Using a basic render of a scene from a 3D package to paint over is a very effective way of creating concepts. However, for those that aren’t familiar with 3D software or don’t have the time to learn a new program, painting normal maps and using 3D layers in Photoshop can be a very handy alternative.
In this tutorial, veteran CG artist Nick Lewis shows how to hand-paint normal maps in photoshop and use them in a 3D layer. Instead of using the normal map as a texture, it’s used to create the full form of the object or scene. As the normal map is added to a 3D layer, it can be lit, textured and painted over within photoshop without having to switch between software packages.
See also these previous tutorials from Nick Lewis about the same subject
Painting with Normals in Photoshop – The Basics
Mech created using Hand-Painted Normal Maps