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In this movie we are going to talk about some ways that you can create the look of realistic human skin using the mental ray Subsurface Scattering shaders. So in this scene I have a character model just a head of a character. and at the moment he has got just the default lambert applied to his surface of his skin. The lighting in this scene is fairly simple. Let's take a look at it really quick though, because it is important. I have an areaLight, which is creating the main key light on the surface of the front of the character.
I have fill light here which is just a directionalLight with a very low intensity value and just a slight bluish color to it, and then most importantly I have a light here which is a directionalLight and it's lighting the character from the back, and this is going to be an important element to creating the realistic skin, as you see as we get into creating the shader and adjusting the settings. Lastly, in the camera1 attributes under Environment, we have the background set to just a bluish color, just so we can see the objects a bit easier.
I'll do a rendering. You'll see that the character at the moment looks fairly simple. And here he is with just a default lambert applied to the skin. I'll minimize the Render View and let's take a look in the Hypershade, and see how we can start creating one of these shaders. So go to Window > Rendering Editors > Hypershade, and under mental ray > Materials, if you scroll down you will find there are number of shaders that use the misss prefix, and this stands for the mental images subsurface scattering shader.
So any one of these shaders will create the subsurface scattering effect. and some of the shaders are very simple and some of them are more complex. The misss physical shader is the most complex and that's really only used for objects like very thick candles or jade or things like that, when you really need an actually physically accurate subsurface scattering effect. But the one that I like to use the most is the misss_fast_skin_maya shader, because this is the simplest to use.
It does have more settings than some of the others, but I find it to be the most reliable. So I'm going to click on this shader to create the node and I'll select the character's head, right-click over the misss_fast_skin_shader, and choose Assign Material To Selection. I am going to select the shader and in the Attribute Editor, let's just rename this oldmanSkin, so we know exactly what it is.
So I would like to create a test render to see what the default settings look like. So let's close the Hypershade for a moment and do another test under. So the way subsurface scattering works in the real world is photons of light penetrate the outer layers of the surface, bounce around inside and then leave again, and they pick up some of the color of the surface as they leave again. In human skin you see it is as sort of this redness, especially in the areas that are strongly lit from the back or thin areas like the ears. Now the default settings right here tend to make the surface look very plastic, so we can start to adjust this to make it look a little bit more realistic.
So I have got the Attribute Editor open for the oldmanSkin shader. Let's take a look at some of these settings. At the top we have the Diffuse Layer. This is the outermost layer of the skin and this is where most of the light bounces off the skin and back into the environment. So this is where you put the basic skin color, adjust the ambience, the strength of the diffuse color and so on and so forth. But the settings that control the actual subsurface scattering quality of the shader are found in the subsurface scattering layers, and for the skin shader, we have three layers.
So when you're working with the shader, what you want to think about is think about the layers of skin. Skin covers up all our muscles and bones and keeps us together for the most part when we are walking down the street. So it's very useful in that respect. But you can sort of think about it as building up layers on top of things like muscle and bone. When I am designing the shader, I think about it from the bottom up, from the innermost layer of the skin and up to the outermost layer of the skin. And the way the shader is organized, is the innermost layers are down here at the bottom, and as we go up in the Attribute Editor, we moved outwards to the outermost layers.
So the Diffuse layer is the outermost layer, and the Back Scatter Color is the innermost layer. The Back Scatter Color is most affected by strong backlighting. So the directional light that I have here, that's shining in the back of the character is going to interact with that back scattering color the most. So let's select the character and go back to the oldmanSkin shader and the way I like to do this design in the shader is I want to turn everything off that I can and work with one channel at a time, and slowly build up the look of the shader that way.
So the first thing I am going to do is I am going to go down to the Specularity layer, this is the specular head highlights in the surface of the shader, and I set Overall Weight to zero. So this turns Specularity off and you can see that in the preview. There's no more specularity. I am going to leave Back Scattering the way it is, but I'm going to set the Subdermal Scatter Weight to zero so that effectively turns that off, and I am going to set the Epidermal Scatter Weight to zero that turns that off, and I'm going to turn the Diffuse Weight to zero.
That turns that off. Think of these weight attributes as kind of like volume knobs for each one of these channels. So, now they're all set to zero with the exception of the Back Scatter Color, so let's do a test render and see how it looks. So this rendering gives you good idea of how back scattering works. It's picking up the color from the directional light that's shining in the back of the character's head and you can see the redness come through. In the thinner parts of the surface we get more red and the thicker parts of the surface, it's darker color.
The tip of the nose of course has the most amount of matter behind it, so it's the darkest. If you started to rotate the camera and had it shining from the profile, the nose might be redder, but from this moment since we are looking at it straight on, that's going to be the darkest. So if we look at the settings, the way this works is as I mentioned before, the Weight setting is kind of like the volume knob. So right now that's set to 0.5. So in terms of the overall shader, this is how much of the contribution the Back Scatter Weight is giving to the overall look of the shader. Now the Depth and the Radius are also important to understand.
