"SSS Direction" -- What Does It Do?

margrave

I spent the past few days searching for a solution (that doesn't involve post-work) to the blue UV seams that appear when using spectral rendering.

And to my surprise, I discovered the solution is as simple as setting "SSS Direction" to -1.0.

But now the question becomes, what is SSS Direction?

After reading the official doc pages, I get it's like the normal vector of the scattering effect. But I don't get what that means in practical terms, and I have no eye whatsoever for whether a skin shader is realistic or not. Is -1.0 a good value? Is it too waxy or ashen? Why does it make the blue UV seams go away if the SSS is facing directly outward? Is this basically just a roundabout way of disabling SSS completely?

Richard Haseltine

I believe -1 is literally back scattering, 1 is forward scattering, and intermediate values are off-axis.

Fishtales

SSS Direction - This property is used to determines whether the surface scatters toward or backwards from the light. 0 is Isometric, or almost no scattering - like water. Negative numbers (-) backscatter to the direction of the light source. Positive numbers (+) forward scatter away from the direction of the light.

Richard Haseltine

Fishtales said:

SSS Direction - This property is used to determines whether the surface scatters toward or backwards from the light. 0 is Isometric, or almost no scattering - like water. Negative numbers (-) backscatter to the direction of the light source. Positive numbers (+) forward scatter away from the direction of the light.

Thank you, I was unsure (and wrong) on the meaning of intermediate values

Fishtales

Richard Haseltine said:

Fishtales said:

SSS Direction - This property is used to determines whether the surface scatters toward or backwards from the light. 0 is Isometric, or almost no scattering - like water. Negative numbers (-) backscatter to the direction of the light source. Positive numbers (+) forward scatter away from the direction of the light.

Thank you, I was unsure (and wrong) on the meaning of intermediate values

I got it from the Document Centre :)

http://docs.daz3d.com/doku.php/public/software/dazstudio/4/referenceguide/interface/panes/surfaces/shaders/iray_uber_shader/shader_general_concepts/start ;

margrave

Richard Haseltine said:

I believe -1 is literally back scattering, 1 is forward scattering, and intermediate values are off-axis.

So how much scatter should a realistic skin shader have? What's the consequence of going too far towards -1.0? And how does reorienting the direction solve the visible UV seams issue?

Anim

margrave said:

And how does reorienting the direction solve the visible UV seams issue?

I am not sure if the direction does help at all. According to my tests the Transmitted Color seems to be the root cause. But I only tested with the standard ubershader not the new skin stuff and only with a simple test scene. It might be that it does not work for other situations.

As long as the Transmitted Color is pure gray (R,G,B channels are all set the same) it seems to be ok.

Anim

Regarding the SSS Direction:

I am trying to understand that as well.

• it is most obvious if the facing surface does not receive much direct light and the angle of the incoming light is relevant as well
• higher values emphasise the transmitted color (at 1 the scattered color seems to be gone)
• lower values emphasise the scattered color
• it reacts to a lot of the other surface settings

I don't have an answer for a "standard" value regarding skin. Some write -0.63 but I think the textures and the other SSS and translucency setting have such a heavy influence that there is not a standard value at all.

The filenames of the attached renders explain the change in the sss direction. E.g. 001_sss-dir_-1.0.jpg ".  ... dir_-1.0" ... means SSS Direction = -1

 

nonesuch00

So if it's scatters towards the light that means if reflective and aways from the light refractive? Or at least superficially seeming so?

margrave

Anim said:

Regarding the SSS Direction:

I am trying to understand that as well.

• it is most obvious if the facing surface does not receive much direct light and the angle of the incoming light is relevant as well
• higher values emphasise the transmitted color (at 1 the scattered color seems to be gone)
• lower values emphasise the scattered color
• it reacts to a lot of the other surface settings

I don't have an answer for a "standard" value regarding skin. Some write -0.63 but I think the textures and the other SSS and translucency setting have such a heavy influence that there is not a standard value at all.

The filenames of the attached renders explain the change in the sss direction. E.g. 001_sss-dir_-1.0.jpg ".  ... dir_-1.0" ... means SSS Direction = -1

I've been doing more research, and based on my (admittedly-limited) knowledge of SSS I think "SSS Direction" is sort-of like the dot product of the light rays hitting the skin.

At 1.0, they go straight through with no scattering.

At 0.0, they are perpendicular to the light rays, and aligned with the surface they're hitting, forming a circle with maximum scatter.

At -1.0, they are perfectly reflected and travel straight back out with no scattering.

