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m_overrideMarkdownText: '# Custom Hair, Cloth, and Skin Shaders
We developed
custom shaders with unique lighting models for scenarios where standard PBR diffuse/specular
was insufficient. These shaders were created in Shadergraph using a custom lighting
node, similar to a regular PBR master node. Initially, we used the Unlit Shadergraph
target, but it lacked some lighting, shadow, and lightmap shader variants. Therefore,
we created a CustomLit target with these keywords enabled.
## Hair Shader
The
hair shader is inspired by [Physically Based Hair Shading in Unreal](https://blog.selfshadow.com/publications/s2016-shading-course/karis/s2016_pbs_epic_hair.pdf).
This technique typically requires an extra texture to define hair strand rotation
for consistent highlight directions. To enhance performance, we oriented the
UVs so the strands run lengthwise from left to right, achieving similar results
at a lower cost.
We implemented the paper''s specular and environment lighting
equations but found the multi-scatter equation unsuitable for our needs. Instead,
we used a simple Lambert diffuse, which also improves performance.
The
shader is implemented in HairLighting.hlsl, with most hair-specific logic in
the "CalculateLighting" function. We simplified vector calculations like cosThetaI/cosThetaR/cosThetaD
using the normal, avoiding re-derivation from sinThetaI/sinThetaR. This approach
yields the same results with simpler dot products instead of sqrt/trigonometric
instructions.
The result improves upon the standard Lit specular equation.
The primary specular highlight stretches across hair strands, creating long bands
instead of focused circles. A secondary transmitted highlight (TT) allows light
to shine through when behind the hair surface. A shifted, tinted reflection (TRT)
replicates the metallic/sheen highlight effect often seen in hair.
The
hair shader can use regular albedo, normal, roughness, and occlusion maps. However,
we found a single color and smoothness value, combined with a packed Normal/AO
map, provided sufficient control. We also included a "micro shadow" technique
using normal and AO data to calculate small-scale direct light self-shadowing,
enhancing depth within the hair.
## Cloth Shader
For the cloth shader,
we replaced the specular term with a "Charlie Sheen" distribution, combined with
the Ashikmin visibility term described in [Cloth Shading | Krzysztof Narkowicz](https://knarkowicz.wordpress.com/2018/01/04/cloth-shading/).
This creates a more natural highlight on cloth, simulating individual fiber lighting
instead of a hard, flat surface. We also replaced the diffuse lighting with a
wrapped diffuse term for a softer falloff and a configurable subsurface color
to simulate scattering in shadowed sections, as detailed in section 4.12.2 of
[Physically Based Rendering in Filament](https://google.github.io/filament/Filament.md.html#materialsystem/clothmodel).
The
cloth shader uses regular albedo, roughness, and normal maps. However, roughness
is interpreted as a distribution of random fibers and their alignment to the
normal direction.
## Skin Shader
The character skin shader replaces
the diffuse term with a subsurface scattering approximation known as pre-integrated
skin shading. A resource on this topic is available here: [Simon''s Tech Blog:
Pre-Integrated Skin Shading](https://simonstechblog.blogspot.com/2015/02/pre-integrated-skin-shading.html).
We chose a lookup table over a function fit, requiring a "curvature map" baked
in Substance Painter to control the effect''s strength. The curvature and dot
product between light and normal index the lookup table to determine scattered
diffuse lighting.
The specular uses a regular GGX function, which produced
satisfactory results for our purposes.
We also added a secondary "deep
scattering" approximation. This involves sampling the shadow map and comparing
depths with the current pixel to estimate geometry thickness along the light
source direction. This is added to the diffuse term using a simple transmittance
formula. While effective in some situations, it is sensitive to shadowmap precision
and flickering. URP has a hardcoded shadow constant bias, so we added bias and
normal bias controls for shadowmap sampling to reduce artifacts.
'
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