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Parameter | Description |
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Color_______________________ | 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. |
Technical Description
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.
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