If that's correct, then I think--think-- the ugly SSS seams are the result of the scattering. Based on some Blender answers for a similar problem and some griping about Iray bugs on other softwares' message boards, I don't think Iray recognizes that the surfaces are supposed to be one mesh. It thinks the arm ends at the shoulder, and that blue seam is caused by the scattering bleeding past where Iray thinks the arm "ends", as if there was an edge there. Hence, when you set the SSS Direction to -1.0, there's no seams because all the light rays are being reflected exactly back the way they came without any scattering.

margrave

Anim said:

margrave said:

And how does reorienting the direction solve the visible UV seams issue?

I am not sure if the direction does help at all. According to my tests the Transmitted Color seems to be the root cause. But I only tested with the standard ubershader not the new skin stuff and only with a simple test scene. It might be that it does not work for other situations.

As long as the Transmitted Color is pure gray (R,G,B channels are all set the same) it seems to be ok.

I tried the (0.99, 0.99, 0.99) trick with Aiko 8, but unfortunately it made her face no pasty white with vivid red lips like a geisha. So unless I figure out how to counter that, it's a no-go, I'm afraid. 

Anim

Hence, when you set the SSS Direction to -1.0, there's no seams because all the light rays are being reflected exactly back the way they came without any scattering.

I am not sure but there seems to be more involved. In the below render the dir is set to -1 but the seam is still there if transmitted is not a pure gray walue.

Which shader are you using - iray uber or the new one that came with G8.1 ?

margrave

@Anim I used the default Aiko 8 materials, and I also tried it with an iSourceTextures figure, Ingeborg. I'll have to experiment with it some more when I get a chancd tomorrow.

Paintbox

This article is very enlightening :

https://www.fxguide.com/fxfeatured/pixar-deep-dive-on-sss-siggraph-preview/

All about SSS

Anim

Informative article. I read it in the past but forgot about it. This time I bookmarked it and understood what it is about

 

Sevrin

I picked up Skin Shading Essentials Tutorial | Daz 3D a while back, and it got into a great amount of detail about what all the various settings do and how they interact with each other.  Turns out there's a lot to this skin business.

nonesuch00

Anim said:

Regarding the SSS Direction:

I am trying to understand that as well.

• it is most obvious if the facing surface does not receive much direct light and the angle of the incoming light is relevant as well
• higher values emphasise the transmitted color (at 1 the scattered color seems to be gone)
• lower values emphasise the scattered color
• it reacts to a lot of the other surface settings

I don't have an answer for a "standard" value regarding skin. Some write -0.63 but I think the textures and the other SSS and translucency setting have such a heavy influence that there is not a standard value at all.

The filenames of the attached renders explain the change in the sss direction. E.g. 001_sss-dir_-1.0.jpg ".  ... dir_-1.0" ... means SSS Direction = -1

Oh, so it is like I thought. Very good explanation and examples you made!

Anim

Sevrin said:

I picked up Skin Shading Essentials Tutorial | Daz 3D a while back, and it got into a great amount of detail about what all the various settings do and how they interact with each other.  Turns out there's a lot to this skin business.

Yes indeed  The more I test the more questions I have. But I start to get it. Its quite interesting how the shader parts work. And it seems the way they influence each other is complex but not a miracle (the MDL handbook helped to get a more in depth understanding http://mdlhandbook.com/).

Anim

nonesuch00 said:

Oh, so it is like I thought. Very good explanation and examples you made!

Thanks, I could share the scene if somebody is interested and tells me how to export it so other can use it. I found save as scene asset but am not sure if that works regarding the obj files and pathes.

cajhin

Interesting topic, and thanks for the renders.

I can only contribute link to the source spec: https://raytracing-docs.nvidia.com/iray/mdl/specification/MDL_spec_1.6.2_05Aug2020.pdf

It says in 21.3 Volume Scattering: "directional_bias– Influence of light direction on scattering. Range:[−1,1], with 0 specifying isotropic behavior,1 forward scattering, and −1 back scattering."

My understanding: this is a bias, so it sets a probability for the direction of a scattered light ray. 0 does *not* mean 'no scattering', but 'scattered totally randomly'. -1/1 reduces SSS to simple transmission(?)

Anim

According to my tests scattering always happens. Even at dir = +1 or -1 the sss and translucency have influence. It never acts like transmission without scattering. The dir indeed controls whether the light scatters away from the light source or to the light source. At -1 it scatters to the light, at +1 it scatters away from the light and values inbetween mix the direction accordingly. I think that is also what you said, right?

This testing is boring (and I am still scratching my head about some things), but I start to have fun in making materials with less guessing and endless trial and error