Houdini 20.0 Nodes VOP nodes

Karma Fur VOP node

A physically-based hair and fur material with medulla support

On this page
Since 20.0


The Karma Fur shader is an extension of the Chiang model - Karma Hair. This model considers a structural feature of fur and thick hair: the so-called medulla. Hair and fur have three main components:

  • Cuticle: the hair’s outermost layer. The cuticle consists of 5-10 overlapping layers and is covered by nested series of scales. The scales protrude slightly from the surface of the hair.

  • Cortex: the hair’s mid-layer. The cortex generally accounts for 80-90% of a hair’s volume, but less for fur. The cortex carries the light-absorbing pigments responsible for hair color.

  • Medulla: the inner core inside the cortex. The medulla is also pigmented, but not present in every hair, but mainly in normal to thick hair. Scattering from the medulla is much more significant than scattering from the cortex.

The main goal of this extension to Chiang model is not just to reproduce fur strands properties on micro-level, but to achieve a photorealistic and controllable light scattering of fur on macro-level. You can control the radius of the medulla with Medulla Radius. When set to 0, the shader matches exactly the Chiang model. Taking the medulla’s geometry into account has several advantages over the original Chiang model

  • Precise and controllable light attenuation due to the analytical refraction of the medulla through the cortex medium, based on cortex IOR and cortex absorption coefficient.

  • Control light scattering of grooms through Medulla Radius, Medulla Phase (Henyey-Greenstain scattering function), and Medulla Diffuse to localize scattering. Hair and fur behave very similar to volumetrics. With the medulla extension you can control light scattering in the same way as volumetrics.


To get full scattering between Chiang lobes and medulla lobes, Caustic paths should be turned on for the fur geometry. Without caustics you’ll experience a loss of energy resulting in darker hair.


In conjunction with Karma Fur, the Chiang model mimics the cortex along with several advanced parameters. You can find more information about cortex parameters on the Karma Hair page. This chapter only describes the extension’s custom parameters.

Cuticle Reflectance

A human hair shaft has cuticle scales similar to roof shingles. However, cuticles on fur can have complex shapes. The outer surface of animal fur fibers is usually rougher than that of human hair. We account for roughness of both longitudinal and azimuthal sections of a fur fiber in our model, assuming they have the same level of simplicity.

You can also create an extra primary (R) specular lobe with its own Longitudinal and Azimuthal roughness and color input. This lobe is mixed with the main R-specular RGB components of Extra R Color.


The index of refraction (IOR) contributes significantly to the hair’s final look. Basically this is a cortex IOR. The IOR affects how the cortex material refracts the medulla and how big the medulla appears in the render.


Medulla allows for a fluffy and diffusive look with controlled back scattering and rim effects. In the Chiang model, those effects are difficult to achieve. Certain looks are even impossible with basic Chiang. The extended medulla model lets create more complex setups with coat and thermal insulation layers, for example undercoat. You also have more control over the fur’s look to get a more volume-like and realistic appearance.

The advanced medulla model is mainly used for fur, but it can also be applied to human hair. Thick human hair also has a medulla, but typically it’s thinner than animal medulla.

Medulla Radius

The parameter which controls radius of medulla inside cortex.

Medulla Phase

Here you control the light’s scattering behavior. For natural strands, this parameter is always positive. Light that scatters through the groom is always forward scattering.

Medulla Diffuse

This parameter doesn’t have a physical background and is meant for artistic control. You can increase Medulla Diffuse to make the material more diffuse. As a consequence, light is reflected from the medulla, not scattered.

Medulla Pigment

Controls the amount of light that is absorbed by the medulla.

Medulla Opacity

This parameter doesn’t have a physical background and is meant for artistic control. Medulla Opacity creates more transparent medulla lobes that can be mixed with main Chiang lobes.


Cortex Blend Normals

Blend between surface normal and custom normal, connected to the node’s Ncortex input, to randomize surface normal.

Medulla Blend Normals

Blend between surface normal and custom normal, connected to the node’s Nmedulla input, to randomize surface normal.


  • The Implementation of a Hair Scattering Model. Physically Based Rendering: From Theory to Implementation. Pharr, Matt

  • A Practical and Controllable Hair and Fur Model for Production Path Tracing. Chiang, Matt Jen-Yuan, Benedikt Bitterli, Chuck Tappan, Brent Burley.

  • Physically-AccurAate Fur Reflectance: Modeling, Measurement and Rendering. Ling-Qi Yan, Chi-Wei Tseng, Henrik Wann Jensen, Ravi Ramamoorthi.

  • An Efficient and Practical Near and Far Field Fur Reflectance Model. Ling-Qi Yan, Henrik Wann Jensen, Ravi Ramamoorthi.

  • Dual Scattering Approximation for Fast Multiple Scattering in Hair. Arno Zinke, Cem Yuksel, Andreas Weber, John Keyser.

VOP nodes