Portrait Shadow Manipulation is an interesting new technology, hot from the Google labs.
It automatically lifts hard shadows in a head-and-shoulders portrait, these being the shadows of the sort commonly seen in pictures and videos casually made by amateurs in sunny places. Here’s the research video and a useful screenshot from it.
It also removes ‘sweat-shine’ highlights.
As you can see, if plugged into a graphics package, this technology might produce better base-portraits from which to then build comic-like art and line-art. Since what you want for your comics ‘colour flats’ layer, under the lineart, is a fairly ‘flat’ lit face or body. To which you add the shadow-render as a separate and adjustable third layer. Otherwise you’re never going to control the shadows so they display in a believable and uniform way, across the 120 panels of a 28-page comic.
What would be even more interesting would be to go the other way with the new technique, and to ‘auto-cast’ shadows onto fairly flat-lit faces, just by knowing where the eyes / mouth / shoulders are.