Caustics is an effect that is produced when light rays are reflected or refracted by the materials within a scene, which can produce light patterns on objects in the scene. The Caustics Render Element displays these effects, generated with Caustics photon mapping. It is enabled from the Caustics rollout under the GI tab in the Render Settings window. This render element is useful for enhancing or toning down caustic effects in the composite.
To use render elements, select from the Available render elements column and click the Add button to add the passes you wish to generate at render time. When you render the scene, the added render elements listed in the column on the right, will also render, which can be viewed from the VFB window's channel drop-down menu.
For more information on the parameters on this dialog, see the Render Elements tab page.
