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I have a hair system on a head - I want to use a translucent shader in the head's skin material because a translucent shader allows the colour of the hairs to be seen when they pass beneath the skin -

however, I also want to use a subsurface scattering shader in the skin material because it's more realistic that a translucent shader for skin - unfortunately, though, the SSS shader does not allow the hair colour to be seen beneath the skin -

I'd also like to use a diffuse shader on top of the translucent/SSS shaders to get nice sharp shadows on the skin (both translucent and SSS shaders soften shadows that fall on/into them) - but, of course, the more the diffuse shader is applied the less the translucency/SSS shows up

using mix shaders to combine all these shaders is not optimal: it's always a balance game, more of one means less of another - and the add shader breaks PBR -

I suspect I can combine these shaders in a more controllable and discreet way using the light path node (perhaps using Is Transmission Ray?) - but I've been unable to get it to work - any suggestions?

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The Translucent and Sub Surface Scattering shaders produce only an approximation of what is actually a volumetric effect - of the light scattering throughout the internal volume of the material - and this makes it difficult to combine them realistically. You cannot use the Light Path node for this as there is no Light Path that can distinguish between SSS and Translucent paths (since they are both emulated as surface shaders, not actual volumetric rays).

Instead, use a Volumetric material mixed with the surface Diffuse. This allows the transparency of the surface to be adjusted and also allows the volumetric density and scatter/absorption to be adjusted to get the desired effect.

Here's the material :

material

Adjust the mix node to vary the surface Diffuse/Transparency. Adjust the Value node to affect the volumetric density. Vary the Diffuse shader to affect the surface color and the RGB node to affect the volume.

This can produce the following effect (note the hair roots showing below the surface) :

final

(Example produced using Branched Path Tracing inteegrator, increasing the Volume samples (to 6) and similarly increasing the number of Volumetric Bounces in the Light Path properties)

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