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I've been using a skin shader primarily based on the Brecht skin shader, which is itself based on the Sum-of-Gaussians approach as seen in 14.4.7 from GPU Gems by nVidia. Its Cycles adaptation, which is essentially what I've been using as well, can be seen in this post. The inside of the skin nodegroup is shown in post #13334 on the next page (sorry, I can't post more than two links).

As you can see, for every row of the GPU Gems table, the image input of the nodegroup is multiplied with the corresponding RGB value and connected to the color input of an SSS shader. The radius vector of each SSS shader is taken from the Variance column of the GPU Gems table (incorrectly labeled as Scale in the screenshot). After that, the SSS shaders are added up and then mixed with two Glossy shaders of varying roughnesses.

Now, with the new Principled BSDF from the upcoming Blender 2.79, I could take advantage of the (as I understand it) more realistic shading as well as implement the two Glossy shaders by using the regular roughness input and the clearcoat option of the Principled shader, but how would I go about adapting the Brecht approach for the SSS-part? I obviously can't just add up a bunch of Principled shaders, so I'd either need to somehow combine all that Sum-of-Gaussians stuff beforehand to use as an input for the Principled BSDF, or I'd need to separate the SSS/diffuse component from the specular component to independently add and/or mix them back together again.

This is where I'm kind of stumped, so if anyone has an idea on how to do this, that would be great.

Thanks :)

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