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Photon Generation
Shoot photons from all light sources:
Point light: Identical position. Uniform spherical distribution of directions.
Directional light: Identical direction. Uniform distribution of positions.
Area light: Uniform distribution of positions on the surface. Cos-thetha weighted distribution of directions.
For multiple light sources with different intensities: Adjust photon energy or photon frequency.
Optimization: Use projection maps to only shoot photons into directions where geometry is. Also requires scaling of energy:


Photon Tracing
On interaction with a surface with diffuse and specular reflection coefficients , :
Russian Roulette: Generate random number

• : Diffuse reflection. Store in photon map. Continue tracing (TODO: Which direction?).
• : Specular reflection. Continue tracing (TODO: Which direction?).
• : Absorption. Store in photon map.

Information stored in the photon map:
: position
: photon power (flux per solid angle)
: angle of incidence

Radiance Estimate
Replace the recursive term in the Rendering Equation with a radiance estimate from the photon map. For a point , find the nearest neighbor samples from the photon map and compute


where


Rendering
Decompose the Rendering Equation

• : Diffuse reflections. Direction independent.
• : Specular reflections.
• : Direct illumination.
• : Diffuse illumination.
• : Caustic illumination.

Evaluate the following factorizations:

• Direct Illumination. Compute using ray casting / hemisphere sampling.
• Specular / Glossy Reflections. Compute using MCPT.
• Radiosity. Compute using radiance estimate from diffuse photon map.
• Caustics. Compute using radiance estimate from caustic photon map.