Hair
Use the Hair material to render hair, fur and curves.
This material is composed of 3 specular components, the primary corresponding
to reflections on the surface of the fibers, the secondary corresponding to internal
reflections inside the fibers, and the back corresponding to transmission through the
fibers.
Additionally, reflections between fibers are simulated with a multiple scattering
approximation. Eventually, a simple non physically based diffuse component is provided.
Tweak the following sliders to see the aspect of each component:
Tweak the following sliders to see the aspect of each lighting contribution (note
that these are not parameters of the material but intensities of the front, back and
environment lights):
Components
Primary
Primary is the amount of primary specular reflected by the fiber.
Secondary
Secondary is the amount of secondary specular reflected by the fiber.
This specular component is reflected through the fiber, and is colored.
Back
Back is the amount of specular transmitted through the fiber.
This component is mostly visible in back lighting.
Multiple Scattering
Multiple Scattering simulates the scattering of light through the
volume of numerous fibers, instead of a single scattering.
Combine this with the previous components to create convincing diffuse and soft hair.
Multiple Scattering requires the Attributes -> Shading -> MS Approximation
attribute to be checked.
Diffuse
Diffuse is a simple diffuse approximation using Kajiya-Kay shader.
Use this to render non photo-realistic hair.
Color
You can specify the hair color in many ways :
- Using the Color attribute, a simple RGB color.
- Using the Melanin attributes to control the hair melanin density, to get plausible human hair colors.
- Using the Absorption attribute with mesured RGB absorption values.
Modulate the color from the tip to the root using the Tip/Root Color attributes.
Randomize the hair color per clump or per fibers using the Clump and Fiber Random attributes.
Add mutant hairs, like white hairs, using the Fiber Mutant attributes.
Color
Color controls the general appearance of the hair. Light
scattering and hue shifting may result in slightly different colors, so this
control is an approximation of the resulting color.
Root Color
Root Color is multiplied to the Color at the root
of the fiber. White leaves the color unaffected.
Tip Color
Tip Color is multiplied to the Color at the tip
of the fiber. White leaves the color unaffected.
Color Shape
Color Shape moves the blending area between Root Color
and Tip Color towards the fibers root or tip. Lower values result in
blending closer to the root, higher values result in blending closer to the tip.
Absorption
Specifies the fiber absorption properties, for red, green and blue. The corresponding
color is multiplied to the general Color. Lower absorption values result in
lighter hair, while higher absorption values result in darker hair.
To render the hair with a specific absorption, set Color -> Color
to pure white.
Melanin Density
Specifies the density of melanin which contributes to the hair resulting color.
This parameter along with Melanin Mix are based on [d'Eon11], and
provides an easy and consistent control of human hair colors. Lower density
values result in lighter hair, while higher density values result in darker hair.
Similarly to Absorption, the corresponding color is multiplied to the
general Color.
To render the hair with a specific melanin density, set
Color -> Color to pure white.
Melanin Mix
Specifies the ratio between eumelanin and pheomelanin in the fibers. Lower values
result in darker and brown hair color, higher values result in lighter and red hair
color.
Clump Hue Random
Clump -> Hue Random controls the amplitude of the hue noise.
Clump Value Random
Clump -> Value Random controls the amplitude of the value noise.
Clump Random Scale
Clump -> Random Scale controls the noise frequency for clumps hue
and value randoms. Lower values result in smooth varations, higher values result in
rapid variations.
The clump random is based on the scalp UV texture coordinates, thus the actual
noise frequency scaling is dependent of the UV layout.
Fiber Hue Random
Fiber -> Hue Random controls the amplitude of
the hue noise per fiber.
Fiber Value Random
Fiber -> Value Random controls the amplitude of
the value noise per fiber.
Fiber Mutant Color
Fiber -> Mutant Color controls the color of mutant fibers.
Use this to create random white hair fibers, for instance.
Fiber Mutant Ratio
Fiber -> Mutant Ratio controls the ratio of mutant fibers.
Fiber -> Mutant Ratio = 0 results in no mutated fibers,
Fiber -> Mutant Ratio = 0.5 results in half mutated fibers, and
Fiber -> Mutant Ratio = 1 results in all fibers mutated.
Specular
These parameters control the appearance of the specular components.
Width
Specular -> Width controls the width of the specular cones.
Small values result in sharper speculars, while higher values result in
wider and smoother speculars.
Shift
Specular -> Shift controls the shifting of the specular cones.
