Subsurface Scattering Materials:

Subsurface Scattering (or SSS) is a process in which when light enters a material, it penetrates beneath the surface, diffusing different colors back from different levels. 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. An SSS shader can simulate this effect. To produce an effective SSS shader, we must construct a network comprised of the Shading Group, An SSS material, an SSS Light Map, and a writable Mental Ray Texture. In Maya since version 2008, Mental Ray will set up this network for you, but I think it is a good idea to examine what is happening in the shading group anyways. See Image Below:

The LightMap Write node in Mental Ray:

The Lightmap write shader gathers information from the geometry and texture. For each pixel of texture that covers the corresponding triangluar face, results are written to the shader so that lighting information, for the purposes of sub-surface scattering, can be computed. The node stores light information on the front and back of a sampled surface, storing information on lighting conditions for use to sample with the scattering material. Thus, without this function in place, there is no way to calculate sub-surface scattering effects.

Plugging in the Textures:

As mentioned above, several textures are needed in combination with the effects of the material to generate the final look. For this demo, here are the specific texture files that are being referenced, and a look at the final render (note, this render is actually a 4-pass render, with the beauty pass of the SSS shader being at the base, composited with an Ambient Occlusion pass, a Reflections pass, and a flat diffuse pass which I used to mute the skin down a bit).

Rendering Enviornment:

For a good looking rendering, I used the setup shown below to generate my lighting.

• Spotlight 1 is my key light, the most intense light source and casting both Shadows and Photons into the scene.
• Pointlight 1 is my back/rim light. Specifically, it is behind the behind the head from the view of the camera. Without a light behind the skin, there will be no rays to beam through from the back, and the effects of backscatter will be imperceptible.
• Pointlight 2 is a fill light from underneath.
• Arealight 1 is a fill light from the stage left (character right) which is being used to soften up the dark side of image.
  
• nurbsSphere1 was cut in half and used as a backdrop for the shot. Curved walls mean that you don't get harsh shadow creases where the floor meets the wall meets the ceiling.
• pSphere1 is a full sphere set to render as single sided. A surface shader has been applied with a panoramic photo. This sphere is set to be visible only in reflections and provides detail for reflections from a photographic enviornment
  
• Final Gather, Global Illumination, and Caustics have all be enabled for this demo.

 

The Shader Setup:

	This image on the left shows you the values I used for my scene after the needed guess/checking experimentation. These values are NOT absolutes as they conform to the size of your scene file and the strength of the textures applied. While you cannot just plug these in everytime for every character, you can start with these relative values, and tweak along the lines that we will follow below in the Shader Tweaks section. Anything highlighted in blue will be tweaked for experimentation in that section.
	In the diffuse layer section, your Color Map gets applied to the Diffuse Color Channel. Ambient and Overall Color are general modifiers that can be tweaked (AT THE END) to modify the overall brightness of the skin. But again, I leave those alone until the end. Diffuse weight can either be textuerd with a Cavity Map, or manually edited as I do here.
	This layer contains information on what happens as light bounces off and through the outermost layer of skin. I usually apply my Color Map to the Epidermal Scatter Color as well.
	This layer contains information on what happens as light penetrates through the outermost layer of skin to the underlayers, and returns through the front. My Subdermal Map gets applied to Subdermal Scatter Color
	This layer contains information on what happens as light comes through the back side of the skin and leaves out the front. This layer is useless without a lightmap (read above). My Back Scatter Map gets applied to the Back Scatter Color, and should reference the luminance value of the thinnest parts of the skin (ears, fingers, nose) where light can make it through.
	General Specularity weighting for how shiny the skin is. If left too high, the skin looks like a wax-candle.
	This contains information for how bright the skin specularity will be, and where it will be concentrated. The Edge Weight can create slight "outline" effects on the skin according the properties of BRDF. My Spec map goes into Primary Specular Color, and can also be applied to Primary Weight.
	A modifier for the Primary Spec, you can blend multiple maps with this setting or use it to tone the spec map as a whole. I set the weight to 0.000, essentially turning it off. I feel this is a "use-it-only-if-you-need-it" section, which I don't.
	Skin Reflectivity settings. Again, this can be weighted to the edge for an outline-like effect.
	Plug in your Bump or Normal Map here.
	This is the hook-up for the lightmap texture as shown in the hypershade diagram above.

