Photon maps

Tip

To see the distribution of photons for global illumination, you can generate a 3-dimensional visualization map in shaded or wireframe scene views. For more information, see See the distribution of photons.

Photon maps are:

Required to produce global illumination and caustic effects. (They are not required for Final Gather.)
A 3D representation of the accumulated light energy at certain photon bounce points.
Conceptually similar to shadow maps, but they capture light instead.
Created during the Photon tracing stage.
Used by the material shaders for those surfaces that participate in global illumination at render time to calculate the contribution of Global illumination and caustics. (This is added to any contribution provided by local (direct) illumination in the scene.)

GI photon maps vs. caustic photon maps

Photons that are specularly reflected or refracted are stored in the caustic photon map; all other photons are stored in the global illumination map.

As the Shinyness parameter of the photon shader attached to an object increases, specular highlights become smaller and the likelihood of specular reflection increases. In this case, more photons migrate from the global illumination map into the caustic photon map, decreasing the density of the global illumination map. (Similar considerations apply to the Translucence parameter, but in the opposite direction: when Translucence is increased, photons migrate from the caustic photon map into the global illumination photon map.)

If there are not enough photons stored in the global illumination photon map, mental ray for Maya warns (in the status bar) that no photons are stored.
If a scene is dominated by specular reflection but caustics are turned off, a significant amount of illumination won’t show in the rendered image.
In these cases, you need to:
- Turn on caustics
- Make sure the light(s) emit caustic photons
  Tip
  You can save a photon map and reuse it, either in later frames in a render, or in a different render at a later time.