I've been looking into various ways to draw wide lines. A couple of times I have run across references to rendering rounded endcaps in the fragment shader, instead of with geometry (by adding a little fan of triangles forming a half-circle). But I haven't seen any concrete examples of how to do this. I get that you would measure the distance away from the line endpoint, and anything beyond that is discarded, which would form a round shape. But how do you do that in practice? I've only used fragment shaders to deal with color and lighting, not anything using distance or position.