V-Ray Next: The science behind the new hair shader

Rendering Hair: A Physiological Approach

Rendering hair correctly in computer graphics is a fairly complex problem to solve. Because hair is made up of thousands of translucent and scaly cylinders, it’s a fairly complex surface. This makes it difficult to determine exactly how light interacts as it transmits, reflects and bounces off it. To make it even more challenging, an average head of human hair contains more than 100K hairs, and these hairs are very fine — usually thinner than a single pixel.

While previous hair shaders have been a good approximation, it can be difficult to get hair color and reflectivity just right. In V-Ray Next, with research based on the paper A Practical and Controllable Hair and Fur Model for Production Path Tracing, we’ve created a new shader that renders hair in a more physiological way. Biologically speaking, hair color is determined by its amount of melanin. So instead of setting an arbitrary color value, we’ve created a simple melanin slider to control hair color realistically. The same goes for shininess. The new settings are now  based on the physiology of real hair. While this may be unfamiliar at first, with a little understanding of the biology of hair, the new shader is much easier to control and much more predictable.

Hair color is all about melanin

Your hair color is determined by its amount of melanin. The more melanin you have, the darker your hair will be. Blonde hair has very little melanin, while black hair has the most. But there are actually two types of melanin that control your hair color: eumelanin and pheomelanin. The ratio of these two types of melanin determine how red your hair is. If you have more pheomelanin, your hair will be more red.

In the new V-Ray hair material you can set hair color with just a single slider control. A value of 0 is white hair, and a value of 1 is black hair. All other hair colors fall somewhere in between. To make red hair, first set the melanin to the desired darkness, then adjust the pheomelanin slider until you reach the desired redness. This sets the ratio of eumelanin to pheomelanin, with 0 being all eumelanin and 1 being all pheomelanin. 

Melanin amount

Melanin with different pheomelanin ratios 

What about dyed hair?

Of course, not everyone keeps their natural hair color. People often use hair dye, which can be just about any color. To help with this we’ve added hair dye controls to the new hair shader. And to use it correctly, you just need to think like a hair stylist. For example, to dye hair blue, you first have to remove the color by bleaching it white. That way the color will show up. To do this in the material, you simply turn the melanin levels down to near 0 and then add dye color as needed. 

Different glossiness levels

Glossiness with different boosts

Examples of random glossiness

The new hair shader is currently available in the V-Ray Next for 3ds Max Beta and can be rendered in both V-Ray and V-Ray GPU. Join the beta to give it a try. 

Reference material

[1] Chiang et al. "A Practical and Controllable Hair and Fur Model for Production Path Tracing" - 2016
      (https://benedikt-bitterli.me/pchfm/)
[2] Pharr "THE IMPLEMENTATION OF A HAIR SCATTERING MODEL" - 2016
      (http://www.pbrt.org/hair.pdf)
[3] d'Eon et al. "An Energy-Conserving Hair Reflectance Model" - 2011
      (http://www.eugenedeon.com/project/an-energy-conserving-hair-reflectance-model/)
[4] d'Eon et al. "An Energy-Conserving Hair Reflectance Model" post publication review - 2011
      (https://publons.com/publon/2803/)
[5] d'Eon et al. "Importance Sampling for Physically-Based Hair Fiber Models" - 2013
      (https://cg.ivd.kit.edu/publications/pubhanika/2013_hairbrief.pdf)
[6] d'Eon et al. "A Fiber Scattering Model with Non-Separable Lobes - Supplemental Report" - 2014
      (http://www.eugenedeon.com/project/a-fiber-scattering-model-with-non-separable-lobes/)
[7] Marschner et al. "Light Scattering from Human Hair Fibers" - 2003
      (http://www.graphics.stanford.edu/papers/hair/hair-sg03final.pdf)
[8] Zinke et al. "Light Scattering from Filaments" - 2007
      (http://cg.cs.uni-bonn.de/project-pages/hairmodeling/documents/BFSDF_preprint_with_copyright_notice.pdf)
[9] Zinke (thesis) "Photo-Realistic Rendering of Fiber Assemblies" - 2008
      (http://cg.cs.uni-bonn.de/en/publications/paper-details/Zinke2008a/)
[10] Jakob (thesis) "Light Transport On Path-Space Manifolds" - 2012
      (https://ecommons.cornell.edu/handle/1813/34189)
[11] Yan et al. "Physically-Accurate Fur Reflectance: Modeling, Measurement and Rendering" - 2015
      (http://people.eecs.berkeley.edu/~lingqi/publications/paper_fur.pdf)
[12] Yan et al. "An Efficient and Practical Near and Far Field Fur Reflectance Model" - 2017
      (http://people.eecs.berkeley.edu/~lingqi/publications/paper_fur2.pdf)