If you don't have an explicit extension to do this, it's really a pain.ĢD graphics simply wants very different graphic and computational primitives compared to 3D. Or, if you want simpler, just draw a line n pixels wide. Whereas, you can divide the rectangle into chunks, send each chunk through the compute pipeline, and it's stupidly straightforward. You have to specify 4 triangles, make sure they don't overlap, specify them in a particular vertex order or annotate them with extra data so you only draw one of the three edges, make sure that you only draw each pixel just once or your blending goes all to crap, etc. On of my favorite examples is drawing a screen/pixel-space rectangle and then drawing a border of the rectangle in a different color and specifying the number of pixels that border should be wide. You can work around the limitations in vertex or fragment shaders, but, in many cases, you're having to unwind a lot of what the vertex and fragment processing does.
Only interpolating based on the edge? Needing to carefully map float coordinates so you don't wind up with fractional pixels mapping incorrectly? Inability to look at pixels to each side to compute something?
