Editing Photon mapping (section)

===Construction of the photon map (1st pass)===

With photon mapping, light packets called ''photons'' are sent out into the scene from the light sources.  Whenever a photon intersects with a surface, the intersection point and incoming direction are stored in a cache called the ''photon map''.  Typically, two photon maps are created for a scene: one especially for caustics and a global one for other light.  After intersecting the surface, a probability for either reflecting, absorbing, or transmitting/refracting is given by the material.  A [[Monte Carlo method]] called ''Russian roulette'' is used to choose one of these actions.  If the photon is absorbed, no new direction is given, and tracing for that photon ends.  If the photon reflects, the surface's [[bidirectional reflectance distribution function]] is used to determine the ratio of reflected radiance.  Finally, if the photon is transmitting, a  function for its direction is given depending upon the nature of the transmission.

Once the photon map is constructed (or during construction), it is typically arranged in a manner that is optimal for the [[k-nearest neighbor algorithm]], as photon look-up time depends on the spatial distribution of the photons.  Jensen advocates the usage of [[kd-tree]]s.  The photon map is then stored on disk or in memory for later usage.