Ah, right. So the vertexes on the projected end of the shadows are not shared, each segment is drawn individually. Normally you have two soft edges that line up and cancel each other out. Not exactly, due to precision errors, but very very close. When you use a radius of 0, the error isn't an slight variation in a shade of grey, but either completely on or off. Any precision error has a good chance to make the pixel completely the wrong color. The simplified shader removes all the penumbra calculations so that two segments that share a point will have *exactly* the same output.