Diffuse SSS

[dsyee]

Trying to figure out exactly what is being simulated with transmission/absorption/scattering. This is the picture I have in my head - can someone tell me if it's right/wrong/inbetween?

I am assuming a diffuse material with some color or image attached to the diffuse texture node. Is this what happens with a ray of full white (RGB 255/255/255) light?

1. Ray hits the surface of the mesh.
2. Diffuse texture color is reflected back (away from the mesh) in some direction.
3. Whatever is LEFTOVER (complement of the diffuse color) is allowed to pass through the mesh.
4. The "leftover" ray is multiplied by the transmission color (as soon as it passes through the surface of the mesh) and travels some distance inside the mesh.
5. The Absorption color is "subtracted" from the ray at some rate dependent on the Scale.
6. The ray has some probability of changing direction, based on the Scattering setting and the Scale. If it changes direction, it is multiplied by the Scattering color (and direction is affected by the Direction setting).

Is that correct? I am wondering, for example:

- If I use the same image for BOTH the diffuse and transmission texture, does that mean there will be NO transmission?
- The Scale is multiplicative, so if for example I set Scale to 100 and Absorption to 0/100/100, is that the same as setting Scale to 200 and Absorption to 0/50/50?
- If the ray exits the mesh (which is typically all we care about), is it affected at all by the diffuse texture?

Any help appreciated!

[face_off]

Your process steps 1-5 are correct in my understanding. Step 6 I'm not sure.

- If I use the same image for BOTH the diffuse and transmission texture, does that mean there will be NO transmission?

Perhaps. I generally set the diffuse image map power to < 1 to get around this.

Your other two question I'm not sure on - sorry. Perhaps someone like Roeland could answer these.

Another complication is I read that either the Transmission, or Scattering/Absorption pins (not sure which) are not UV mapped - so plugging a texture map into them simply takes the color at UV 0,0. But I have not confirmed this. This is something Rikk The Gaijin raised - so perhaps PM him for details.

Paul

[dsyee]

Thanks, Paul. Trying to get finer control over skin materials here.

FYI, I just checked it out, and it seems Transmission works with image maps, but Absorption and Scattering don't. I suppose that makes sense. So, don't waste memory and possibly render time by plugging an image into an Absorption or Scattering node!

[face_off]

FYI, I just checked it out, and it seems Transmission works with image maps, but Absorption and Scattering don't. I suppose that makes sense. So, don't waste memory and possibly render time by plugging an image into an Absorption or Scattering node!

Great to know - thank you. I had issues where eyebrows were getting too much SSS and turning red - so trying to stop that by putting the diffuse map into the scatter pin, but the transmission is a better place for it it seems.

Paul

[dsyee]

That's exactly what I've been trying to do! FYI, since a picture is worth a thousand words (diffuse color set to 0 for all of these):

[face_off]

Given the diffuse texturemap will be in the Transmission pin, I have been setting it up as follows:

Code: Select all

Diffuse = black, or diffuse texturemap (power = 0.1)

Transmission = Color Correction
  texture = multiply
    texture1 = diffuse texturemap (power = 1)
    texture2 = 255/186/100
  brightness_scale = 1.3

What this does is to tint the transmission diffuse map to red, then amplify it so that the scatter node has something to work with.

Paul

[dsyee]

Nice, yes, that's something I've tried, too, and it seems to work pretty well. Depending on the image map, you can sometimes mess with the gamma correction and get something bright enough, so you can just use the multiply node directly. (Not sure how much more "costly" a color-correction is.)

[gazukull]

Heya Paul,

Is this tutorial still relevant http://poserphysics.blogspot.com/2012/1 ... hader.html with all the changes to materials in the 2.0 plugin?

I am looking to redo my SSS shaders under the new system...

Thanks in advance!

-G

[face_off]

Is this tutorial still relevant http://poserphysics.blogspot.com/2012/1 ... hader.html with all the changes to materials in the 2.0 plugin?

Yes, although my current settings have evolved a bit and are still very very WIP. You may be better with RedSpec, but if you want to develop your own......I would recommend the following:

1) Transmission pin -> use a multiply node with texture1 = diffuse texturemap and texture2 = red'ish color. You can put a color correction in there to amplify the effect too if you want (described in a post above).

2) Diffuse pin (on the diffuse material) - reduce the "power" of the texturemap (I've been using 0.2). This is mean there is a reasonable amount of light absorbed so that the scatter/transmission can do it's stuff.

3) I've be using a scatter scale of 50, absorption = 100/255/255 and scattering = 255/0/0. Then I am adjusting the amount of SSS with the Transmission color correction "brightness_scale" mentioned in 1 (currently using a value of 1.4). These are probably not the right levels, but a rough starting point.

4) Glossy material - you can adjust the roughness and specular color with an Falloff Map - so it's roughness = 0.01 for polys facing the camera, and roughness = 1 for polys glancing the camera.

If I ever get around to getting some good results I'll post the shader.

Paul

[gazukull]

As always, thanks much for the reply! It is appreciated.
-G