This process involves taking a sketch of the object and using it to model the design in a 3D application. A simple greyscale render is then used in Gimp/Photoshop above set to multiply which creates the lighting and form.
Points of interest:
* Can speed up the painting process, especially for simple designs easily created in 3D.
* Results in crisp paintings.
* Frees up the mind to concentrate on design.
* Is a good guide.
* Creates very good perspective.