SSS talk

Comments

• Rashad Carter Posts: 1,830

  September 2019

  These conversations always intrigue me. I think these types of threads bsaically requre images to accompany them at all stages. Do you have any test images?
• SnowSultan Posts: 3,778

  September 2019

  I don't have any test images because I'm really just hoping to understand why these settings are the way they are. I managed to make some decent corrections to one skin texture yesterday, but it was mostly just by making small adjustments to the Translucency and Dual Lobe specularity values. Sometimes you can tell that even vendors aren't *exactly* sure how to set their values (like when both the Dual Lobe and normal reflectivity dials are above 0, or when they copy the DAZ SSS values exactly), and I think knowing how these things work would help more than just my curiosity.   :)
• JonnyRay Posts: 1,744

  September 2019 edited September 2019

  I'll give this a shot based on what I believe is going on. Don't ask me for references because I can't recall where I heard / read / learned any of this, it's just how it appears to be working from my point of view. I've whipped up a quick diagram of light interacting with a skin surface.

  

  1. I believe that translucency is affecting how much light is passing through the skin surface to be processed by the rest of the model. This is why they use light colors, they're trying to pass as much light to the SSS and Transmit layers as possible, but allowing some of it to be either reflected (diffuse and specular) or absorbed by the skin surface.

  2. Transmit and scattering distances sound about right. Transmit especially gets really weird with high values as it starts to pass light completely through the body parts. As for the SSS direction, according to Sickleyield's tutorial on DA...

  SSS Direction tells the shader which direction the light is scattered, toward or away from a light source. Negative numbers go toward the light source, and positive numbers scatter away from it (0 will result in almost no scattering).

  Technically, as I understood it, it's actually a modification to the vector used to determine the angle from which the scattered light is gathered. In my diagram the thin black line shows a zero deviation from the "perfect" reflection angle. A negative value would pull the reflection point back to the left while a positive one would push it to the right. Kind of like and angle of refraction. But I could be wrong on that. Personally, if I were a 3rd party reading this, I'd trust Sickleyield more than JonnyRay on this one. :)

  3. For the transmitted and scattering colors, consider the diagram. Scattered light is filtered by the translucancy of the skin, then adjusted by the color in the SSS channel and goes back out through the skin again. Transmitted light has already passed through the surface on the other side (translucancy) bypassed the other side's scattering layer, then transmits and exits the other side. In both cases the light is being multiplied against the translucency and then is weighted against the reflected (diffuse and specular), so it can have a radical effect, especially since many people put the same diffuse map in the SSS color and Transmit color channels (which is a mistake) so they're multiplying the effects of that map significantly.

  

  

  IraySkinModel.png

  815 x 728 - 49K

  Post edited by JonnyRay on September 2019
• SnowSultan Posts: 3,778

  September 2019

  Thank you for that Jonny, I can't say I understand all of it, but you certainly sound like you know what you're talking about.   :)

  1. Ah, I probably forgot that Translucency maps were acting as masks and not color maps. Sometimes I see Diffuse maps put in there and that's likely what made me think that you can also set the underlying color with the map as well.

  2. So leaving the angle at 0 allows light to behave in a physically correct manner, while changing that value forces it to scatter in one way or another? That would make sense.

  3. I guess the thing I'm still not understanding about all of this is since that the mesh itself is only pixel-thin, where are these thickness values coming from? Is that what the distance values are for, to artificially set the thickness and colors of the model's interior to influence light behavior? Does this mean the colors on your graphic also represent the colors set in those values, and why even a small change to them gets multiplied so much? Guess we should just always leave those alone, haha.

  Thanks again for the detailed explanation. This is complicated stuff but pretty important to try and understand.
• nonesuch00 Posts: 18,744

  September 2019

  SnowSultan said:

  Thank you for that Jonny, I can't say I understand all of it, but you certainly sound like you know what you're talking about.   :)

  1. Ah, I probably forgot that Translucency maps were acting as masks and not color maps. Sometimes I see Diffuse maps put in there and that's likely what made me think that you can also set the underlying color with the map as well.

  2. So leaving the angle at 0 allows light to behave in a physically correct manner, while changing that value forces it to scatter in one way or another? That would make sense.

