custom adjustments (as in MR glossy_reflections).

The Interference properties also depend on the refractive index. A Thin Surface Coat, as described above, can be selected, as well as a Bi-Layering mode. Bi-Layering corresponds to two thin coats separated by an InterLayer Spacing.

This structure creates new interference properties which are simple ways of creating interference structures.
Such structures are responsible of Biological Iridescence, for example in butterfly wings, insects, or fishes.

Download T2S Coating - 35.71 KB for Win32
Download T2S Material - 1.61 KB allows to open the T2S Shaders directly in Get->Materials

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

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.

This xsiaddon file is a wrapper containing several unexposed Mental Ray shaders for XSI. The interface has been improved according to the Mental Image documentation.

These shaders are included : Car Paint – Ambient and FG Occlusion -  Glossy Reflection and glossy refraction – Physical Lights – D2S_Lights, which combines “physical_light”, “mib_blackbody” and “mib_cie_d” in order to reproduce realistic illumination, and last MR versions of “DGS_Material”, “Dielectric_Material”, and Volumetric Shader “Participating Media”

Mental ray 3.4 provides several new shaders that are not yet exposed in xsi 5.1 user interface.

This xsiaddon just installs a wrapper which gives you an easy access to these shaders.
The interface has been improved according to the Mental Image documentation.

Here is the list of the shaders that are exposed through this addon

• Car Paint   (see doc at LAmrUG : [link])
• Ambient and FG Occlusion   (see doc at [link] or [link])
• Glossy Reflection and glossy refraction
• Physical Lights
• New version of D2S_Lights, which combines “physical_light” ,  “mib_cie_d” and “mib_blackbody” in order to reproduce realistic illumination.
• Last MR version of Physics Shaders (”DGS”, “Dielectric” and “Participating Media”)

D2S_Lights V.2.0

This version of D2S_Lights includes the possibility to switch between “mib_cie_d” and “mib_blackbody” to get the Light Temperature Color.
This is useful when temperatures below 4000 K are needed, since “mib_cie_d” (which is more accurate than blackbodies between 4000K- 25000K), does not work below 4000 K as it is specified in the MR Documentation.
The CIE Color Chart is now included in the interface.

Warning :

- If Area Lights don’t seem to work, just turn your light source by 180°  (XSI bug)
- When opening an old scene with previous version of d2s_light, it will be necessary to set the light shader again

Final Gather Contribution

Download: Unexposed Mental Ray shaders - 1.2 MB (version 1.1)
(Old version available: Unexposed Mental Ray shaders - 993.91 KB (version 1.0))

You can also download Zip T2S Material - 1.61 KB to open the T2S Shaders directly in Get->Materials

Release notes:

This release doesn’t provide SSS spdl anymore until we fix a naming issue.
Fixed an issue with Edge Factor parameter that was not correctly linked.
Included pdf from xsi TD Love Tour.

You can send your feedback to contact@tek2shoot.com

****** IMPORTANT NOTE ********

In case you have an issue uninstalling the previous version of the addon (xsi might not render after this stage).
Just open a shell, go to “c:softimagexsi_5.0applicationbin” and then run “runonce.bat”, this solves the issue about SSS spdl pointing to the wrong dll.

The Fresnel Mask determines the reflectivity of a surface depending on its refractive index (IOR).

When used as a Mask on the reflectivity of a shader, the grazing-angles surfaces become more reflective than those facing the camera, adding realism to the material illumination.

Contrarily to other shaders (such as Dielectric, provided by XSI) the FresnelMask is based on the “real-physics” Fresnel Laws, and doesn’t have to be set arbitrarily.  Three mathematical models (Schlick, Fresnel and complex Fresnel) can be used to evaluate this function, allowing you allow to choose between accuracy and speed.

The Fresnel Laws, discovered by Augustin Jean Fresnel (1788-1827), describe the behaviour of light at the interface between two different media.

A part of the incoming light is reflected, while another part  is trasmitted (i.e. refracted or absorbed).  The proportion of reflected light only depends on on the (complex) refractive index and on the angle of incidence.

The Fresnel equations determine the Reflection coefficient  R  and the Transimission coefficient  T  = 1 – R,  depending on the light polarization (s or p).

and

where n1 and n2 are the “In” and “Out” complex refractive index,
qi is the incident angle and qt the transmitted angle deduced by the Snell Law  ( n1 sin(qi)  = n2 sin(qt) )
Rs and Rp correspond therefore to the fraction of s-polarized and p-polarized light which is reflected.

If the refractive index are complex, an absorbtion coefficient k , which determines how fast the light is absorbed, needs to be determined.

The Schlick approximation can be made for fast calculations.

For more details concerning the Fresnel equations, please refer to http://www.odforce.net/wiki/index.php/ReflectanceFunctions#The_Fresnel_Effect

Download our xsiaddon T2S Fresnel Mask - 6.86 KB (Framework .NET 2.0 needed )

Illustration of the Fresnel Effect

- The first image has been rendered with a simple Lambert with a Reflectivity of 0.5.
- The second image has been rendered with the T2S_FresnelMask plugged into the reflectivity port of the Lambert node, using a refractive index of 5 with Schlick’s approximation. It can be seen how the grazing angles become much more reflective, adding realism to the scene.

Car Reference

Car Fresnel Mask

• Porcelain preset (522)
• Green glass preset (475)
• Wet metal preset (449)
• Waxy preset (397)
• Cap preset (423)
• Several Presets by Christian Goetzinger (342)
• Soap bubble preset (317)