Softimage XSI uses Mental Ray (which supports photon mapping) to render scenes. Caustics can be displayed in the render pass by selecting caustics in the photon tab of the Render Options Property Editior. Mental Ray renders the effects using the photon mapping technique and various options can be adjusted at this point to control effect of the caustics at the rendering stage.

Selecting caustics activates the rendering of indirect illumination only for caustic effects. Illumination only occurs for photons which follow the path of a caustic, that is, only for specular reflections and the transmission of light onto diffuse surfaces. Also because caustics are treated as a separate form of illumination by Mental Ray activating the global illumination section only will not produce any caustic effects.

The scene opposite is rendered with both the global illumnination and the caustic effects activated in the render pass. In the render options box it is possible to adjust the effects at the rendering stage with the accuracy, radius and filter controls. In order to get a precisely detailed effect a high accuracy value must be combined with a small radius.

Increasing the number of photons in the scene will result in the effective radius becoming smaller as Mental Ray does not have as much distance in which to search for photons. To decrease the grainy appearance without blurring the detail adjusting the number of photons in the scene light property editor will be more effective than only adjusting the render pass options.