Melanin Redness
Dye Color
Melanin
Specular Level
Roughness Along
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Overall Look | |
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Color | This is the overall color of the hair clump. |
Longitudinal Roughness | Longitudinal roughness (i.e. scattering) determines the specular lobe along the length of the hair. This parameter plays an important role on the final look as shown in the images below. |
Azimuthal Roughness | |
Primary Multiplier | |
Components 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 | |
Index of Refraction | The index of refraction of each hair strand. |
Cuticle Angle | This values, expressed in radians, expresses the slope of each cuticle on a hair strand. |
Glint 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. |
Glint Softness | Wether the glints are sharp or soft. |
An interesting aspect of hair rendering is that all the intricate visual details seen in hair stem from the scattering of light in a multitude of hair strands and is not due to the complexity of the BRDF on a single hair strand. This seemingly simple observation explains why so many complex shaders are not successful at conveying a realistic look for hair. This includes the widely used shaders based on the Marschner model not using volume scattering. The shader simulates 3 scattering events from one single hair strand and then proceeds with a Monte-Carlo simulation to follow light paths inside the hair volume.
Two observations:
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R is the light directly reflected from the surface of a hair strand. In many other hair materials this is referred to as the primary highlight. TT is the transmission of light through the hair strand and interacting with the interior of the strand (absorption). This is responsible for the back lighting visible through the hair when a light is behind a hair clump. TRT is the secondary reflection of light traveling through the hair twice before going back to the same side it entered. As this light path gets absorbed twice it is dimmer than R and TT. In many other hair materials this is referred to as the secondary highlight. The following images show a final image and each component rendered separately.
Final | R | TT | TRT |