  3. I guess the thing I'm still not understanding about all of this is since that the mesh itself is only pixel-thin, where are these thickness values coming from? Is that what the distance values are for, to artificially set the thickness and colors of the model's interior to influence light behavior? Does this mean the colors on your graphic also represent the colors set in those values, and why even a small change to them gets multiplied so much? Guess we should just always leave those alone, haha.

  Thanks again for the detailed explanation. This is complicated stuff but pretty important to try and understand.

  I don't think the transmitted part of these model's skin shader materials is quite right, ad least not in the Genesis 3 settings, because I've had some HRDIs make the entire model translucent like ears with bright light behind them.
• JonnyRay Posts: 1,744

  September 2019 edited September 2019

  DOH! I can't believe I neglected to mention @V3Digitime's awesome Skin Shading Essentials Tutorial! Also, the Skin Shading Bundle which includes her Ultimate Iray Skin Manager script interface. I use this ALL the time when adjusting skins because it breaks things down so logically and is a real time saver when I'm adjusting skin values.

  SnowSultan said:

  Thank you for that Jonny, I can't say I understand all of it, but you certainly sound like you know what you're talking about.   :)

  [snip]

  3. I guess the thing I'm still not understanding about all of this is since that the mesh itself is only pixel-thin, where are these thickness values coming from? Is that what the distance values are for, to artificially set the thickness and colors of the model's interior to influence light behavior? Does this mean the colors on your graphic also represent the colors set in those values, and why even a small change to them gets multiplied so much? Guess we should just always leave those alone, haha.

  Thanks again for the detailed explanation. This is complicated stuff but pretty important to try and understand.

  The surface of the mesh is infinitely thin, however these are volumetric settings; so the render is taking into account the whole volume of the figure. This is how it can make things like ears and fingertips more transmissive, while keeping the torso more solid. I recall seeing a good video on explaining how transmission (in particular) works. It is a strange property as "transmission distance" is basically saying "this is how far into the object light needs to travel before it takes on the transmission color properties". It's actually easier to see in a simple example with a sphere.

  [Edit : here it is, it's part of Daz's Iray Uber Base Shader video. I've linked to where they start talking about Transmission, but the whole video is worth a watch. ]

  It's also why things like SSS and Transmission only work when Thin Walled is turned off. Thin Walled tells Iray that it's dealing with a single, thin surface; so there is no depth to it to work with. REALLY strange things can happen when Thin Walled is turned OFF, but there isn't actually any volume to the object (like with a single simple plane primative). Basically in this case, Iray treats everything behind that surface as if it was within the object for purposes of calculating volumetric properties like SSS and Transmission.

  nonesuch00 said:

  SnowSultan said:

  Thank you for that Jonny, I can't say I understand all of it, but you certainly sound like you know what you're talking about.   :)

  1. Ah, I probably forgot that Translucency maps were acting as masks and not color maps. Sometimes I see Diffuse maps put in there and that's likely what made me think that you can also set the underlying color with the map as well.

  2. So leaving the angle at 0 allows light to behave in a physically correct manner, while changing that value forces it to scatter in one way or another? That would make sense.

  3. I guess the thing I'm still not understanding about all of this is since that the mesh itself is only pixel-thin, where are these thickness values coming from? Is that what the distance values are for, to artificially set the thickness and colors of the model's interior to influence light behavior? Does this mean the colors on your graphic also represent the colors set in those values, and why even a small change to them gets multiplied so much? Guess we should just always leave those alone, haha.

  Thanks again for the detailed explanation. This is complicated stuff but pretty important to try and understand.

  I don't think the transmitted part of these model's skin shader materials is quite right, ad least not in the Genesis 3 settings, because I've had some HRDIs make the entire model translucent like ears with bright light behind them.

  I totally agree. I've seen some very bizzare settings before where I've had to question if the person who set them up actually understood how the Uber shader works. They may look okay in the right light, but they can look bizzare once you start changing from simple, low contrast, flat lighting that most vendors use for figure setup. During V3D's tutorial that I linked above, she even points out a couple of places where common mistakes are made (like with the glossy settings).