0 leaves the specular components unshifted, positive values shift the primary
towards the root and secondary and back towards the tip.
Since Multiple Scattering is based on the specular components, Specular -> Width
and Specular -> Shift also affect the appearance of Multiple Scattering.
Self Shadow
Self shadowing attenuates the hair color at the root of the fibers.
Self Shadow does not affect the primary specular.
Amount
Self Shadow -> Amount controls the amount of attenuation. 0
doesn't attenuate the color, 1 results in black color at the root.
Position
Self Shadow -> Position controls the end of the attenuation along
the length of the fibers. 0 doesn't attenuate the color, 1 attenuates until the tip
of the fiber.
Opacity
Opacity controls the opacity of fibers.
To faster render hair with opacity, it is recommended to activate opacity
baking using the Attributes -> Surface -> Opacity Mode attribute.
Use either Vertex or V modes for curves.
The User Guide/Shading/Opacity section for more information
on improving performances with opacity.
Opacity
Opacity -> Opacity controls the opacity of hair in primary
visibility. This setting does not affect the hair opacity in shadows, so
decreasing this value and keeping the Opacity -> Shadow Opacity to
1 results in darker hair.
Tip Fade
Opacity -> Tip Fade controls the starting point of opacity
fading along the length of the fibers. 1 doesn't fade opacity at all, 0.5
fades the opacity from the middle of the fibers, and 0 fades from the root.
When using tip fading, the fibers are fully transparent at the tip.
Shadow Opacity
Opacity -> Shadow Opacity controls the opacity of hair in shadows.
This setting does not affect the hair opacity in primary visibility. Decreasing this
value result in brighter hair.
Advanced
Primary -> Color
Multiplies the final color of the primary specular.
Primary -> Width
Multiplies the width of the primary specular.
Primary -> Shift
Multiplies the shift of the primary specular.
Secondary -> Color
Multiplies the final color of the secondary specular.
Secondary -> Width
Multiplies the width of the secondary specular.
Secondary -> Shift
Multiplies the shift of the secondary specular.
Glints -> Color
Multiplies the final color of the glints specular.
Glints -> Width
Controls the azimutal width of the secondary glints.
Glints -> Half Angle
Controls the azimutal shift of the secondary glints.
Back -> Color
Multiplies the final color of the back specular.
Back -> Width
Multiplies the width of the back specular.
Back -> Shift
Multiplies the shift of the back specular.
Back -> Azimutal Width
Controls the azimutal width of the back specular.
Multiple Scatter -> Back Direct
Multiplies the Back Direct component of the multiple scattering component.
Multiple Scatter -> Single Scatter
Multiplies the Single Scatter component of the multiple scattering component.
Multiple Scatter -> Back Scatter
Multiplies the Back Scatter component of the multiple scattering component.
Multiple Scatter -> Density
Controls the density of fibers for multiple scattering. 0 results in no multiple
scattering (no fiber to scatter light with), 1 results in maximum multiple scattering.
Human hair typically have 0.7 density.
Diffuse -> Color
Multiplies the Diffuse component color. Note that Diffuse uses the general color
settings.
Diffuse -> Power
Controls the exponent of the Kajiya-Kay diffuse component. 0 results in omni
directional diffuse, higher values result in sharp specular-like component.
Ray Depth
Controls the number of bounces the Hair material allows. -1 leaves this value
unchanged from the Diffuse Bounces setting of the RenderPass.
Ray Length
Controls the maximum ray length for bounces. Shorter values result in less
indirect illumination and shorter rendering time.
Direct Sampling
Multiplies the number of rays shot towards the lights for direct lighting.
Indirect Sampling
Multiplies the number of rays shot towards surrounding objects for
indirect lighting.
Trace Set
Specify the raytracing set for direct and indirect lighting.
References
[Marschner03] "Light Scattering from Human Hair Fibers", Stephen R. Marschner, Henrik Wann Jensen, Mike Cammarano, Steve Worley, Pat Hanrahan
[Zinke08] "Dual Scattering Approximation for Fast Multiple Scattering in Hair", Arno Zinke, Cem Yuksel, Andreas Weber, John Keyser
[d'Eon11] "An Energy-Conserving Hair Reflectance Model", Eugene d’Eon, Guillaume Francois, Martin Hill, Joe Letteri, Jean-Marie Aubry
The hair model is provided by Cem Yuksel, and can be downloaded at http://www.cemyuksel.com/research/hairmodels/.