Light Transport Models

Light transport models are mathematical frameworks and equations that simulate how light interacts with surfaces and propagates through a 3D scene. These models form the foundation of rendering techniques, enabling realistic visualization of materials, shadows, reflections, and global illumination.

Key Components

  1. Emission: Light emitted directly from sources (e.g., lamps, emissive materials).
  2. Reflection: Light bouncing off surfaces (diffuse, specular, or glossy).
  3. Absorption: Light energy absorbed by materials (converted to heat or other forms).
  4. Transmission: Light passing through translucent/transparent objects (refraction, subsurface scattering).

Common Light Transport Models

1. Local Illumination (Direct Lighting)

  • Models light interactions at a surface point without considering indirect light.
  • Phong Reflection Model:
    • Combines ambient, diffuse, and specular components: Ambient/diffuse/specular coefficients, and Specular exponent (shininess)
  • Limitations: Ignores indirect lighting and complex light paths.

2. Global Illumination (Indirect Lighting)

Accounts for light bouncing multiple times between surfaces.

Popular Global Illumination Models:

  • Path Tracing:
    • Monte Carlo method to simulate light paths recursively.
    • Handles complex effects like caustics and soft shadows.
  • Radiosity:
    • Solves diffuse interreflections using energy conservation.
    • Uses form factors to compute light transfer between patches.
  • Photon Mapping:
    • Two-pass method: photon emission + density estimation.
    • Efficient for caustics and subsurface scattering.
  • Bidirectional Path Tracing (BDPT):
    • Combines light and camera paths for better sampling.
  • Metropolis Light Transport (MLT):
    • Focuses on difficult light paths (e.g., specular-diffuse-specular).

Cons

  1. Computational Complexity: Full spectral simulations are prohibitively expensive.
  2. Accuracy vs. Speed: Trade-offs between real-time (rasterization) and offline (ray tracing) methods.
  3. Material Complexity: Handling anisotropic, layered, or volumetric materials.
  4. Sampling Noise: Monte Carlo methods require high samples to reduce variance.

Speed considerations

  • Offline Rendering: Physically-based models (e.g. BSDF, GGX microfacet).
  • Real-Time Rendering: Approximations like screen-space reflections, light probes, or Lumen (Unreal Engine and Unity).

Modern Advances

  • Hybrid Models: Combine ray tracing with rasterization (e.g. native support in NVIDIA RTX).
  • Neural Rendering: ML-based denoising and path guiding (e.g., NVIDIA OptiX, Intel Open Image Denoise).
  • Participating Media: Models for fog, smoke, and atmospheric scattering.