Outlines contribute a lot to the clarity of an anime-style character. And to add them to their characters ASW uses a technique first used in the year 2000, called Inverted Hull Outlines.
Imagine looking at the scene from the light source's perspective. You'd only ever see the lit surfaces. The unlit ones are never visible and might as well not exist.
In Videogames they use this idea for performance optimization and it’s called Backface Culling. If you apply the same idea to the player camera instead of the light source, any geometry that is not visible to that camera doesn’t get rendered.
Videogames simplify this calculation by only focusing on the Normal Direction of the object. Technically each Face has two Normal Directions because you can look at it from two directions. To separate them, one side is called “Frontface” and the other “Backface”. On a closed mesh, the Frontface is visible from the outside, and the Backface is visible from the inside.
Using backface culling, videogames save performance by making the Backfaces permanently transparent, because you can’t look inside most models anyway.
But what if you invert the Normal Direction while backface culling is enabled? All of a sudden, you can see the inside of the model, because the Backfaces are now on the outside.
Now combine the original mesh with this inverted copy and make the copy slightly bigger. The inverted version is only visible around the edges of the original. Add a black color to the copy and you have an outline.
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This method has existed since Jet Set Radio (published in the year 2000).
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ASW chose Inverted Hull over post-process outlines for two reasons: