PBR Shading Models

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What is PBR?

• Respond realistically to different lighting conditions
• Are portable across different engines and tools
• Produce consistent results with measurable parameters
• Conserve energy (don’t reflect more light than received)

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Standard Lit Shading Model

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Dielectrics vs. Conductors

• Have chromatic diffuse reflection
• Have achromatic specular reflection (white/gray)
• Examples: plastic, wood, stone, fabric

• Have no diffuse reflection
• Have chromatic specular reflection (colored)
• Examples: iron, gold, copper, aluminum

// Plastic (dielectric)
material.baseColor = vec3(1.0, 0.0, 0.0); // Red diffuse
material.metallic = 0.0; // White specular highlights

// Gold (conductor)
material.baseColor = vec3(1.0, 0.85, 0.57); // Gold specular
material.metallic = 1.0; // No diffuse

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Energy Conservation

reflected_light = diffuse_component + specular_component <= incident_light

• Diffuse contribution decreases
• Specular contribution increases
• Total energy remains conserved

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The BRDF Components

• Uses Lambertian diffuse with Disney diffuse for improved edge behavior
• Controlled by baseColor (for dielectrics)
• Modulated by (1 - metallic)

• Uses GGX (Trowbridge-Reitz) microfacet distribution
• Controlled by roughness
• Fresnel term uses Schlick approximation

BRDF = (1 - metallic) * diffuse + specular

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Roughness and Specular Highlights

• Micro-facets aligned with surface
• Sharp, mirror-like reflections
• Small, bright specular highlights

• Randomly oriented micro-facets
• Blurred, diffuse reflections
• Large, dim specular highlights

// Mirror-like
material.roughness = 0.0;

// Matte
material.roughness = 1.0;

// Typical values
material.roughness = 0.3; // Glossy plastic
material.roughness = 0.6; // Semi-rough metal

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Perceptual Roughness

perceptual_roughness = user_input (0..1)
alpha = perceptual_roughness²

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Fresnel Effect

• At grazing angles (near 90°), all surfaces become highly reflective
• At normal incidence (0°), reflectance depends on material

// Fresnel is automatic in Filament
// Controlled by:
material.reflectance = 0.5; // For dielectrics
material.metallic = 1.0; // For metals
material.baseColor = goldColor; // Specular color for metals

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F0 (Fresnel at Normal Incidence)

F0 = 0.16 * reflectance² 
F0 = ((ior - 1) / (ior + 1))²

F0 = baseColor (measured specular color)

• Water: 0.02 (2%)
• Plastic: 0.04 (4%)
• Glass: 0.04-0.05 (4-5%)
• Diamond: 0.17 (17%)

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Clear Coat Layer

final_color = base_layer + clear_coat_layer

• Is always isotropic (not anisotropic)
• Is always dielectric (IOR = 1.5)
• Has independent roughness
• Can have its own normal map

material.clearCoat = 1.0;
material.clearCoatRoughness = 0.1; // Glossy coat

// Optional: different normal for coat
vec3 coatNormal = texture(materialParams_clearCoatNormal, uv).xyz;
material.clearCoatNormal = coatNormal * 2.0 - 1.0;

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Clear Coat IOR Change

// This is handled automatically
// Can be disabled with clearCoatIorChange : false

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Anisotropic Reflections

material.anisotropy = 0.8; // Strength
material.anisotropyDirection = vec3(1.0, 0.0, 0.0); // Tangent direction

• GGX distribution with anisotropic extension
• Separate roughness along tangent and bitangent
• Direction map to control highlight orientation

// Positive = tangent direction
material.anisotropy = 0.8;

// Negative = bitangent direction  
material.anisotropy = -0.8;

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Sheen Model

material.sheenColor = vec3(0.5, 0.3, 0.1); // Color and intensity
material.sheenRoughness = 0.8; // Typically high for cloth

• Uses Charlie sheen distribution (Estevez 2017)
• Sits below clear coat if present
• Energy-conserving with base layer

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Subsurface Scattering Model

material.thickness = 2.0; // Material thickness
material.subsurfacePower = 12.0; // Scattering falloff
material.subsurfaceColor = vec3(1.0, 0.8, 0.7); // Scattering color

