This node is a fully featured model of hair/fur shading. It gives you control over highlights, glints, and back-lighting, which are the essential visual components of real hair. Similar to the Surface Model, the Hair Model computes direct lighting (from light sources) and indirect lighting (lighting bouncing off other objects in the scene).

This node is intended to be the starting point for hair/fur shader networks. Since the Hair Model itself takes care of lighting and simulates most physical properties, simply connecting the Hair Model VOP to the output would create an "uber-shader" capable of simulating most hair/fur surfaces.

For more information on building a material shader with VOPs, see creating materials.

• Physically-based hair model

• Works with PBR, ray-tracing, and micropolygon rendering engines

• Easy-to-use controls for the size and position of highlights

• Energy conservation

In non-physically based renders, this node adds support for the following:

• Per-light exports

• Variance antialiasing support for area lights

• Variance antialiasing support for raytracing

The hair model takes a physically-based approach to shading hair. It does this by taking into account the different ways that light rays can bounce off of an individual strand of hair.

The hair model simulates three light paths: reflected (primary or R), transmitted-reflected-transmitted (secondary or TRT), and transmitted-transmitted (transmission or TT). The primary path hits the hair and is reflected. The secondary path enters the hair, reflects off the back of the hair, and exits out of the front of the hair. The transmission path enters the hair and exits through the back. The following diagram depicts the paths:

Each light path produces a distinctive visual feature. For example, primary paths tend to be scattered diffusely, resulting in a uniform color. You can think of the primary component as being similar to a diffuse component. In fact, you must have Diffuse Contribution enabled on your light sources to get contribution from the primary light paths.

The secondary component is more directional in nature than the primary component, resulting in highlights. You can think of the secondary component as a specular highlight.

The transmission component controls how much light passes through a hair strand. This makes it useful for controlling the look of hair/fur in situations where the hair/fur is back-lit.

For a discussion of reflectivity, see the Surface Model.

The size parameters control the angular spread of a reflection or transmission lobe. Low values produce a concentrated lobe that reflects/transmits light over a narrow cone of angles, whereas high values produce a lobe that reflects/transmits light over a wide cone of angles. For example, if you want to produce a diffuse look, use a large size. For a sharp highlight, use a small size.

The shift parameters control the general direction of a reflection or transmission lobe. Values range from -1 (towards the hair’s root) to 1 (towards the hair’s tip). A value of 0 results in a direction perpendicular to the hair. The Reflection Shift parameter on the Secondary Reflection tab is typically most useful for controlling the position of a secondary reflection.

This node exports a large number of variables you can use to create extra image planes when rendering to the OpenEXR format. It is often useful for compositing and special effects to have the primary and secondary reflections output by the shader as separate images.

For example, you can export the shader’s direct/indirect/combined diffuse/reflect/refract output, samples, and so on. You can export the total output for each variable, or separate per-light exports.

See the Extra image planes parameter of the mantra render node for more information.

The Hair Model VOP combines the effects of the other three hair VOPs: Physically Based Hair (Primary Reflection), Physically Based Hair (Secondary Reflection), and Physically Based Hair (Transmission). This is analogous to the way the Surface Model combines the effects of the Physical Based Diffuse and Physically Based Specular VOPs.

The Hair Model VOP handles higher level functionality such as:

• Deep raster exports

• Conservation of energy

• Opacity

The other Physically Based Hair VOPs simply create BSDFs. You could use them separately, but usually you would want to combine them. The Hair Model VOP uses these BSDFs to get the overall hair effect.