Hair Shader (Pre-3.6)

Hair Shader

The hair shader is a dedicated rendering technique for rendering hair and fur. Hair rendering is a relatively hard task to achieve in real-time with high quality results due to the very fine geometry and specific lighting behavior.

The most recent implementation of hair shading strongly implies the use of alpha test in conjunction with soft alpha test enabled so the use of opacity is strongly discouraged since its visual result is prone to issues especially when sorting multiple polygons using opacity.

Shader Params

Shader Params	Description	Shader Gen Option
Alpha test softening	Sets amount of alpha test softening to apply.	Soft alpha test
Ambient spec multiplier	Controls over specular intensity.	Default
Bump scale	Sets hair bumpiness.	Default
Detail tilling	Sets detail tilling scale.	Default
Diffuse wrap	Sets diffuse wrapping.	Default
Hair color	Sets hair color.	Default
Indirect bounce color	See Illum Shader for more information.	Default
Primary spec color	Sets primary spec color.	Default
Primary spec glossiness	Sets primary specular glossiness.	Default
Primary spec shift	Sets primary specular shift.	Default
Rim multiplier	Sets rim strength.	Default
Secondary spec color	Sets hair secondary spec color.	Default
Secondary spec glossiness	Sets secondary specular glossiness.	Default
Secondary spec shift	Sets secondary specular specular shift.	Default
Self-shadow softening	Sets geometry self-shadowing multiplier.	Default
Wind frequency	Sets the speed at which the verts are deformed.	Wind bending
Wind phase	Sets hair animation phase and randomizes the deformation.	Wind bending
Wind wave0 amp	Sets the amount or amplitude at which the verts are deformed.	Wind bending
Wind wave2 amp	Sets the amount or amplitude at which the verts are deformed on a different curve.	Wind bending

Shader Generation Params

Param	Description
Vertex Colors	See Illum Shader for more information.
Soft alpha test	Enable for alpha test to have soft edges and for more stable rendering.
Wind bending	Simulate wind effects.

Guidelines for Modeling Hair

Due to limitations in hardware regarding alpha blending sorting, we cannot at this point have per-pixel sorting which would save a lot of work, so we need to use a very cheap sorting approximation which works something like this:

Render hair opaque areas (this is where your alpha channel is 1 (white)). Then render hair backfaces, and finally render hair frontfaces.

This works for many cases but it has a lot of limitations, regarding modeling for it to work:

Start by making an initial hair cap (imagine a hair helmet), this will give missing shading (anisotropic, etc.) details from base head while making sure each hair patch should be modeled as single sided.
Use vertical UV patches so that they follow the hair strands vertically. This adheres to the anisotropic shape.
Then add all of the little details by adding hair patches.

When "Soft alpha test" is enabled, the hair will be rendered using the usual opaque rendering pipeline. In a post process we apply alpha test smoothing. You can also enable double sided option at will.

Hair Texturing Guidelines

Regarding hair texture, the trick for having very thin hair is to make texture as pixel perfect and with as little distortions/rotations (else filtering will kick in more and look blurrier). One way of achieving this is in Photoshop making a uniform noise, then using strong Motion Blur.
Then add little details to hair texture, colored noisy, alpha blended areas, etc.
Important note regarding alpha blended areas, we are trying to avoid them as much as possible, so try to make alpha blended areas only at the bottom tip of the hair.
For better "thinner" results enable Anisotropic texture filtering in diffuse texture filter options.

Diffuse texture	Diffuse texture alpha channel