Help to understand SSS (my brain hurts)

[gordonrobb]

Been working to get a good skin shader. Attached is my latest attempt. Been also reading the manuals and watching video tutorials, however I'm confused.

Can someone explain to me the fucntions and connections between.

-Transmission
-Absorption
-Scattering
-Scale

I thought I had it sussed, but can't seem to get it to work the way I think it should. My initial view was the following, but sometimes it doesn't seem to work this way.

- Transmission - assign a colour or texture that the sss will show
- Absorption - the amount that light is absorbed, therefore 0 is no absorption and 1 is total (ie effect will not show at all)
- Scattering - not really sure what this means
- Scale - seems to be 'tweak' to get the right effect

As I said, if anyone can explain these things in a way that actaully makes sese, that'd be great.

[gordonrobb]

Anyone? Or anyone point me in the direction of an explanation that means something? Manual doesn't really tell me anything

[FooZe]

Hi Gordonrobb,

The best thing to do to learn what each one does is to make a cube or sphere and make it a specular material. Add some lighting and then set the specular materials index and reflection to zero. This will help in minimizing any other effects while you are isolating each control. Also make sure you are on PT or PMC.

The transmission is what light is transmitted through the material.
If you have no transmission SSS will not do a thing.
Just as similarly, if you have a really big index and full reflection, all you see is the reflection and SSS won't do a thing.

Absorption i think you have sorted out already, it's how much light is absorbed. So if you set this to say yellow (full Red and Green) then you should see that your sphere goes blue, because all red and green light is being absorbed in the material.

Play with the scale after this, and you will see it changes the strength of the effect (both absorption and scattering).

The scattering is literally this - it scatters the color light you specify as it goes through the material.

While leaving the absorption at 1 for R and G, try putting the scattering up to 1 for R and see the effect (you may want to move the light source or camera around.). Play with the scale and amount of red scattering. You should notice that the side of the sphere facing the light source goes red, this is because the light is being scattered around (including back to the camera).

The last parameter is the scattering direction, which is a bit tricky.
At a value of -1, the scattering is weighted so rays go back towards way they came, making the effect more noticeable when the camera is between the object and the light. With a value of 1 the scattering is weighted to go straight through, so the effect is more noticeable when your camera is looking through the object towards the light source.

Usually you wouldn't need to change this.

So in a nutshell, transmission is the colors that SSS has to work with, if you cut anything out from the transmission you are going to have less to work with when using SSS.
Absorption color is the opposite (since the specified color is getting absorbed) of the color you will see deep in the material, and the scattering color is the color you will see on the edges of the material facing the light.

I hope this isn't an overload of info!


Thanks
Chris.

[mbetke]

Absorption color is the opposite (since the specified color is getting absorbed) of the color you will see deep in the material, and the scattering color is the color you will see on the edges of the material facing the light.

THis is what I always diliked with the Octane SSS. Why not making the color selection the way that I select the SSS color I need. Not what I want to exclude?

[gordonrobb]

FooZe: an excellent explanation. Thank you an immense amount.

I don't understand it all

But it wil allow me to explore the effect with a lot more intelligence than I had on the subject.

[sigmamoon]

Hi Gordonrobb,

The best thing to do to learn what each one does is to make a cube or sphere and make it a specular material. Add

......

I hope this isn't an overload of info!


Thanks
Chris.

Very nice description frooze

and to complete with a nice practical video gordonrobb, you can see the nice tutorials of "radiant" at this topics
http://render.otoy.com/forum/viewtopic.php?t=7566

the video number 22 explains your wonders

be carefull to use the v1.13 libs of octane cause between the 1.10 to 1.12 there were some broken value inside the diffuse SSS

have good tests

[gordonrobb]

Yeh, I had watched that. Although it helped, it's not great at explaing what is actually happening. Fooze has done a great job though.