There is more than one way to create outline or contour renders in 3D applications, and the task in Blender is no different in that regard. Daniel Kreuter notes that each technique has its shortcomings and shares his technique for creating outline or contour renders in Blender that is the easiest and creates the best result.
In this tutorial I’m showing you how I add contours to a rendered image
The key to the entire process is using the Normal and Set Pass of Blender’s Render Engine. The by finding the Normal Direction you can obtain information of the direction of the face, and the Set Pass will provide the distance or Z-Depth information that you can use to generate contours. This is all done by connecting a series of nodes together in Blender, and, if by chance you feel like nodes looks much like a spider’s web, you are able to download the Blender Node Setup for your own use, courtesy of Daniel Kreuter here.