50 shades of grey or iridescence secrets

Taking part in SIGGRAPH means having the possibility to access a wide range of technical papers that gives us hints about what our jobs will be in the future.

Some of these papers or technics are adaptable straight away in our workflow in order to enhance the photorealism of our frames. Among them, I would like to point out the “Reflectance and Scattering” conference, Nicolas Holzschuch and Romain Pacanowski’s work: “A Two-Scale Microfacet Reflectance Model Combining Reflection and Diffraction” but also Laurent Belcour and Pascal Barla’s “A Practical Extension to Microfacet Theory for the Modeling of Varying Iridescence”.

Our frames often make us wonder about the shades of grey and their neutrality: Cool? Warm? To what extent? Is there any room for a neutral grey in real life?

Papers that were developped at SIGGRAPH take up an idea first encountered on Maxwell Render blog: this concept that we can add a coating and iridescence effect to the structure of our materials in order to grant subtle colored variations to our diffuse color. Basically, you lightly shade the material according to the angle of incidence of the light. This gives a pearly effect which intensity and presence have been really lowered. The result is extremely realistic! Even though this is remarkable concerning grey shades, it can also be applied to other colors with more saturated diffusions.

The process is quite simple:

• With rendering engines which support “thin films” or “coatings”, you just need to affix a very diffuse coating layer in addition to reflection and diffuse parameters. Then you have to adjust this layer weight correspondingly to the effect that you want to create. This process is well explained in this article from Maxwell Render blog.
  
• For other rendering engines, there are plugins that are able to stimulate this effect like Siger Studio’s “Thin Film” plugin for instance. The concept is to load the frame with a rainbow shading and to tailor the falloff and fresnel settings to your will in order to modulate shimmer effects.
  Download the plugin 

To know more about this subject, consult the Siggraph technical papers:

• A Two-Scale Microfacet Reflectance Model Combining Reflection and Diffraction
• A Practical Extension to Microfacet Theory for the Modeling of Varying Iridescence (Pdf)
• A Practical Extension to Microfacet Theory for the Modeling of Varying Iridescence (video)