Claudia Herbst
Associate Professor
Pratt Institute
Department of Digital Arts


Surfaces & Sub-Surface Scattering

Many "opaque" objects absorb light into the surface to some degree; the 3D term for this is sub-surface scattering.
Light penetrates the surfaces and, depending on the material, bounces around. In the words of the Maya folks:

"Subsurface scattering refers to light penetrating an outer surface and then being scattered diffusely beneath.
This creates the look of a soft surface, often with a very subtle glow. An example of subsurface scattering is
human skin, where the soft, porous quality and semi-translucent glow are essential to achieving a realistic look.
Other surfaces that require scattering are leaves, wax and gel."

Here are a couple of examples (figure 1, 2, 3, 4).



Figure 1




Figure 2 and 3




Figure 4



In Maya, the following materials feature a scatter attribute in the Mental Ray section: Anisotropic, Blinn, Lambert,
OceanShader, Phong and PhongE surface. Tuning this scatter attribute is one way to create the look of subsurface
scattering. Note that the scatter attribute interacts with the Ray Depth Limit attribute of your scene's light; try a value
of 2 or higher and compare the results.



Exercise:

The purpose of this exercise is to learn the basics of how to simulate surfaces with sub-surface scattering and,
more importantly, to create luscious detail using multiple maps. This exercise has two parts, one that focuses on
realism, one on creating an impressionistic effect.

Open the file grapes_blinn.ma located on DDA transfer. Notice that I placed a spotlight behind the grapes and a
point light off to the side. I assigned a blinn material to the grapes and further set the scatter attribute to a value of 5,
added a color map, translucency map, and a specular map. Take a look at the different maps; it's the same map in
each instance but I rotated it to create some offset and changed the contrast depending on type of map. For example,
the specular map is less bright because grapes are usually somewhat matte. Using Mental Ray do aquick render,
figure 5 is what I got.


Figure 5


Your task is twofold:

1) Create your own shader network using your own maps and account for light penetrating an object's surface and for
the texture beneath the surface -- different types of fruit (real or imagined) are great for this. Feel free to use the grape
model but if you do, make them green (or any other color, just not red...) Alternatively, you may use an object from the
assignment you are currently working on.

2) Once you've created something that looks realistic, create a second version of the same object(s) in which you still
capture the essence of sub-surface scattering but are creating a more stylized, painterly effect. Thus, the second image
should be more impressionistic. Create a simple but nicely designed composition; account for the background/environment.

Finish both parts of this exercise by next week and put a copy of both images in my folder on DDA transfer. Save ".tif"
images and use the following naming convention:


Firstname_Lastname_Real.tif and Firstname_Lastname_Impr.tif



(Next week we'll look at Mental Ray subsurface scattering shaders.)