Holy shit...I think I figured out how they are doing this and its definitely not a bullshot if so.
- render your model with its normal texturing in a front and back view.
- Take screens into photoshop, duplicate, turn black and white and up the threshold to get the crazy black and whites you need for the comic book contrasting blacks and clone that layer.
- Take the cloned threshold layers and save them out as various maps like diffuse, specular, etc...generally these are black and white maps that tell the engine how to light, or shade things based on whether its black or white. Some of these effects only appear in direct lighting too like with specular maps.
- Trace over the original renders 2d Pencil detailing like the hatching and such on anywhere you want detailing to be more prominent. This includes various outlines. Duplicate this file and use one on top of your UV maps for the model's texture map. Use the other as a bit of an extra for your specular. Likely with a lot of opacity so it doesn't always show and look traced on. This way you can use an outliner option in your engine to generate outlines, but use the specular to add a bit more emphasis on the areas you want with these traced effects.
- Create ramp shaders based of the threshold layer duplicates and map these with additional low opacity so that they flow as a black and white shadow/lighting effect as light passes over the model on don't look painted on. This is the only part I feel may not be necessary because you can hook a ramp shader up to the engine lighting nodes and generate this effect quite possibly more cleanly.
Trace/Threshold tactics are something I've seen folks do with a 3d reference model for 2d images to create amazingly detailed Cel Shaded looking comics, but it never occurred to me till now looking at it that this may be a way to apply those results backwards onto the model's maps for animation.
Gonna try this. I think I'm right or at least in the ballpark on how to generate these results. I'll work on this to see if it works out well or needs more or less maps/techniques to pull it along.
EDIT: Found it! I remembered a guy using this stuff before for a 3d to 2d work flow.
This is what I think they have adapted as a technique to create maps for 3d cel shading of this level
Basically trace in the line work, save some of it for texture maps and other layers of it for specular/diffuse map assistance to help the toon shader outline tools to gain line weight. Setup either you own ramp shaded maps for musculature from threshold layers converted to specular/diffuse maps or if it looks too painted on even as a highly opaque map just rely on rampshader node properties added to your light nodes in the engine. Should only have to draw in details on anything inside the general outline of the character and the engine should generate the rest of your lines on its own and even get assistance from any opaque diffuse/Specular maps you throw in.
Gotta go. Zbrush is calling me.