I have not used OpenGL much and don't know if you can actually output 2D device coordinates, but I am sure it supports the same format the hardware uses nativel

I have not used OpenGL much and don't know if you can actually output 2D device coordinates, but I am sure it supports the same format the hardware uses natively - 4D device coordinates. With the 4D device coords it can do things, which make no sense in 2D: clipping against view frustum (e.g. how do you clip against the front plane in 2D?) and perspective mapping.