  Post edited by JonnyRay on September 2019
• SnowSultan Posts: 3,778

  September 2019

  I'll check a few of those links soon Jonny, thanks again.

  because I've had some HRDIs make the entire model translucent like ears with bright light behind them.

  Yeah I've had this too, which is why I still wonder why we aren't using hand-painted translucency maps that are mostly dark gray with light areas on the ears and such. 
• magog_a4eb71ab Posts: 769

  September 2019

  JonnyRay said:

  DOH! I can't believe I neglected to mention @V3Digitime's awesome Skin Shading Essentials Tutorial! Also, the Skin Shading Bundle which includes her Ultimate Iray Skin Manager script interface. I use this ALL the time when adjusting skins because it breaks things down so logically and is a real time saver when I'm adjusting skin values.

  SnowSultan said:

  Thank you for that Jonny, I can't say I understand all of it, but you certainly sound like you know what you're talking about.   :)

  [snip]

  3. I guess the thing I'm still not understanding about all of this is since that the mesh itself is only pixel-thin, where are these thickness values coming from? Is that what the distance values are for, to artificially set the thickness and colors of the model's interior to influence light behavior? Does this mean the colors on your graphic also represent the colors set in those values, and why even a small change to them gets multiplied so much? Guess we should just always leave those alone, haha.

  Thanks again for the detailed explanation. This is complicated stuff but pretty important to try and understand.

  The surface of the mesh is infinitely thin, however these are volumetric settings; so the render is taking into account the whole volume of the figure. This is how it can make things like ears and fingertips more transmissive, while keeping the torso more solid. I recall seeing a good video on explaining how transmission (in particular) works. It is a strange property as "transmission distance" is basically saying "this is how far into the object light needs to travel before it takes on the transmission color properties". It's actually easier to see in a simple example with a sphere.

  [Edit : here it is, it's part of Daz's Iray Uber Base Shader video. I've linked to where they start talking about Transmission, but the whole video is worth a watch. ]

  It's also why things like SSS and Transmission only work when Thin Walled is turned off. Thin Walled tells Iray that it's dealing with a single, thin surface; so there is no depth to it to work with. REALLY strange things can happen when Thin Walled is turned OFF, but there isn't actually any volume to the object (like with a single simple plane primative). Basically in this case, Iray treats everything behind that surface as if it was within the object for purposes of calculating volumetric properties like SSS and Transmission.

  nonesuch00 said:

  SnowSultan said:

  Thank you for that Jonny, I can't say I understand all of it, but you certainly sound like you know what you're talking about.   :)

  1. Ah, I probably forgot that Translucency maps were acting as masks and not color maps. Sometimes I see Diffuse maps put in there and that's likely what made me think that you can also set the underlying color with the map as well.

  2. So leaving the angle at 0 allows light to behave in a physically correct manner, while changing that value forces it to scatter in one way or another? That would make sense.

  3. I guess the thing I'm still not understanding about all of this is since that the mesh itself is only pixel-thin, where are these thickness values coming from? Is that what the distance values are for, to artificially set the thickness and colors of the model's interior to influence light behavior? Does this mean the colors on your graphic also represent the colors set in those values, and why even a small change to them gets multiplied so much? Guess we should just always leave those alone, haha.

  Thanks again for the detailed explanation. This is complicated stuff but pretty important to try and understand.

  I don't think the transmitted part of these model's skin shader materials is quite right, ad least not in the Genesis 3 settings, because I've had some HRDIs make the entire model translucent like ears with bright light behind them.

  I totally agree. I've seen some very bizzare settings before where I've had to question if the person who set them up actually understood how the Uber shader works. They may look okay in the right light, but they can look bizzare once you start changing from simple, low contrast, flat lighting that most vendors use for figure setup. During V3D's tutorial that I linked above, she even points out a couple of places where common mistakes are made (like with the glossy settings).

  That's pretty much the case imo. I forget which G8F character it was, it might have been Hinako for Mei Lin or a different one, but I was always plagued by black specks with the skin in renders until changing some of the settings. I think some of these issues are just down to incorrect surface settings.

  I remember playing around with the Transmitted Color and SSS Color together a while back and found that the closer they are in similarity, the more transparent the skin becomes in renders.