How does the VRayAlsurfaceMtl work with SSS ?

[stevejjd]

[Introduction and Problem]

Hello,

I'm really struggling to grasp the concept of SSS and how to art direct the exact skin I want.

I did find a diagram over on the Render Man help page But thats for their "Multiple Mean Free Paths" SSS. Is there a diagram of the VRayALSurfaceMtl that we can refer to?

[VRayALSurfaceMtl Scene]

- I made a scene you can download here
- Cube 1 represents the human head scale with 1 cm markers.
- Cube 2 represents the human eye scale with 1 mm markers.
- Back lit primarily for SSS
- An alsurface material is applied to both cubes. you can focus on the small or big cube as you please.

basically I have been trying to control the colors and create a beautiful skin material but because I don't understand what is happening, its impossible to control. Before I continue I wish to show you how I want SSS to be controlled (My pre-conceived notions);



Looking at the diagram you can see that I think, or want to think that controlling the SSS is basically choosing the colors of a gradient ramp. Ultra simple as each layer is the color you choose. Image you have a cube which represents a cutout of human skin from the arm, back or face. We are looking at the side profile so we see the layers.
Layer 01 = SSS 1, oil reflection layer
Layer 02 = SSS 2, skin surface reflection layer
Layer 03 = Diffuse color and texture
Layer 04 = SSS 1
Layer 05 = SSS 2
Layer 06 = SSS 3

The color you assign to the SSS layers will be exactly as you would expect to see. I the diagram I put Red to Blue to Green. So that is exactly the kind of gradient we would see on the cross section of skin. This is how my mind thinks and why its so confusing to try and use real world SSS models like ALSurface.

[Example of Un-intuitive controls]

I'm trying to mimic the gradient in the diagram above - meaning that.....I wish to have 1 color transition to another with gradient blending between but its not working in ALSurface;

- I turnoff all SSS and starting 1 lobe at a time.
- I make the SSS 1 a Blue color with a depth of 1 and it looks fine.
- I make the SSS 2 a Red color with a depth of 2 and it makes everything go purple with a diffuse of Green (WTF?)

The result is a Purple cube with a green diffuse color. - I have no idea why this results in a green diffuse.

Anyway, I'm struggling to explain my problem, and I'm probably just not smart enough to 'get it'. Ill try to clarify more later but this turned out to be so hard to explain. I just want simple SSS controls that are intuitive.

[stevejjd]

Is it possible to connect a gradient ramp to one of the SSS lobes and map the gradient to be depth based?

I just want to try and see if I get the intuitive SSS controls with a gradient ramp where there is color blending not color mixing.

Is this possible?

[stevejjd]

Hi Guys,
This is very hard to explain. Here is another diagram showing how I want to control the SSS

Can SSS be controlled this way? - with gradient ramp points instead of lobes that overlap and add to each-other.

[stevejjd]

here is a diagram of how I suspect ALSurface of working and why I think its hard to control.

Why is additive mixing used? wont that always make the first lobe white/transparent?




Does anyone know how the color is calculated?

I suspect each lobe will blend additively as it overlaps the next.

I think that the value of a color will determine how transparent or opaque it is. Black values will be Opaque and White Values Transparent. I think SSS needs to have translucency to allow light to penetrate down into the skin. White values allow light in and Black values stop them.



I also suspect that the diffuse is calculated by a subtraction blending mode. Isolate the first SSS by turning the other two off. Now enter Red, Green or blue and note the Diffuse color. Its as if there is some kind of subtractive calculation.




A Note on Depth: Its important to understand that Depth is actually "Ray Depth". Before, I raised the issue of color being expressed in a gradient ramp and mapped to the "Depth" coordinate. Now I understand that there is no such "Depth Coordinate" and that , in the case of ALSurface, "Depth" is actually the Depth of the light ray traveling through a volume.

That is why all SSS lobes start at Depth of 0 and end at the specified Depth value. It is Light Depth Based. I would still ask the question, is there a UVW Mapping coordinate based on Volume Depth?

I'm going to do more testing but I think I figured it out. I think I know how to control and art direct Alsurface.

[PIXELBOX_SRO]

Its such a shame this post is being overlooked by the devs :-/

[stevejjd]

hey PIXELBOX_SRO

Thanks for the comment. Yeah I felt a bit embarrised that the post had a lot of views and no comments. Like I farted in a room and everyone pretended it didn't happen haha.

I'm think really starting to understand it now - I will need to do a few more tests but I may be getting the hang of it. Ill update it later

I am not very smart. But the strength of not being smart is that once you understand a complicated thing through extreme effort you are more likely to be able to explain that complex thing to a child

If by any chance I master Alsurface SSS, then I'll make a video tutorial explaining it. That will prevent the horror I had to go through to understand it for others

Also I'm annoyed that one of my uploaded pictures displays as a link and not a picture.

[PIXELBOX_SRO]

this happens to me a lot, recently with a post concerning strange LUT behaviour in VFB.
I dont know why, for some reason it just has been completely overlooked by everyone :-/

It would be great to see how you go on with the Alsurface material!
I probably would not have the patience and time to go deep into it and understand how it works.

[stevejjd]

No worries. The most probable answer is that people are just way to busy to tackle this subject. That's okay though. Im using this public forum to figure the answer out myself and if anyone makes a contribution it would really help. I'm sure Ill nail it down soon. If I do master the concept of this shader Ill definitely share it with a video tutorial.

You know its just so funny, I really pushed for a lot of changes in the VFB - so that you see the final result while look developing.....Well I have almost totally abandoned that stuff. I leave that stuff for Nuke. hahaha crazy. The only thing I do is add filmic tone mapping.

Steve ~

[PIXELBOX_SRO]

No worries. The most probable answer is that people are just way to busy to tackle this subject. That's okay though. Im using this public forum to figure the answer out myself and if anyone makes a contribution it would really help. I'm sure Ill nail it down soon. If I do master the concept of this shader Ill definitely share it with a video tutorial.

You know its just so funny, I really pushed for a lot of changes in the VFB - so that you see the final result while look developing.....Well I have almost totally abandoned that stuff. I leave that stuff for Nuke. hahaha crazy. The only thing I do is add filmic tone mapping.

Steve ~

I took a different approach when tonemapping and postnig renders, i started using Coronas image editor Just open vrays exr in their image editor and you get access to all the nice glares/flares, filmic and LUTs Works like charm and the difference between wha we coudl do with the renders in PS/Arion and what we coudl do in Corona Image editor is simply stunning

Fingers crossed with the Alsurface! keep us in the info loop