In the following two images, both of the GI and FG Rebuild features have been turned off, forcing Mental Ray to re-use existing maps. Notice that when I move objects or the camera, things don't quite look right. However, in both renders, the render time was an amazingly fast 8 Seconds! This is because I am not rebuilding anything at all, and thus the render takes just as long as if FG and GI were turned off (Fig.30 - 31).
Rendering Caustic Effects:
Caustics are light patterns that are created when light from a light source illuminates a diffuse surface via one or more specular reflections or transmissions. Examples are the light patterns found on the bottom of a swimming pool as light is refracted by the water surface; light being focused by the refractive value of glass onto a diffuse table; light emanating from the headlights of a car as light is refracted from the bulb off the back parabolic mirror; light being refracted inward by gold, a precious metal. Notice the common theme here. Refraction is the key to seeing caustic effects. In the following image, the red dots show several occurrences of caustic light (Fig.32).
To Enable Caustics, let's go into the Render Settings window and turn on Caustics (Fig.33).
We are also going to need to tweak our settings. Usually, I will make my Caustics Radius the same value as my GI radius (Fig.34).
Next we are going to need to make a refractive/transparent material to let light through. It's not enough to just set up refractivity. If you are using a Maya Material (Blinn, Anisotropic, Phong, etc.), you will need to add a photon shader to your material so that it can focus the photons into a caustic highlight. If you are using a Mental Ray material (DGS, Dialectric, MIA), these already have built in photon shaders so you won't have to do any extra work. Here is a look at the differences between the Shading Groups for a Maya Blinn, and a Mental Ray MIA material (Fig.35).
If you are using a Maya material, you can input the photon shader like this (Fig.36). And Render!
Notice how we now have a beautiful green tinted highlight in the middle of the shadow for the green crystal ball (Fig.37).
That's it for Caustics, and for this tutorial. I hope you were able to find it useful. Now get out there and render something pretty!