i was experimenting with NPR rendering recently and found this issue in the method i use for outlines, i just can't get my head round it.
So, what i did was to use the solidify modifier with flipped normals along with a material that has backface culling on to get a nice real time outline on my model.
However this caused the backfaces of the flat geometry used for eyeline and eyebrow stuff to get completely filled with the outline material. I tried using vertex groups to exclude those vertices from the effect of the modifier but it doesn't appear to work, the modifier still creates another layer of geometry with the outline material, just not as thick.
I also tried to use the knife tool to cut out only the important parts of the texture but that ridiculously breaks completely the shape keys of the whole face.
I don't know what to try anymore, how can i NOT get the eyeline affected by the modifier like this?