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

RenderMan For Maya lets you create subsurface scattering effects by attaching special RenderMan attributes to Maya Materials. When these attributes are attached to a Material a subsurface scattering pre-pass will be generated automatically, much like creating a shadow map in a pre-pass.

Before we get started let's render the Maya scene, subsurface_scatter.ma, without subsurface scattering. 

Render-> Render Current Frame

You'll get an image like the one below:

The basic scene

To add subsurface scattering, select the Maya Material called Mushroom1 in the Hypershade. Next, we'll add the subsurface scattering attributes to this Material. To attach the attributes follow these steps:

1) Select the Material in the Hypershade (in this case named "mushroom1").

2) Open the Material in the Attribute Editor.

3) From the Attribute Editor Menu:

	Attributes-> RenderMan-> Add Subsurface Scattering 

Now that you've added the subsurface scattering attributes, you will find them at the bottom of the Material under the Extra RenderMan Attributes tab. There are a number of subsurface scattering parameters that you can use to control the effect. Right now we'll render with the defaults, so simply render again:

Render-> Render Current Frame

You'll see the pre-pass generated first. After that is processed (in about 30 seconds), you should get an image like the one below:

Default subsurface scattering

You can see the effect is there, but it needs to be tweaked for this particular model. We'll do that next.

In the Extra RenderMan Attributes section of the Attribute Editor you'll find the parameters for controlling the subsurface scattering effect. The most important setting is the Scattering Free Path parameter, which determines how far light penetrates into an object. In this case we'll set the Scattering Free Path to 2 (as measured in world space). Render again:

Render-> Render Current Frame

You should get an image like this:

Scattering Free Path set to "2"

By messing around with the other parameters (for instance, connecting a Marble to the Albedo), you can create some interesting effects, like this:

With Marble added to the Albedo

 

Because subsurface scattering is a potentially expensive effect, RenderMan for Maya allows you to reuse these calculations. If the light doesn't change and the object isn't moving, reusing these calculations can save a lot of time. To reuse subsurface scattering do the following:

1) Open the Render Globals.
2) Select the RenderMan Passes tab.
3) You'll see a list under Pass Settings, select rmanSSMakeBrickmapPass.
4) Switch the Caching Behavior from Compute to Reuse.

The Passes tab gives you a high degree of control over the individual passes that are part of a render job. By selecting a different pass in the Pass Settings window, parameters will automatically appear for that pass. In this case, you've switched the mode of the Subsurface Scattering pass from Compute to Reuse. Now that you've set the Caching Behavior to Reuse, try rendering again and you won't have to wait for the pre-pass. 

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Remember you'll want to switch the Caching Mode from Reuse to Compute if the lights change, the object moves, or if you're tweaking most of the subsurface scattering parameters.

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You are able to create subsurface scattering effects with RenderMan for Maya by attaching special RenderMan attributes to Maya Materials. This effect is generated in a pre-pass which is referenced in during the rendering of the final image, just like shadow maps.

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If you are rendering subsurface scattering on multiple objects, like a clump of mushrooms (where each object is clearly distinct from, yet close to, the others) you'll want to apply a separate material to each object. If a single material is used the subsurface scattering effect with be blurred across the objects.

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