Having a photorealistic GPU renderer such as Redshift doesn’t mean that you are only bound to create photorealistic images and animations. Non-photorealistic (npr) stuff is still possible. Ideally, creating a cel shader that will give a more hand painted and pen outlined natural look to a 3D scene isn’t too much different when working with Redshift.
There are two schools when it comes to creating cel shaded rendering. It can be a post process effect, or it can be done in real-time. Christophe Desse is a Lead Technical Artist at Naughty Dog who has offered some insights into how he creates a cel shaded look using Redshift, that is the non-post-process variety.
The setup uses Maya’s PaintFX outlines for the strokes and the old stand-by of using facing ratio modified ramps that are then channeled into the diffuse of a redshift material. Christophe uses a couple “in-camera” effects too, using a bokeh lens effect as well as basic tone mapping.