Additional points about SSS

Since I opened the discussion about subsurface scattering (SSS) with my Light and SSS and SSS – Why Should I Care? posts, I’ve received some good feedback / additional information. I wanted to capture those here.

Other Uses

First, the point has been made that although we think of SSS as adding realism to surfaces which don’t reflect 100% of the light that strikes them, the effect of SSS can be used for other purposes. It can add some depth to the surfaces for toon style rendering, and can even completely change the look of an object. For some examples, see the following product pages at the DAZ 3D store.

Note: I don’t get any commission if you choose to buy any of these products. :) I’m actually referencing them because they have example images that show the effects.

DAZ Studio – SSS Shader

We’ve had a couple of good discussions about the Subsurface Shader Base that is available for free for DAZ Studio. These discussions have largely been about how the shader works. It was actually one of these discussions which spawned my initial blog posts. I wanted to capture a couple of important points here.

Pre or Post?

The first point was asking for some clarification about how the selection of either Pre or Post processing of the SSS effect changes the resulting calculations. Age of Armour (Will) was kind enough to provide us with some information in this thread on the DAZ 3D forums.

The choice to Pre or Post application of the SSS effect has to do with how the surface values are calculated. For the Pre option, the calculation is:

(
(Diffuse map * Diffuse Color * Diffuse strength)
* Lighting
) +
(Subsurface Calculation * Lighting)

This basically means that the Diffuse surface color is calculated, then the SSS effect is added to the result.

When chosing the Post option for the SSS effect, the calculation looks significantly different.

(
(Subsurface Calculation * Lighting)
* (Diffuse map * Diffuse Color)
) +
(
(Diffuse Map * Diffuse Color * Diffuse strength)
* Lighting
)

In this case, there are two calculations that use the Diffuse surface settings. In the first part, the SSS effect is multiplied by the diffuse color. Note that the diffuse strength is not factored in at this point, it is simply creates a version of the diffuse color which is tinted by the subsurface effect. The second part of that equation is a standard diffuse surface calculation. The two diffuse colors are then added together to arrive at the final color for the surface.

The Origins of SSS

The ideas and concepts around subsurface scattering for the purpose of computer graphics were first described in a paper titled “A Practical Model for Subsurface Light Transport” presented to the ACM Siggraph conference by Henrik Wann Jensen, Stephen R. Marschner, Marc Levoy, and Pat Hanrahan. Warning for those who seek to understand SSS at that level, this is NOT trivial mathematics by any stretch. I cannot be held responsible for any damage to your brain from trying to read the paper.

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1 Comment »

  1. […] Marschner is also the primary author on the Subsurface Scattering paper that I linked over in my Additional SSS Information […]


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