Cheap Realistic Skin Shading Stephen Clement Overview Popular

Cheap Realistic Skin Shading Stephen Clement

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_ch 16. html • Texture-space diffusion – http: //http. developer. nvidia. com/GPUGems 3/gpugems 3_ch 14. html • Texture-space diffusion (12 -tap) – http: //advances. realtimerendering. com/s 2010/Hable-Uncharted 2(SIGGRAPH 2010 Advanced Real. Time Rendering Course). pptx • Screen-space diffusion – http: //giga. cps. unizar. es/~diegog/ficheros/pdf_papers/TAP_Jimenez_LR. pdf • Blended Normals – http: //advances. realtimerendering. com/s 2010/Hable-Uncharted 2(SIGGRAPH 2010 Advanced Real. Time Rendering Course). pptx • Offline “Fast-Skin Shaders” – http: //www. google. ca/#hl=en&source=hp&biw=1920&bih=965&q=fast+skin+shader&aq=f&aqi=gm 5&aql=&oq=&gs_rfai=&fp=bc 38547320 fe 36 d 4

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 • Trick is to improvise! • Perceptual rendering, not physical. – Doesn’t have to be correct, just has to look good. • Use core concepts of different techniques to approximate. – Simulate multiple layers. – Use a good (physically based) BRDF. – Approximate common visual properties. • Red bleeding on shadow edges. • Soft appearance. • Harsh falloff.

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 • Kelemen/Szirmay-Kalos BRDF Optimizations. – Save beckmann distribution to texture. – Saves a crap-load of instructions. – Save fresnel to texture. – Math version may be cheaper depending on the system. • Why use just 1 specular term? – You can have 4 channels in your beckmann texture. Why not use them? – Calculate the shader as usual but use a 4 -value float instead. – Weigh them at the end using a simple dot product. – Additional cost isn’t very high and you get good varied specularities. • Use energy conservation if you think it’ll help.

New Skin Model – BRDF (Code) Code modified from NVIDIA’s implementation. // Computes beckmann distribution // To bake to texture: tex. Coord. x = Ndot. H, tex. Coord. y = Exp float 4 Get. Beckmann. Distribution( float Ndot. H, float Exp ) { // Some roughness weights float 4 m = half 4(1, 0. 12, 0. 023, 0. 012) * (Exp * Exp); float alpha = acos( Ndot. H ); float ta = tan( alpha ); float 4 val = 1. 0 / (m * pow(Ndot. H, 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: Hdot. V = tex. Coord. x, tex. Coord. y = F 0 float Get. Fresnel. KS( float 3 Hdot. V, float F 0 ) { float base = 1. 0 - Hdot. V; float exponential = pow( base, 5. 0 ); return exponential + F 0 * ( 1. 0 - exponential ); } Don’t use these textures, compute them yourself for better precision.

New Skin Model – BRDF (Code) float Kelemen. Szirmay. Tex( float 3 N, float 3 L, float 3 V, float Exp, float F 0 ) { // Pretty straightforward float Ndot. L = saturate(dot(N, L)); float h = L + V; float H = normalize(h); float Hdot. V = dot(H, V); // Get fresnel from texture; 0. 028 is a good value for F 0 float f. Fresnel = tex 2 D(fresnel. Tex, float 2(Hdot. V, F 0)); // float f. Fresnel = Get. Fresnel. KS(Hdot. V, F 0 ); // Math version. // Get beckmann distributions from texture float 4 f. Beckmann = pow(2. 0 * tex 2 D(beckmann. Sampler, float 2(Ndot. H, Exp)), 10); float 4 f. Spec = max( (f. Beckmann * f. Fresnel) / dot( h, h ), 0 ); // Weight results using dot product float result = saturate( Ndot. L ) * dot(f. Spec, half 4(1. 0, 0. 625, 0. 075, 0. 005)); } return result;

New Skin Model – BRDF (Result) (Image intensified for clarity)

New Skin Model - Layers • Simulating multiple layers. – Create new textures for each layer. • Can be simulated using diffuse map with simple color operations in real-time. – Use mip-map for subdermal map (should be blurrier than others). – May actually be cheaper than using normal textures, depending on the resolution. – Light each layer. • • • Instead of blurring, just use lightly wrapped lighting for each layer. No wrapping for diffuse layer. ~0. 8 – 0. 9 for epidermal. ~0. 7 – 0. 8 for subdermal. Don’t wrap too much! – You’ll get ugly ambient like the standard wrap method. – Apply lighting to each texture and weigh accordingly. • Make sure the result equals 1! – Good values: Epidermal = 0. 3, Subdermal = 0. 2, Diffuse = 0. 5

New Skin Model – Layers (Textures) Diffuse Epidermal Subdermal Back Scattering Specular Normal

New Skin Model – Layers (Image)

New Skin Model – Back Scattering • Extremely simple. – Just a few calculations with N·L. • Mask by translucency texture. – Store in subdermal map alpha channel. – Alternatively, use vertex colors/alpha for mask. • Use non-sharpened shadows. – Soft shadows let the backscattering through more. • Other techniques can be used instead. – Translucent shadow maps. • Issues: – Might not properly be occluded in a shadowed area. • Technique still needs work (incomplete).

