Editing Photon mapping (section)

====Calculating radiance using the photon map====

In order to calculate surface radiance at an intersection point, one of the cached photon maps is used.  The steps are:

# Gather the N nearest photons using the nearest neighbor search function on the photon map.
# Let S be the sphere that contains these N photons.
# For each photon, divide the amount of flux (real photons) that the photon represents by the area of S and multiply by the [[Bidirectional reflectance distribution function|BRDF]] applied to that photon.
# The sum of those results for each photon represents total surface radiance returned by the surface intersection in the direction of the ray that struck it.