Practical difference between SSS and Medium?

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SarahC
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Thu Jul 02, 2020 4:45 am

Hiyah everyone!

I was exploring materials recently, and making many different renders of single materials I've put together on simple models - eggs, STL models of ghosts - things like that that I can see the pure material by itself, and how the folds and creases in the model look for translucency and scattering.

Medium is for things like milk, clouds, and wax. Thick things that let light through them.

Subsurface Scattering does what it says, bounces light around the outer layers of the object then back out. Skin, strawberries, and such.


I made an interesting observation and wanted your thoughts.....

Through my experiments I've discovered that subsurface scattering can only be used once in a material, and as such is much like Medium. I've been unable to have two SSS layers - only one ever influences the material.

In this regard the Medium and SSS behave the same way : there's absorption and scattering.

I tried setting a Medium to produce a SSS material, and it looked identical side by side. This was setting the absorption and scattering high, so the light only penetrated the "sub surface".

I tried making a SSS material that behaved like a Medium - setting them both low - and I got two renders that looked identical.

They both have "asymmetry", they've both got IOR, the only difference I can see is that Medium has a co-efficient file feature, and a phase function! A little bit more than SSS.

Lastly - the orange juice material (on the Thea material download section) I expected to be a medium... deep light penetration. But when I took a look it was SSS!

This has blurred the lines for me mentally - can SSS and Medium be interchanged completely? Is there something one can do but not the other (besides Phase which is damn subtle anyway).....?

Medium and SSS can't be used on the same material, only one ever renders. So for example I can't make a pink core as a Medium, and then apply a SSS green effect for a ghostly melon... I just get the pink.

So for me now - SSS and Medium are more "contextual" rather than distinct entities. By that I mean if I see a SSS, it's going to be set by the designer so that light doesn't penetrate deep, and a medium in use is going to be much more transparent...
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Hieru
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Thu Jul 02, 2020 10:13 am

I think that part of the issue here is scale.

Medium is usually used for liquids and environmental effects and the settings are geared towards creating things like swimming pool water or fog. But if you apply very dense settings to small objects then the appearance will be very similar to SSS.

However, if you applied SSS settings to very large objects or materials like pool water, you would not see similar results to Medium.

So, whilst they may look identical in some circumstances, Thea will no doubt being treat them differently in the way light penetrates those materials. And those differences will only become obvious at different scales.

The change in material system from V1.5 to V2.0 is probably muddying the waters also. SSS was probably the best way to approach orange juice with V1.5, but Medium is probably more appropriate for V.20.
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