Cheap Realistic Skin Shading

Overview• Popular Skin Models• New Skin Model

– Ideas– BRDF– Layers– Back Scattering– Blended Normals– Shadows– Extras

• Results• Conclusion

Popular Skin Models

Popular Skin Models• Red wrapped lighting

– http://http.developer.nvidia.com/GPUGems/gpugems_ch16.html

• Texture-space diffusion– http://http.developer.nvidia.com/GPUGems3/gpugems3_ch14.html

• Texture-space diffusion (12-tap)– http://advances.realtimerendering.com/s2010/Hable-Uncharted2(SIGGRAPH 2010 Advanced RealTime Rendering Co

urse).pptx

• Screen-space diffusion– http://giga.cps.unizar.es/~diegog/ficheros/pdf_papers/TAP_Jimenez_LR.pdf

• Blended Normals– http://advances.realtimerendering.com/s2010/Hable-Uncharted2(SIGGRAPH 2010 Advanced RealTime Rendering Co

urse).pptx

• Offline “Fast-Skin Shaders”– http://www.google.ca/#hl=en&source=hp&biw=1920&bih=965&q=fast+skin+shader&aq=f&aqi=g-m5&aql=&oq=&gs

_rfai=&fp=bc38547320fe36d4

Popular Skin Models (cont.)

• The diffusion approximation techniques are the most popular when it comes to high-fidelity realism.

• Each model is in the extremes.– Either extremely cheap/poor approximation or really

good/expensive.• Wrapped lighting can work in practice but fails in sharp lighting.

• Need a good alternative that’s somewhere in the middle.– Has to be cheap but still look up-to-par.

New Skin Model

New Skin Model

New Skin Model - Ideas• Use concepts from other models.

– Use Kelemen/Szirmay-Kalos BRDF.• Looks great.• Can be relatively cheap with proper optimizations.

– Simulate “Fast-Skin Shaders” Multiple layers.• Each layer has it’s own texture (epidermal, subdermal).• Epidermal layer is blurred lightly.• Subdermal layer is blurred a lot.• Diffuse is not blurred at all.• Sum the layers at the end.

– Soften normal map using blended normals.• Gives soft look without washing out lighting.

New Skin Model - BRDF

New Skin Model – BRDF (Code)Don’t use these textures, compute them yourself for better precision.// Computes beckmann distribution

// To bake to texture: texCoord.x = NdotH, texCoord.y = Expfloat4 GetBeckmannDistribution( float NdotH, float Exp ) { // Some roughness weights float4 m = half4(1, 0.12, 0.023, 0.012) * (Exp * Exp); float alpha = acos( NdotH ); float ta = tan( alpha ); float4 val = 1.0 / (m * pow(NdotH, 4.0)) * exp(-(ta * ta) / m);

// Scale the value to fit within [0-1] return 0.5 * pow( val , 0.1 ); }

// Computes fresnel reflectance (can be computed on the fly no problem)// To bake to texture: HdotV = texCoord.x, texCoord.y = F0float GetFresnelKS( float3 HdotV, float F0 ) {

float base = 1.0 - HdotV; float exponential = pow( base, 5.0 );return exponential + F0 * ( 1.0 - exponential );

}

Code modified from NVIDIA’s implementation.

New Skin Model – BRDF (Code)float KelemenSzirmayTex( float3 N, float3 L, float3 V, float Exp, float F0 ){

// Pretty straightforwardfloat NdotL = saturate(dot(N, L));float h = L + V;float H = normalize(h);float HdotV = dot(H, V);

// Get fresnel from texture; 0.028 is a good value for F0float fFresnel = tex2D(fresnelTex, float2(HdotV, F0));// float fFresnel = GetFresnelKS(HdotV, F0 ); // Math version.

// Get beckmann distributions from texturefloat4 fBeckmann = pow(2.0 * tex2D(beckmannSampler, float2(NdotH, Exp)), 10); float4 fSpec = max( (fBeckmann * fFresnel) / dot( h, h ), 0 );

// Weight results using dot productfloat result = saturate( NdotL ) * dot(fSpec, half4(1.0, 0.625, 0.075, 0.005));

return result;}

New Skin Model – BRDF (Result)

(Image intensified for clarity)

New Skin Model - Layers

New Skin Model – Layers (Textures)

Diffuse Epidermal Subdermal

Back Scattering Specular Normal

New Skin Model – Layers (Image)

New Skin Model – Back Scattering

New Skin Model – Back Scattering (Code)

float3 BackLighting(float3 lightColor, float NdotL, float shadowMap, float transTex){ // Calculate back scattering. float backLight = lerp(NdotL, 1.0, transTex) - lerp(NdotL, 1.0, 0.4);

float3 result = saturate(backLight) * lightColor * shadowMap * backScatterStrength * backScatterColor; return result;}

New Skin Model – Back Scattering (Image)

New Skin Model – Blended Normals• Use blended normals to soften bump-mapping.

