The material samples the depth buffer to find the edges of objects and place an outline around them that thins with distance. An in-depth breakdown is available for the basic method and depth biased method. The materials are available in upk format and should be added to the post process chain. The environments used to test this were created by Patrick Loughman and Epic Games.

Basic Method
(Lines all the same )

Depth Biased Method
(Line weight decreases by distance)

Material nodes breakdown for basic effect
(click for larger image)