I am making a 3D RTS game with Direct3D. Basically, I want to make 2D health bars hovering over my units that show how many hitpoints they have left after receiving wounds. I'm not sure what a good way to go about this would be because the thing is, the health bars have to always face the camera, no matter what angle they are at from the center of the viewport. I also want them to look as close to perfectly rectangular as possible, but I want them to look smaller if they are farther away.
I don't want to post code, so I'm just looking for a "general concept" type of answer.