This page provides details on the settings available for the Hair Material in V-Ray.
VRayHairMtl is a material that is primarily designed for rendering hair and fur. It can be used to render geometry generated through Maya's own nHair system, the VRayFur primitive, or other third party hair and fur plugins.
VRayHairMtl is frequently used with the VRayHairSampler node to randomize hair color per strand as well as set the color along each strand of hair for the most options in designing the look of hair. Similarly, this material is frequently used with the VRayFurSampler node to customize the coloring of fur.
Image courtesy of Emilie Stabell & Brandon T. Cebenka
||Hypershade|| > Create panel > VRay section > VRay Mtl Hair 3
||Create Render Node window|| > VRay section > VRay Mtl Hair 3
||V-Ray Shelf|| > Right-click Create V-Ray Materials button > VRay Mtl Hair 3
Components of VRayHairMtl
The material is based around three components - primary specular component, secondary specular component, and a transmission component. A diffuse component is also provided for rendering of materials made up of cloth threads or other non-translucent fibers.
The primary specular component represents light that is reflected off the outer surface of a hair strand. The secondary specular component represents light that goes through the hair strand and is reflected off the back surface. The transmission component represents light that goes through the hair strand.
The image below illustrates the meaning of the three components:
Preset – Allows you to choose one of several available preset materials. For more information, see the VRayHairMtl Presets example below.
Overall Color – A common color multiplier for all color components in the material (except for opacity). Used to easily change the look of the entire hair material.
Transparency – Controls the transparency of the material. The transparency can be mapped along the hair strands using the VRayFurSampler texture. Making the hair more transparent towards the tip may produce more smooth and realistic anti-aliasing at the expense of increased render times. If you map the tip opacity, make sure you don't make the strands thinner at the tip as well - transparency already creates the effect of thinning strands. You can enable the Opaque for Shadows and Opaque for GI options to reduce render times for transparent hair. Note that with transparent strands, it may be necessary to increase the Max. Transp. Levels option in the global V-Ray settings too.
Diffuse Color – Controls the diffuse component of the shader. Use this for materials made out of cloth threads or other non-translucent fibers, as well as for dirty hair. Note that clean hair or fur does not normally have a diffuse component, so in that case leave the parameter black.
Diffuse Amount – The amount for the diffuse component of the material.
The primary specular component corresponds to light that is reflected off the outer surface of hair strands (see the figure above).
Primary Specular – The primary specular color component. Normally this is dark gray.
Primary Specular Amount – A multiplier for the primary specular color.
Primary Glossiness – The glossiness for the primary specular component. Values closer to 1.0 make the hair more shiny and sleek. Lower values give it a matted look.
The secondary specular component corresponds to light that is reflected off the back surface of the hair strands.
Lock to Secondary – When enabled (the default), the color for the secondary specular component is derived from the color of the transmission component. Since a ray of light goes twice through the hair width, the color of the secondary specular component can be computed by multiplying the transmission color with itself. When this option is enabled, the hair color is mostly determined by the transmission color component.
Secondary Specular – The color of the secondary specular component. If Lock to Secondary is enabled, this value is ignored and the secondary specular color is derived from the transmission color.
Secondary Specular Amount – A multiplier for the secondary specular component. If Lock to Secondary is enabled, this value is ignored and the secondary specular amount is derived from the transmission amount.
Secondary Glossiness – The glossiness for the secondary specular component. Values closer to 1.0 correspond to shiny and sleek hair. Lower values correspond to matted hair.
The transmission component corresponds to light that goes through the hair strands.
Transmission – The color for the transmission component. When Lock to Transmission is enabled, this color determines the overall hair color.
Transmission Amount – The amount of the transmission component.
Transm. Glossiness Length – The glossiness of the transmission along the hair strand length. See the Examples for a demonstration of the effect of this parameter.
Transm. Glossiness Width – The glossiness for the transmission component across the hair strand width. For more information, see the Transmission Glossiness Length and Width Parameters example below.
Example: VRayHairMtl Presets
Here are some examples of the same hair geometry rendered with different presets. The scene is illuminated with one spherical V-Ray light, with GI set to brute force for the primary GI bounces, and light cache for the secondary, with retrace enabled.
Example: Transmission Glossiness Length and Width Parameters
This example shows the effect of the Transmission Glossiness Length and the Transmission Glossiness Width parameters. It shows a number of vertical strands (in this case, produced by VRayFur) lit from behind by a spherical light.
Note how each parameter changes the way light scatters along the length and the width of the strands. Higher values for the length glossiness compress the transmission highlight along the strand length, while lower values expand it.
Opaque for Shadows – When enabled, the hair material is always opaque for shadow calculations. This speeds up the rendering of transparent hair.
Opaque for GI – When enabled, the hair material is always opaque for GI calculations. This speeds up the rendering of transparent hair. For more information, see the Strand Taper vs Transparency Mapping example below.
Simplify for GI – When enabled, a simplified diffuse version of the BRDF is used for GI calculations. This may speed up the rendering of hair but may significantly change the final look.
Use Cached GI – Similar to the Use Irradiance Map option for the VRayMtl material; if it is disabled, the hair material will always be calculated with brute force GI. For more information, see the Effect of GI on Hair example below.
Transmission in Specular – When enabled, V-Ray calculates the transmission as part of the Specular Render element instead of Refraction one.
Example: Effect of GI on Hair
This example shows how important GI (multiple scattering) is for the appearance of hair, especially bright hair. The top row shows several of the presets rendered without GI, and the bottom shows the same scene when GI is enabled. Again, GI is brute force and light cache has retrace. The scene GI environment is black so that a spherical area light is the only light source.
GI is Disabled
GI is Enabled
Example: Strand Taper vs Transparency Mapping
This example shows how transparency mapping can be used to simulate hair thinning towards the tips with improved quality. Since transparency also increases render times, the VRayHairMtl material provides two options to optimize this - the Opaque for Shadows and Opaque for GI options allow to skip the transparency calculations for shadow and GI rays.
For the first image, the thinning of the strands towards the tips was done by setting the Taper parameter of VRayFur to 1.0. The opacity of the hairs for the 2nd, 3rd and 4th image is done with the VRayHairSampler texture in the Opacity slot of the VRayHairMtl material, while keeping the Taper parameter in VRayFur to 0.0. The scene is illuminated with one point light with sharp raytraced shadows to better show the difference between the options. Note how opacity mapping improves the anti-aliasing of the hair, while still preserving the same overall look, at the expense of somewhat increased render times. However, the first image also required increased AA samples compared to the rest.
Note that using transparency for the strands may require increasing of the Max. Transp. Levels option in the global V-Ray settings, although it was not necessary for this particular case.
Hair geometry with taper.
Render time: 2m 28s
Hair geometry without taper, instead using opacity mapping.
Render time: 2m 19s
Hair geometry without taper, with opacity mapping, Opaque for GI enabled.
Render time: 2m 14s
Hair geometry without taper, with opacity mapping,
both Opaque for Shadows and Opaque for GI enabled.
Render time: 2m 31s
Light Multiplier – In the absence of GI, the hair may look very dark. GI is the proper way to correct this, but if GI must be really avoided, this multiplier can be used to brighten the hair.