Step 1
Reference.

Anyone who has held a frog in their hand can tell you that a frog's skin is rough and scaley on the back, but smooth and leathery on their belly. Creating a procedural shader which can automatically "detect" what part of the body it is currently shading, and adjust accordingly is a very powerful technique which can be used in many shaders. This tutorial will show you one method of doing this. You will also learn a simple technique which quickly simulates the effects of ambient occlusion in displaced geometry.

Step 2
Surface Normals.

How can we create a shader that automatically changes from hard and scaly on the top to soft and leathery on the bottom? Simple. We use the Surface_Angle node. This returns the x, y or z component of N (the surface normal).

Step 3
How it works.

If we just use the y component of the surface normal (Ny), we get a value that varies from 1 at the top, to 0 on the side and -1 on the bottom. This is almost exactly what we need.

Step 4
A little math (very little).

We don't want the scales to start tapering off right away. We want them to be very scaley across all of the top and part of the side, with just a little tapering at the end. To do this, we simply multiply Ny by three. Then, to prevent our value from going outside of the 0-1 range, we clamp the result. That wasn't too bad, was it?

Step 5
A simple shader network.

The image at left shows a shader network that implements the ideas discussed so far (click image to enlarge). We are simply using a scaled, clamped Ny to blend between green (Ny >= 1) and blue (Ny <= 0).

Step 6
Displacement.

Now we can take what we have learned so far and build a displacement shader that creates the scales. Click on the image at left to enlarge.

Step 7
Skin.

The shader so far is using a simple diffuse model. Real skin reflects light much differently than this simple model can reproduce (see this paper for details). Fortunately we have provided a shader node which was designed to render skin. Oddly enough, it is called "skin". The image at left shows the effects of replacing the clay node from the shader above, with a skin node.

Step 8
Coloring the skin.

The first thing we will do is change the color of the skin to match the change in texture. The scales will be a dark green, while the underbelly will be a light tan. Click on the image at left to enlarge.

Step 9
Simulating ambient occlusion.

We can use the same procedural function that was used to displace the surface to simulate the effects of ambient occlusion. Basically, deep cracks in the surface will receive less ambient light. In this shader, we have to use another blender node to taper this effect off as the surface wraps around to the belly. Click on the image at left to enlarge.

Step 10
Tweak the shader.

To give the skin a little more "glow", I increased kd to 2.0 and ks to 1.5.