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To get started, let's open the Maya scene, caustic_dragon.ma. 
(Where are the tutorial files?) 

RenderMan for Maya uses a classic two-pronged approach to rendering caustics, utilizing “shading models” and photon maps to create the effect. Caustics can be created using Maya materials or RenderMan shaders with equal ease and just a couple of “tricks”.

First, let's render the Maya scene as is. 

Render-> Render Current Frame

You'll get an image like the one below:

Direct lighting, with transparent Deep Shadows.

 

The aforementioned two-pronged approach boils down to two things: surfaces and lights. The “shading model” is an attribute that you attach to your surfaces and the photon map is generated, using a special pass, from attributes added to your light.

As you can see from our scene, with the good ol' reliable dragon sitting on our oogly grid-plane-thing, we have two types of surfaces: one translucent surface filtering light to create the caustics and another opaque (and matte) surface that caustics are cast upon. We need to add RenderMan attributes to each to let the renderer know how to treat each during the caustics pass.

1. Select the Dragonberry material attached to our dragon (it's a Phong E, but a Blinn would do just as well) using your method of choice.

   From the Attributes menu in the Attribute Editor, select RenderMan -> Add Caustic Controls. In the Extra RenderMan Attributes section that is added there is now a drop-down list of Shading Models to choose from (note that it appears after the Refraction attributes that we added surreptitiously…); for our purposes, the “Glass” Shading Model will work best.

2. Select the Gridbert material and add the caustic controls to that as well. In this case, you want to select the “Matte” Shading Model.
3. Now select the Key light. Before we add the appropriate attributes, note that we've already added RenderMan Shadow Attributes. Deep Shadows are not required for caustics, but they sure are nice. We'd be remiss if we did not point out that our Key light's falloff is set to Quadratic. This is not negotiable — you simple must do this…please. Because of the change in the falloff, we needed to compensate a bit by increasing the light's intensity. This light is, like, totally intense.

   So…with that out of the way, go ahead and select RenderMan -> Add Caustic Attrs from the Attributes menu. They'll look a little something like this:

   

   The first thing you'll notice is that there's a Caustic Map parameter, and an rmanRenderCausticPass has been created. There are also parameters controlling the strength and coloring of the caustics, and a parameter (Caustic Estimator) that controls how many photons are “consulted” in the caustics calculations.

   For our purposes, to make our a smidgen more overt and to pretty things up a bit, let's bump the Caustic Strength up to 3-ish, make our filter a lovely shade of lavender, and lower the Caustic Estimator to 50.

4. Now let's take a look at that pass we've created. Click on the widget next to the Caustic Map parameter to navigate to the pass tab (or you can access it from the Passes tab in the Render Settings). You've got your usual pass settings, for cameras, objects, lights, and caching, plust you have some caustic-specific settings. For now, the important ones are the Emit setting, which dictates the actual number of virtual photons to emit, and the Shading Rate. We want to bump the number of photons up to 500,000, and we'll leave the Shading Rate alone for now, but we want to remember it's there, for future reference.

5. That seems like a lot of stuff to do, but it went pretty smoothly, right? Okay then, let's render…

   

   Voilà! A wee smudge of a bright spot in the middle of our shadow. Um…let's see if we can't spruce that up…go back to the caustic pass and spike the photons setting, then lower the shading rate. It will take a wee bit longer to render, but the results are nice — a bit more definition to the caustics and some nice spread, plus additional caustics under the front feet…

   

      

You can get the same results with a RenderMan Shader using essentially the same workflow. RenderMan Studio users can build their appearance in Slim, add it to their scene, and then add the necessary attributes via the Attribute Editor, or anyone can create a RenderMan Shader node, reference an appropriate .slo file, and proceed as above. The image below is using a funky little Delux appearance created with Slim 7.0.

Caustics with a RenderMan Shader

 

Although there are a few important things to remember, creating caustic effects is an essentially simple process. After you're comfortable with the simple scene we've set up here, run through things again with a different caustic shading model, like Chrome. The workflow is the same, but you get the funky caustics of sunlight bouncing off your bling-bling.

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