Corona SSS for dummies ?

[selant]

Can someone please explain SSS material philosophy step by step (like the famous series ".... for dummies" books  :)

For example : which color in the slots indicates what in the beauty picture, and the scattering distance affects what, etc..What if we chose full black in the absorption with the same setup, or what happens if we choose full white. Which level should we consider for these color slots..  Because i can not understand such as human skin is scattered with blood color(redish).. I cant see how exactly light travels on SSS assigned materials and how it leaves the surface in what color.. I want to know the philosophy rather to look for prepared materials and i am sure lots of people are confused with sss materials..

To start with the following example :

* Why is diffuse color level 0 ? (does orange juice not having a color in real life ?)
* Why is reflection 1 and glossiness 1 ?  (is orange juice reflecting all the light ? )
* Why is refraction 1 ? (Orange juice is transparent and lets all the light pass inside to other side ?)
* Is absorption color full white or a little greyish, and why ?
* Can somebody show which area exactly the scattering color slot affects in the glass picture ?
* If distance is increased/decreased what would be the result ?

[maru]

I already started preparing a better SSS guide, but it had to be delayed because of other important things. I hope I will  be able to return to it soon. There seems to be a lot of confusion with the SSS settings and I understand that it has to be fixed.

I will try to answer on your questions, but I may be wrong at some points. Would be good if someone inspected it and pointed possible mistakes.

* Why is diffuse color level 0 ? (does orange juice not having a color in real life ?)

Same reason why you set diffuse level 0 for water. It is the color of the surface. Water's surface is transparent, unless there is some kind of dust on it. You can simulate that dust by using diffuse color. Same goes with metals.
Juice has no surface color unless it is dirty/rotten/covered with something. In fact, if you look really close at the juice (for example the area where it touches the glass in real life - you will notice that it is fully transparent like water, and the color comes from the tiny particles inside it.

* Why is reflection 1 and glossiness 1 ?  (is orange juice reflecting all the light ? )

It reflects (almost) all light at grazing angles. Reflectivity is set to 1, but Fresnel IOR is not 999, which means that value of 1 will only be applied to grazing angles. You could set it to 0,8 or 0,9, but it should not make much difference and I am not sure if it is more correct in any way.
Reflection glossiness is @1 because the surface of the juice has no roughness (if you looked at it under microscope there would be no bumps).

* Why is refraction 1 ? (Orange juice is transparent and lets all the light pass inside to other side ?)

Yes, if you remove the light-scattering media from its inside. Absorption and scattering colors take care of this.

* Is absorption color full white or a little greyish, and why ?

Absorption color fully white would mean no absorption at all. It is greyish so that some light is absorbed and scattering can kick in.

* Can somebody show which area exactly the scattering color slot affects in the glass picture ?

If everything else was set to black, the only color you would see would be the scattering color. You can imagine it as the color of the tiny particles which are inside of the object. The tiny particles bounce the light in the inside.

* If distance is increased/decreased what would be the result ?

Light traveling inside will get darker faster.

You can read some helpful tooltips by hovering over each setting. Maybe this will help.

[selant]

it really was useful to understand, at least now i have a better idea which specific setting is like that.. Thank you very much for step by step explanation!
I always had understanding problems with the SSS but this explanation helps in big scale.

[Ludvik Koutny]

In case of orange juice, you can think of absorption distance as the density of juice. Basically the greater distance value is, the less dense the juice will appear, and it will appear as if it was more mixed with water. The lower distance value is, the greater will the density be, so making that value really low could give you something like thick orange smoothie, and extremely low distance will result in pretty much solid orange material. :)

Also, since juice is water based, make sure your reflection and refraction IOR is set to 1.33 (IOR of water).

[romullus]

While we're at it, i'd like to ask when colourful absorbtion make sense?

[maru]

While we're at it, i'd like to ask when colourful absorbtion make sense?

I think it does, but I have problems imagining when it should be used, and when it shouldn't. I know that setting absorption to pure black and changing distance only is a nice and easy way. But not sure if correct.

I guess if there is something like colored glass and it has some tiny particles of different color inside - then absorption color + scattering color should be used.

Here is an excerpt from a top secret discussion with Ondra and something which I would like to include in some official guide at some point:

absorption:
color: red
distance: 5cm

this means that light will get multiplied by the selected red color after travelling 5cm
and after travelling another 5 cm it will again get multiplier by red
so it will be getting more and more red, and also darker and darker (unless the red is 255)

if you set absorption to 50% gray and distance to 10cm, then 50% of light will disapperar after travelling 10cm
and after another 10 cm 50% of the new 50% will disappear, so only 25% will remain
and so on
it goes like this exponentially, so getting the right distance can be tricky - when scaling the object and/or changing the distance, the color changes exponentially, so it is really simple to get 100% black or 100% while (which means no SSS at all)
but this is the way it works in nature, if you want physically correct results, it has to work like this, and nobody invented better controls for it yet

[Ricky Johnson]

I think the terminology for absorption is counter-intuitive when it comes to colours because a pure red colour (at R1.0, G0, B0) would be absorbing all of the colours other than red wouldn't it? I know this is the same as many other properties, i.e. diffuse/reflection colour, is the colour that's being returned by something, but as the word absorption is used you expect it to work to positively remove that colour rather than remove it. It still makes way more sense to define the colour that's not being absorbed though. It's just the wording.

Is there any definitive description for how scattering colour operates as well? The description you've pasted there for how the absorption works is really useful, something along those lines. I always think of the scattering colour as an internal multiplier but now I don't think it can be technically as it would be doing something similar to absorption colour if it were.