Why is it important to use three levels of subsurface scattering for human skin?

Mark Dawson, UK Tituoan replies To interpret the skin, a subsurface scattering shader is generally composed of three levels; I prefer to think of them as layers: Epidermal scatter, subdermal scatter and back scatter. To understand their roles and the importance of each of these layers, it’s essential to know what they do and how the lighting impacts and interacts with the shader.

The epidermal scatter is simply the first layer and will represent the epidermis, the first thing you can see Think of the three levels as layers: epidermal scatter, subdermal scatter and back scatter on the skin. That’s where you can put your colour map. The subdermal scatter is the second layer and will represent the depth of the skin; it’s where we’ll find the veins and vessels, for example – that’s why it’s important to create a texture featuring those elements. The third and final layer is the back scatter, and is intimately related to the lighting you have in your scene.

Here you can set the amount as well as the depth of the lighting you need through the skin.

It won’t work properly if you don’t have suitable lighting in your scene and so you’ll need to have a back light set correctly to be able to see the lighting going through and crossing the skin. You also choose the colour of the back scatter in this layer.

At this point you should be able to fully understand the importance of having three layers and what difference it would make if we’d used just two. Each layer has its own function and interacts directly on the other; that’s also why it’s sometimes tricky to set a subsurface shader correctly since each change you do on a layer influences the others layers.

We know that a subsurface shader lets light go through the surface, so it doesn’t make much sense to use it on a car or on a wall, but it can be useful to use different layers for a car paint, for example, to have more control on the reflectivity of the car paint’s surface.

In a nutshell, you should only use subsurface scattering when what you want to render lets the lighting go through the surface.