New Skin Model – Back Scattering (Code) float 3 Back. Lighting(float 3 light. Color, float Ndot. L, float shadow. Map, float trans. Tex) { // Calculate back scattering. float back. Light = lerp(Ndot. L, 1. 0, trans. Tex) - lerp(Ndot. L, 1. 0, 0. 4); float 3 result = saturate(back. Light) * light. Color * shadow. Map * back. Scatter. Strength * back. Scatter. Color; 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) float 3 Blend. Normals(float light. Diffusion, float vertex. Ndot. L, float bump. Ndot. L, float 3 light. Pos) { // Tweak max values as you see fit. float red. Intensity = lerp(0. 0 f, 0. 6 f, skin. Diffusion. Amount); float green. Blue. Intensity = lerp(0. 0 f, 0. 4 f, skin. Diffusion. Amount); float red = lerp(vertex. Ndot. L, bump. Ndot. L, red. Intensity); float green. Blue = lerp(vertex. Ndot. L, bump. Ndot. L, green. Blue. Intensity); green. Blue = min(red, green. Blue); // remove unwanted green/blue // Put it all together. float 3 result = float 3(red, green. Blue. xx); return saturate(result); }

New Skin Model – Blended Normals (Image)

New Skin Model – Shadows • Need to compensate for not including shadows in diffusion process. • 2 Options: – Blur shadow map for each layer. • Blurring shadow maps is expensive. • Gets worse if done per-object. – Cheat. Instead of blurring the shadow maps for each layer, sharpen them.

New Skin Model - Shadows (cont. ) • Sharpening shadows. – – – Use pow() function to sharpen shadows. Subdermal shadow has no pow() applied. Epidermal has small pow() applied (~2 -4). Diffuse has huge pow() applied (~8 -16). Assumes soft shadows. • Should be relatively soft/jittered/blurred. – Don’t forget to compensate for the sharper shadow when applying shadows to specular! • Otherwise you’ll get specularity bleeding into shadowed areas. • Mileage may vary (depends on shadow map, scale of object, etc).

New Skin Model - Shadows (cont. ) • Blended Shadows. – Simply varying the sharpness of the shadows for each layer might not give enough bleeding color (artistic preference). – Take idea from blended normals . • Calculate 2 different powers for the shadow map. • Blend between them for red and green/blue channels. – This gives a nice red edge. – Desaturate if the edge is too red. • • • Make difference higher for subdermal layer (1 – 2). Make difference lower for epidermal layer (4 – 8). Make them both exactly the same for diffuse ( 16 – 16). • Can be ignored if results were already good.

New Skin Model – Shadows (Code) float 3 Blend. Shadows(float 2 shadow. Pow, float shadow. Map) { // Calculate 2 different power factors. float shadow. R = pow(shadow. Map, shadow. Pow. x); float shadow. GB = pow(shadow. Map, shadow. Pow. y); // Blend shadows float red = lerp(shadow. GB, shadow. R, skin. Diffusion. Amount); float green. Blue = lerp(shadow. GB, shadow. R, skin. Diffusion. Amount * 0. 5); float 3 result = float 3( red, green. Blue. xx ); } // Result may be a bit too red, desaturate it a bit. result = lerp(result, dot(result, float 3(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 • Rim Lighting – Calculate fresnel (N·V) – Calculate rim term • Rim = smoothstep(0. 0, 0. 5, fresnel) * rim. Strength; – Add to specular during lighting pass. • spec += rim * light. Color * pow(N·H, rim. Power) * N·L * shadow. Map; • Melanin – Calculate luminance of diffuse texture. • lum = dot(diffuse, float 3(0. 33, 0. 59, 0. 11)); – Blend between 1 and diffuse*luminance. – Multiply new result with original diffuse • diffuse * lerp(1. 0, diffuse*lum, melanin); • “Oily” specular. – Second independent specular term to give a bit of “oily” shine to the surface. • Artists will probably thank you.

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 Ndot. L + 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. ) • The Good: • New Skin Model is cheap and looks good. • Simulates various scattering effects (bleeding shadows, back lighting). • Scales well with multiple characters. • The Bad: • Not as accurate as diffusion methods. • Still early in it’s development (experimental). • Wrapped lighting can start to show with very bright lights. • However, lights that bright shouldn’t occur in the first place. • Good shadow maps should help hide this.

Thanks for viewing! • References: • Screen-Space Perceptual Rendering of Human Skin, Jorge Jimenez, Veronica Sundstedt, Diego Gutierrez, 2009 – • Efficient Rendering of Human Skin, Eugene d'Eon, David Luebke, and Eric Enderton, Eurographics 2007 – • http: //advances. realtimerendering. com/s 2010/Hable. Uncharted 2(SIGGRAPH%202010%20 Advanced%20 Real. Time%20 Rendering%20 Course). pptx Crafting Physically Motivated Shading Models for Game Development, Naty Hoffman, 2010 – • http: //http. developer. nvidia. com/GPUGems/gpugems_ch 16. html Uncharted 2: Character Lighting and Shading, John Hable, 2010 • • http: //http. developer. nvidia. com/GPUGems 3/gpugems 3_ch 14. html Real-Time Approximations to Subsurface Scattering, Simon Green, 2004 – • http: //giga. cps. unizar. es/~diegog/ficheros/pdf_papers/TAP_Jimenez_LR. pdf http: //renderwonk. com/publications/s 2010 -shadingcourse/hoffman/s 2010_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

Fin Head model available at Infinite-3 D