Photon Depth value controls the number of objects a photon can hit
before it is absorbed. This has the effect of reducing unecessary
rendering time as photons which reflected or transmitted many times
will retain little energy and often have relatively no visible
effect on the final image.
Different values can be specified
for reflections, refractions and both together. XSI allows a maximum
depth for reflection and refraction of 20 and the sum for both of
If the specified photon trace depth is too small no
photons will be stored in the scene therefore if a value is set to
zero the caustics are disabled and an error message
The default values in XSI are set to 2 and the
default values in Mental Ray are 5. In this instance two layers of
glass were used to investigate photon depth. A photon depth value of
1 does not penetrate the glass and produces no caustic effects. A
minimum of depth of two is required for caustics.