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Approximation Method

1. Wrap lighting: Light wraps around to back side
2. Thickness-based attenuation: Uses thickness parameter
3. Color scattering: Modulated by subsurfaceColor

subsurface = subsurfaceColor * pow(saturate(wrapNdotL), subsurfacePower)

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Cloth Shading Model

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Key Differences from Lit Model

1. Softer specular lobe - Better matches fabric appearance
2. Forward/backward scattering - For velvet-like materials
3. Two-tone specular - Via sheenColor
4. No metallic - Fabrics are always dielectric

// Velvet-like
material.baseColor.rgb = vec3(0.1); // Dark base
material.sheenColor = vec3(0.8, 0.2, 0.3); // Bright rim

// Denim-like
material.baseColor.rgb = vec3(0.2, 0.3, 0.5);
material.sheenColor = vec3(luminance(baseColor)); // Natural

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Ashikhmin-Shirley BRDF

• Softer highlights than GGX
• Better edge scattering
• Optimized for cloth appearance

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Image-Based Lighting (IBL)

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Split-Sum Approximation

specular_IBL ≈ pre_filtered_environment × BRDF_LUT

• Stored in cubemap with mip levels
• Each mip represents different roughness
• Generated offline with cmgen tool

• 2D lookup table (NdotV, roughness)
• Pre-computed Fresnel integral
• Shared across all materials

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Diffuse IBL

// Automatic in Filament
// Controlled by indirect light intensity

• 1 band (1 SH coefficient) - Fastest
• 2 bands (4 SH coefficients) - Good
• 3 bands (9 SH coefficients) - Best (default)

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Advanced Lighting Features

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Specular Ambient Occlusion

material {
    specularAmbientOcclusion : simple // or bentNormals
}

• Fast approximation
• Based on horizon occlusion

• More accurate
• Requires bent normal map
• Higher cost

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Multi-bounce Ambient Occlusion

material {
    multiBounceAmbientOcclusion : true
}

• Reduces over-darkening
• Adds color to AO (based on baseColor)
• More realistic occlusion

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Refraction

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Solid Refraction

material.ior = 1.5; // Glass
material.transmission = 1.0; // Fully transmissive
material.absorption = vec3(0.5, 0.8, 0.5); // Green tint
material.thickness = texture(materialParams_thickness, uv).r;

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Thin Refraction

material.ior = 1.5;
material.transmission = 1.0;
material.microThickness = 0.001; // 1mm shell

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Dispersion

material.dispersion = 0.36; // Diamond-like

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Specular Anti-Aliasing

material {
    specularAntiAliasing : true,
    specularAntiAliasingVariance : 0.15,
    specularAntiAliasingThreshold : 0.2
}

• Analyzes normal map variance
• Increases roughness where needed
• Preserves highlight shape at distance

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Custom Surface Shading

material {
    customSurfaceShading : true
}

fragment {
    vec3 surfaceShading(
        const MaterialInputs materialInputs,
        const ShadingData shadingData,
        const LightData lightData
    ) {
        // Custom lighting implementation
        float NdotL = lightData.NdotL;
        vec3 diffuse = shadingData.diffuseColor * NdotL;
        return diffuse * lightData.colorIntensity.rgb;
    }
}

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Performance Considerations

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Material Complexity Costs

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Optimization Tips

1. Don’t set unused properties
   // Bad - costs even at 0
   material.clearCoat = 0.0;
   
   // Good - don't set at all
   // (just omit it)
   
2. Use appropriate shading models
   • Cloth model for fabrics (not lit + sheen)
   • Unlit for UI elements
   • Subsurface only when needed
3. Limit variants
   material {
       variantFilter : [skinning, ssr] // Skip unused variants
   }
   
4. Optimize textures
   • Pack roughness/metallic/AO into single texture
   • Use appropriate precision (medium for mobile)

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Validation and Debugging

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Common Issues

• Check ambient occlusion is in [0,1]
• Verify IBL is set on scene
• Ensure colors are in linear space

• For metals, specular = baseColor
• For non-metals, check reflectance value
• Verify metallic is 0 or 1 (not in-between)

• Check baseColor is in [0,1]
• Verify emissive is reasonable (< 10,000 nits)
• Don’t add to baseColor AND emissive

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Further Reading

• Real Shading in Unreal Engine 4 - Foundational PBR paper
• Physically Based Rendering in Filament - Complete technical documentation
• Material models reference
• Material properties reference