I guess there are a number of things that cause that. The most intended part of the effect is that I use anti-aliasing filter and sometimes an overlay of 0.5px blurred mid-tone masks on the sprite sheets before having GIMP generate the map. Since they're trying to generate for model textures they treat everything like a bump, but all it takes to get a standing sprite effect like this is to apply a convex curve on the green channel and presto the light goes behind the character / prop