# Photon Map Construction Radiance Method

Photon map construction in action.

Rendering with a photon map is done in two passes: In the first pass photon maps are constructed (global and caustic maps) and in a second pass the scene is rendered with stochastic raytracing using these photon maps.

A 'Radiance' method called 'Photon Map' controls the first pass. The raytracing method 'Stochastic Raytracing' is used for the second pass. Options for the second pass are described in the documentation about stochastic ray tracing. This page describes the Photon Map Construction Radiance method options.

Photon Map Construction Control Panel

Different options are provided for controlling construction of global and caustic maps:

A global map stores all illumination information (on glossy and diffuse surfaces) and is used to limit recursion depth in stochastic raytracing:

- Global Map (Toggle): If on, photons are traced each iteration for storage in the global map.
- Samples / Iteration: Number of paths traced each iteration for the global map. (Note that the number of stored photons can be lower or higher than this number)

A caustic map stores caustics (LS+D paths) which are visualized directly in the second pass:

- Caustic Map (Toggle): If on, photons are traced each iteration for storage in the caustic map.
- Samples / Iteration: Number of paths traced each iteration for the global map. (Note that the number of stored photons can be lower or higher than this number)

Other options:

- Maximum Path Length: Limits the length of traced light paths and as such the number of interreflections calculated. (Length = number of segments in the path)
- Render Particle Hits (Toggle): If on, the photon hits are visualized on screen (= particle/light tracing). The image however does not necessarily reflect a correct (unbiased) view on the scene, since e.g. caustics are present in the global as well as the caustic map.
- Photons in reconstruction: Determines how many nearest photons are used in the radiance reconstruction. This parameter is in effect only during the second pass. A higher number causes more smoothing and takes longer.