Advanced Lighting Techniques in DirectX

This section delves into sophisticated lighting models and techniques that go beyond the basic Lambertian and Phong models, enabling more realistic and visually compelling scenes in DirectX applications.

Physically Based Rendering (PBR) Fundamentals

Physically Based Rendering (PBR) aims to simulate the behavior of light in the real world more accurately. Instead of artist-defined parameters, PBR uses material properties that are based on real-world measurements.

Key PBR Concepts:

Albedo: The base color of the surface, representing diffuse reflectivity.
Metallic: Defines whether a material is a metal or a dielectric (non-metal).
Roughness: Controls the smoothness or roughness of the surface, affecting specular reflections.
Specular: (Often derived from metallic/roughness) Controls the intensity of specular highlights.
Normal Map: Simulates surface detail by altering the surface normal.

Comparison of PBR lighting with traditional lighting

Visual comparison between traditional lighting and PBR results.

Common PBR Shading Models

Several micro-facet based models are popular for PBR:

Cook-Torrance: A widely used model that combines diffuse and specular components based on surface micro-geometry.
GGX (Trowbridge-Reitz): A more modern distribution function that provides more realistic highlights, especially for rough surfaces.

Shader Implementation of PBR

Implementing PBR typically involves a custom pixel shader that calculates lighting based on the PBR material properties and light information. A common PBR shader structure involves:

Input: Vertex data (position, normal, UVs), material properties (albedo, metallic, roughness), light direction, camera position.
Texture Sampling: Fetching albedo, normal map, metallic, and roughness textures.
Normal Mapping: Transforming the normal vector from tangent space to world space using the sampled normal map.
BRDF Calculation: Implementing the Bidirectional Reflectance Distribution Function (BRDF) for both diffuse and specular components. This often involves Fresnel equations, normal distribution functions (like GGX), and geometric shadowing terms.
Light Integration: Summing up contributions from multiple lights.


// Example: Simplified PBR BRDF calculation (conceptual)

float3 CalculatePBR(float3 albedo, float metallic, float roughness,
                    float3 N, float3 V, float3 L, float3 lightColor) {

    // Fresnel, NDF, Geometric Shadowing terms (simplified placeholders)
    float F = FresnelSchlick(max(dot(N, H), 0.0), metallic);
    float D = NormalDistributionGGX(N, H, roughness);
    float G = GeometrySchlickGGX(N, V, L, roughness);

    // Diffuse component
    float3 kD = (1.0 - F) * (1.0 - metallic);
    float3 diffuse = albedo / PI * kD * (1.0 - D * G / 4.0); // Simplified diffuse

    // Specular component
    float3 kS = F;
    float3 specular = (D * G * kS) / (4.0 * max(dot(N, L), 0.0) * max(dot(N, V), 0.0) + 0.001);

    return (diffuse + specular) * lightColor * max(dot(N, L), 0.0);
}

Advanced Lighting Effects

Image-Based Lighting (IBL): Uses environment maps (like HDRi) to provide realistic ambient lighting and reflections, capturing the complexity of real-world lighting environments.
Screen Space Reflections (SSR): A real-time reflection technique that computes reflections based on depth and color information available in screen space.
Global Illumination (GI): Simulates indirect lighting bounced off surfaces, providing more natural and soft lighting. Techniques include Radiosity, Ray Tracing, and Light Propagation Volumes.
Subsurface Scattering (SSS): Simulates light that penetrates a translucent surface, scatters within it, and exits at a different point (e.g., skin, wax).

            Note: Implementing advanced lighting techniques requires a good understanding of shader programming (HLSL/GLSL), linear algebra, and the physics of light. Performance considerations are crucial for real-time applications.
        

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