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Parameters Description
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- 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.
- 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.
Overall Look | |
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Color | This is the overall color of the hair clump. |
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. |
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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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:
- 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.
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