Shader Tweaks:
In this section I will break out certain values (the ones highlighted in blue in the image above) and show you what modifiing these numbers will produce in terms of render results.
The renders will illustrate my base setting (in the center) which has all of the values in the image above left alone. There will be 1 lower and 1 higher value per channel, but note that
only 1 value will be tweaked a time. There are an infinite amount of varriations that can occur here if you start tweaking more than 1 thing. It is VERY STRONGLY RECOMMENDED
that in testing your own shaders as well, you minimize the amount of information you change from test to test, so that you can better understand the effects of the changes you just made.
Sometimes altering 2 sliders will cancel out the apparent change, so again, alter 1 thing at a time, which is what we do now.......


Diffuse Weight:
My base setting was at 1.0
On the left, it was lowered to 0.2
On the right, it was raised to 3.0

These values can have a dramatic effect on the frontal brightness of the skin. Notice how a lower value reveals more back scatter (the ear), where the higher values blow out the skin altogether.



Epidermal Scatter Weight:
My base setting was at 0.4
On the left, it was lowered to 0.1
On the right, it was raised to 5.0

I think it is interesting to notice here how changing the Epidermal Scatter Weight so closely mimics the Diffuse Weight changes. Since this is the outermost layer of skin scatter, the results indeed should be similar,
as if we were comparing a the first layer of skin (here) to a layer of makeup just on top of the skin (diffuse weight above). Importantly though, notice how the higher value here looks creamier than the higher value
shown in the Diffuse Weight section, thats the extra scatter.



Epidermal Scatter Radius:
My base setting was at 5.0
At the bottom, it was lowered to 1.0
At the top, it was raised to 40.0

Subtle effects here, but you can notice that the higher radius, the more the skin seems to glow from underneath. If coupled with a higher Scatter Weight as well, you can imagine that the results tend to look really soapy.

Subdermal Scatter Weight:
My base setting was at 0.4
At the bottom, it was lowered to 0.1
At the top, it was raised to 5.0

Ok, so now we get deep into the underlayers of the skin, controlling how much glow they add to the top. I think the results here are pretty evident, weight the under-skin too high, and blows out through the top. Too low, and the skin looks lifeless and dull.

Subdermal Scatter Radius:
My base setting was at 5.0
At the bottom, it was lowered to 1.0
At the top, it was raised to 4.0

This setting refers not to how much brightness comes through (thats the weight section above) but how FAR that weight will spread outward visually across the across the surface once it reaches the front. Again, notice here how this effect more subltely lightens the surface not by adding more light, but by blending the light that does make it through outward farther on the face.

Back Scatter Weight:
My base setting was at 2.6
At the bottom, it was lowered to 0.1
At the top, it was raised to 8.0

What we are looking at here is the brightness value of light penetrating from BEHIND the surface coming out the front. Notice how the effect is dramaticaly seen in the ear, since that area is thinner (and represented as brigher in the back scatter color map).




Back Scatter Radius:
My base setting was at 140
At the bottom, it was lowered to 10
At the top, it was raised to 400

These changes effect how much the BACK effects come through on the front of the surface. Very similar to the Subdermal Scatter Radius a few sections above. Notice though that if too low, the ear loses it's glow.




Back Scatter Depth:
My base setting was at 70
At the bottom, it was lowered to 10
At the top, it was raised to 200

This slider essentially says "if the surface is less than this thick as the camera looks directly back through it, then light will be able to pass through". Thus, low numbers limit the amount of light, high numbers create a brighter/redder glow.





Specularity Overall Weight:
My base setting was at .550
At the bottom, it was lowered to .05
At the top, it was raised to 3.0

This is the overall brightness of the skin's shine. Too high produces a wax-like look. Too low and the skin looks dry and pale.





Specularity Primary Edge Weight:
My base setting was at .2
At the bottom, it was lowered to .01
At the top, it was raised to 2

Ok, this one is REALLY subtle. Look on the left side of the head, just above the ear. You see that brighter outline on the edge of the head? That is this effect. Follow it downwards, notice how it mutes itself as it passes by the ear, and disapears alltogether at the lowest value.

Specularity Primary Shininess:
My base setting was at 30
At the bottom, it was lowered to 2
At the top, it was raised to 100

This is the rough equivalent of "eccentricty" on a Blinn. The higher the value, the further the spread of the highlight as it goes outward from the center. The lower the value, the tighter the highlight.



Reflection Weight:
My base setting was at .2
At the bottom, it was lowered to .02
At the top, it was raised to 3

This stuff is super hard to see, I know. But in this separate stages, a change in the overall amount of reflectivity is being added. This is generally modified by the Specularity Overall Weight as well.




It is my hope that these side-by-side-by-side images give you a basis for comparision and a better understanding of the power of SSS shading networks as you create your own materials and textures.
And the final image, one more time for comparison: