gray skin fix

[Alessandro Padovani]

daz studio 4.15.0.30, blender 3.0.1, diffemorphic 1.6.1.0890

This is related to #24 #117 #349 #573 #857. It is reported in #857 that some skins appear "gray" compared to iray. It turns out that in the skin implementation in #24 I fine tuned the skin absorption #117, but I never fine tuned the skin scattering. This results in some tint variation in some cases especially when the skin uses a low density volume. So I used the idea by @engetudouiti #573 to fine tune the skin scattering and the new factor is 200, both for the uber skin and the pbr skin #349.

# if there's transmission then use volume absorption
if iray transmission enable
    use volume absorption
    color = transmitted color
    density = 100 / transmitted distance

# if there's sss then use volume scatter
if iray sss enable
    use volume scatter
    color = invert(sss color)
    density = 200 / scattering distance
    anisotropy = sss direction

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Below the comparison first iray then cycles then the fix. Test scenes included skin-uber.duf and skin-pbr.duf.

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Then below an example with the fixed "gray" skin, first iray then cycles. It's the Evangeliya figure reported in #857. Please note that from a pbr point of view a low density volume makes little sense since the skin is solid. But some PAs use a low density volume with a low translucency, that results in a highly sensitive skin tint.

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[Thomas Larsson]

The scatter density of the DAZ Volume node is now set to 200/distance. This was the only change compared to before, right?

[Alessandro Padovani]

Commit 635dd7b works fine thank you for the fast fix.

Yes it's the only change, I just explained why and how since it wasn't easy to get. Please note that there are other two factors that come into play in general and also for the skin tint. They are the cycles settings and the tone mapping. Since the skin is volume based the more volume bounces the more the precision. Also tone mapping of course plays its role with tints.

edit. note. This is better fixed with the MDL volume #1125 and MDL translucency #1091.