The Back Scatter Depth determines how far this red color is going to penetrate the surface. So in other words, when this is set to 25, this is how much the red color is going to come through the entire character. So as I lower this value, I am going to set this down to 10. What's going to happen is we are going to see the thicker parts are going to become even darker, and that it's only going to be the thinner parts that we are going to see this red color. Now the Radius setting, you can sort of think about this as if I rotate the view and imagine light hitting the surface from the back, think about as the photons of light are hitting the surface and browsing around within the surface, think about the radius as about how far they spread as they go through the surface.
So this is kind of like a spread value, and I want to lower this down a little bit too, because right now I think it's a bit too strong. So I want to lower this down to let's say about 8. I usually think about this in terms of same units. So if the grid is showing this is 10, same units, then I'm thinking about the Radius is being in about a volume of about 8 units, and same with the Depth. So let's do another rendering. So as you can see by lowering those values, the Depth and the Radius, we get more of a subtle shading here on the ears.
This look is starting to look a little bit more realistic in terms of how it actually affects human skin. But of course, if you make it too subtle then it's going to disappear altogether. So what I might do is increase the Back Scatter Weight to about .8 and let's give it a little bit more of a Radius to it. I will put this up to about 10 as well. So once you understand how these works, the Depth being how far the light penetrates the surface and the Radius being spread across the surface, then you'll understand how these other settings work, because it's basically the same way.
Once again, each one of these is layers. So this is the innermost layer and now this is the next layer up. the Subdermal Scatter layer. Someone talking about human skin, I think about this layer of being like muscle and bone, and this layer being the deepest layer of the skin where you might see like veins and that kind of thing. So again, I am going to turn the volume up on this, so I am going to increase the Subdermal Scatter Weight to let's say, I'm going to start with 0.5 and generally when I'm working with the shader, I will just start plugging in when I think it would be appropriate values and adjusting from there.
The main difference between the Subdermal Scatter settings and the Back Scatter settings is there is no Depth setting; there is only Radius. So I am going to lower this down to about, let's do the setting of 10, and I am going to keep these colors to where they for the moment, but that's another thing that you can adjust as you are tuning the skin. You can start to adjust the colors. It's a bit heavy-handed, but you can easily see now how the two colors are working together, so they're being added together, which is helping to increase the brightness on the ears, maybe a little bit too much but you can also see how these colors are combining on the various parts of the surface.
So what I might do is lower this Weight down to about 35. I tend to like to have a higher Back Scattering Weight, because I like that back scattering effect, and I lower the Subdermal Scatter Weight a little bit, and I might bring the Radius down to about 8, and let's darken the Back Scatter Color a little bit and darken the Subdermal Scatter Color a little bit. Once you understand how they work, it's just a matter of tweaking to get the settings that you want. The Epidermal Scatter Color is the part of surface that's just below the very top of the skin.
It's just going to usually have a lighter color, and again with this one I like to have a bit lower value than the other two. So I might set this to about 5, just to start with, and as terms of the weight, let's do 25 and see how that looks. Now you can see how the three layers are starting to work together to create the subsurface scattering effect. Once again, a little bit heavy-handed, but at this point I'm mostly interested in making this as obvious as possible, so you can see how these things work.
It really takes a bit of tweaking to get the setting that you want. Finally, at the top we have the Diffuse Weight, and this is the color that directly interacts with the lights at the very surface of the skin. So this is where you think about things like freckles and moles and that kind of stuff, they are actual surface color of the skin, and I am going to set this to about 35, do another rendering. And there you can see how they're working together. The last thing that I like to adjust as I'm designing the shader is the Specularity settings and this is going to determine the shininess of the skin.
Now what's interesting about the shader is that it essentially has two different specular channels. We have a Primary Specular Color and a Secondary Specular Color, and then an Overall Specular Weight. So this is going to be the overall volume knob for Specularity. So let's put this up to about 0.5 or maybe a little bit higher. Let's put it at 0.8. What I want to point out about the way these two work is if you can think about this as being like the broad specular highlights, well, imagine the light that is shining on the surface of the skin. This specular highlight is going to be fairly broad and spread out across the skin as it reflects the light in the room.
The Secondary Specular Color, think about this is adding kind of the quality of like oily skin or wet skin. So you have a very shiny type of highlight on top of a broader highlight and that's basically the way these two works. As you can adjust their Weight and their Shininess. Higher values are going to make these more shiny. So if I set this up to 50 and increase the Weight up to 0.5, what we are going to get is a wetter looking skin. If I lower this, lower the Shininess setting, lower the Weight, we are going to get more of a duller skin.
So depends on the effect that you're trying to achieve, and by default this has a slight bluish color and it kind of looks like the skin might in like a cool lighting setting. It has a nice contrast to the warmer colors to the rest of the skin. So I am going to render this and look at the end result and of course, we have a ways to go before we create generally realistic skin, but it's important to understand how the shader works, because in the next step, what you would want to do is start to plug textures into the various channels to start to control the overall colors of each channel as you design the skin.
So you can see here is this specular coloring and you can see this is nice and shiny in the waxy parts of the ear and on the other parts of the face. So once you have established the overall values for the shader, the next step would be to go in and start plugging textures into these various color channels to start to fine-tune the look of it. So rather than having these sort of unrealistic colors, you can have a much more sophisticated texture in there controlling the color for each channel.
That's generally the way that I go about designing the shader.
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