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Note the glow caused by light scattering inside blond and light coloured red hair. Also note how black hair has nice silky features. The "glint" is caused by caustics inside hair strands and is not an after thought of the shading model. All these features necessitate proper simulation of light scattering in hair clumps.
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Melanin
This parameter, alongside the Melanin Redness bellow, controls the generation of natural-looking hair colors. It sets the amount of melanin in each hair strand. Colors will range from white at around 0, to blonde at around 0.2 to red and brown around 0.5 and to black at the maximum value of 1.0.
Melanin Redness
Controls the redness of hair. In more technical terms, this controls the the proportion of red pheomelanin (as found in red hair) relative to the amount of eumelanin.
Dye Weight and Dye Color
It is sometimes desirable to render non-natural looking colors for hair. Using a dye color allows setting the color directly, without using melanin parameters. The dye weight allows mixing both methods using a simple mix. Setting tthe Dye Weight to 1 will use only Dye Color and setting it to 0 will only use Melanin and Melanin Red, all values in between will use a mix of both.
Specular Level
Sets the overall specular level for hair. Values closer to 0 will render rough looking hair, values closer to 1.0 will render very shiny hair.
Roughness Along
Roughness Around
White Hair
This parameter controls the amount of white hairs in the clump of hair.
Synthetic Fiber
Dye Hue Variation
Technical Notes
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 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.
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- There are is diffuse component. 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.
- 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.
Melanin
Melanin Redness
Dye Color
Specular Level
Roughness Along
Roughness Around
White Hair
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Synthetic Fiber
Dye Hue Variation
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