Understanding Photon Mapping for a Deterministic Simulation
The Deterministic simulation is fast and targeted but is best suited to analyze simple optical paths. When contributions and optical interactions are multiple, you should use Monte Carlo algorithm or generate a photon map.
Photon Mapping
Photon Mapping is a luminance algorithm that allows you to realistically simulate and render the interaction of light with objects.
This algorithm takes into account diffuse inter reflections, caustics and surface contributions of the optical system.
The Photon Mapping process is the following:
The first step of photon mapping is a photon propagation phase. A first pass is done using a Monte Carlo direct simulation to send photons from sources into the scene. Photons are then stored in a map.
The second pass, called the Gathering phase, is a deterministic inverse simulation. The photon map from the first pass is used to compute local radiance.
At the end of the simulation, a noise map is generated and can be reused for future simulations.
Simulation Results
As the first pass corresponds to a Monte Carlo direct simulation, photons are randomly drawn. As a result, photon deposition on scene parts are different from one simulation to another, implying different photometric results in localized measurements as shown below.
For example, considering a 10x10 mm² ellipsoid measurement area, and running 20 simulations, a 2.3 cd/m² standard deviation is obtained on this measurement series.
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