– Calculate N·L for vertex normals and bumped normals.– Blend between them with different strengths for different color

channels.• Use “lerp(0.0, max, intensity)” for intensity of each channel

– Prevents perfectly smooth normals (we don’t want those).– Good values for max:

» Red = 0.5 – 0.7» Green/Blue = 0.15 – 0.4

– Intensity is contstant for all» 0-1

– Use new value for N·L for diffuse lighting.

New Skin Model – Blended Normals (Code)float3 BlendNormals(float lightDiffusion, float vertexNdotL, float bumpNdotL, float3

lightPos){

// Tweak max values as you see fit.float redIntensity = lerp(0.0f, 0.6f, skinDiffusionAmount);

float greenBlueIntensity = lerp(0.0f, 0.4f, skinDiffusionAmount);

float red = lerp(vertexNdotL, bumpNdotL, redIntensity); float greenBlue = lerp(vertexNdotL, bumpNdotL, greenBlueIntensity);

greenBlue = min(red, greenBlue); // remove unwanted green/blue

// Put it all together.float3 result = float3(red, greenBlue.xx);return saturate(result);

}

New Skin Model – Blended Normals (Image)

New Skin Model – Shadows

New Skin Model - Shadows (cont.)

New Skin Model - Shadows (cont.)

New Skin Model – Shadows (Code)float3 BlendShadows(float2 shadowPow, float shadowMap){

// Calculate 2 different power factors. float shadowR = pow(shadowMap, shadowPow.x); float shadowGB = pow(shadowMap, shadowPow.y);

// Blend shadows float red = lerp(shadowGB, shadowR, skinDiffusionAmount); float greenBlue = lerp(shadowGB, shadowR, skinDiffusionAmount * 0.5); float3 result = float3( red, greenBlue.xx );

// Result may be a bit too red, desaturate it a bit. result = lerp(result, dot(result, float3(0.33, 0.59, 0.11)), 0.75);

return saturate(result);}

New Skin Model – Shadows (Image)

Pure diffuse layer Pure epidermal/subdermal layers

New Skin Model - Extras

New Skin Model – Notes

• Shadow sharpening doesn’t need to be done for every layer.– Can simplify and apply single blended shadows

beforehand.– Can still provide good bleeding.

• Can do blended normals more than once.– Create variety between bump strengths for each layer.

• Not limited to constant color for backscattering.– Subdermal texture.– Translucency ramp.

Results

Results

Results (cont.)

Standard NdotL + Blinn-phong (physical model) Skin Shading, no SSS Skin Shading, full SSS

Conclusion• Use Kelemen/Szirmay-Kalos BRDF.

– Bake beckmann distribution and fresnel into textures.– Use 4 specular terms instead of 1.

• Approximate subsurface scattering with lightly wrapped texture layers.– Epidermal, subdermal.– Keep wrapping at a minimum to avoid washing out the lighting.

• Use blended normals to soften normal maps.• Sharpen shadows for layers using pow() to create bleeding shadows.

– Faster than blurring.

• Use simple masked N·L calculations for backscattering.– Really cheap and easy to do.

• Can add rim lighting or melanin for extra effect.• Might prove more effective if mixed with more methods (diffusion

maybe?)

Conclusion (cont.)

Thanks for viewing! • References:• Screen-Space Perceptual Rendering of Human Skin, Jorge Jimenez, Veronica Sundstedt,

Diego Gutierrez, 2009– http://giga.cps.unizar.es/~diegog/ficheros/pdf_papers/TAP_Jimenez_LR.pdf

• Efficient Rendering of Human Skin, Eugene d'Eon, David Luebke, and Eric Enderton, Eurographics 2007

– http://http.developer.nvidia.com/GPUGems3/gpugems3_ch14.html

• Real-Time Approximations to Subsurface Scattering, Simon Green, 2004– http://http.developer.nvidia.com/GPUGems/gpugems_ch16.html

• Uncharted 2: Character Lighting and Shading, John Hable, 2010• http://advances.realtimerendering.com/s2010/Hable-Uncharted2(SIGGRAPH%202010%20Advanced

%20RealTime%20Rendering%20Course).pptx

• Crafting Physically Motivated Shading Models for Game Development, Naty Hoffman, 2010– http://renderwonk.com/publications/s2010-shading-course/hoffman/

s2010_physically_based_shading_hoffman_b.pdf

• Real-Time Realistic Skin Translucency, Jorge Jimenez, David Whelan, Veronica Sundstedt, Diego Gutierrez, 2010

• http://giga.cps.unizar.es/~diegog/projects/IEEE/ieee.html

FinHead model available at Infinite-3D