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Info

Before explaining the parameters of the material in detail, take note that :

  1. There are no diffuse terms. Most shaders have an ad-hoc diffuse term to approximate the soft look of hair. In reality, this soft look is caused by light scattering and is not related to a classic diffuse BRDF.
  2. No control over directly reflected color. Direct reflections have the same color as the incoming light. Any coloured visual features are due to light passing through a strand of hair. 
  3. There are no "glint" parameters other than strength and softness . This visual feature is caused by caustics inside each hair. Depending on hair roughness, light gets periodically concentrated in some areas of each hair strand and this causes the so-called glint.


Parameter

Description
Color

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This is the color of one hair strand. It is defined as: 1.0 - Absorption. Absorption inside a hair strand is the main parameter driving the overall look of hair. Fair hair has small absorption levels (Hair Color is closer to white), while dark hair has high absorption (Hair Color iscloser to black). When absorption levels are low, light scatters more so "glow" is more apparent.
Cuticle Angle
This values, expressed in radians, expresses the slope of each cuticle on a hair strand.
Index of Refraction
The index of refraction of each hair strand.
Roughness
Longitudinal

Azimuthal

Primary Multiplier

Weights
Reflection Weight
The contribution of Reflection (R) to the final result.
Transmission Weight
The contribution of Transmission (TT) to the final result.
TRT Weight
The contribution of 2nd Order Reflection to the final result
Note that the different weights are actually a balance between the different components. In other words, the weights are normalized so as to conserve the overall energy in the system.
Glints
Strength
The intensity of the glint effect, which are caused by caustics reflections inside the hairs and are typically driven by the roughness of the reflections.
Softness
Wether the glints are sharp or soft.

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