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====== Photon mapping ======

The aim of photon mapping is to determine the global
global illumination of a scene efficiently and thus to create realistic images in less time.
It can be divided into two phases. In the first phase, photons are sent into the scene from a light source. With every interaction with a diffuse surface, an entry is stored in a photon map. This map is then used to calculate the illumination of the scene.


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There are four important parameters that influence the quality of an image rendered using photon mapping:
<key>Global photon count</key> determines how many photons are to be emitted into the scene.
The higher this value, the more precisely the scene is illuminated. For scenes where only the caustic effects are of interest, it can be advantageous to set this value to 0.
<key>Caustic photon count</key> defines how many caustic photons are to be emitted into the scene. Caustic photons are only saved in the photon map when the first interaction with a reflective or transparent object takes place.
As the name suggests, these photons are responsible for caustic effects in the rendered image. An increase in this value ensures stronger and more detailed visibility of caustic effects. For a scene
with only diffuse surfaces, this value can be set to 0 to save computing time.
<key>Photon area</key> determines the size of the area that is scanned for photons when rendering the image.
The higher this value, the brighter the scene is illuminated. A lower value ensures more precise lighting. Negative values are not possible.
<key>Recursion depth</key> determines the recursion depth at which the calculation of the photons is started.