The 3Delight Hair shader is a physically plausible shader that renders hair and fur using a Monte-Carlo simulation. A Monte-Carlo simulation explores the different light paths inside a medium (in this case, a clump of hair) to produce an image.
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 a single hair strand. This seemingly simple observation explains why so many complex shaders are not successful at conveying the look of realistic hair. This includes the widely used shaders based on the Marschner model.
Example renders of blond, red and dark hair. Note the glow caused by light scattering inside blond and red hair. Also note how black hair has All theses features necessitate proper simulation of light scattering in hair clumps. | |||
Before explaining the parameters of the shader in detail, let's explain the fundamentals :
- There is no diffuse term. Most shaders have an add-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.
- The is no primary/secondary highlight parameters. As is the case for diffuse, these parameters are an approximation to directly relfected light and to light that has been reflected after bouncing twice inside a hair strand.
- No control over directly reflected colour. Direct reflections have the same colour as the incoming light. Any coloured visual features are due to light passing through through a strand of hair.
- There are no "glint" parameters. 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.
Parameters
| Parameter | Description |
|---|---|
| Hair Colour | This is the colour of one hair strand. It is defined as (1 - Absorption). Absorption inside a hair strand is the main parameter driving the overall look of hair. Fair hair has small absorption levels (Hair Colour is closer to white), while dark hair has high absorption (Hair Colour is closer to black). When absorption levels are low, light scatters more so more "glow" is more apparent. |
| Hair Thickness | The diameter of one hair strand. The wider the hair the more it absorbs light. 3Delight first computes the absorption levels from Hair Colour and then multiplies that absorption by this parameter to get the actual absorption inside one hair strand. Note that is parameter doesn't affect the geomtric thickness of hair |
| Reflection Roughness | This is the roughness of the direct reflection. The range is [0..1] with values closer to one meaning rougher hair. |
| Reflection Weight | |
| Transmission Roughness | |
| Transmission Weight | |
Output Variables