title: Possible to draw OpenGL HUD overlay over a different application's window (without flickering)?
body: <p>I need to draw an overlay consisting of lines and text on top of another application. The application in question is a 3D outside world viewpoint, and the overlay is a head up display.</p>
  
  <p>I don't have access to any type of callback from the outside world application to execute draw code in it's draw loop.</p>
  
  <p>Drawing directly over the application's window will result in flickering as the draw loops will not be synchronized, so to me that doesn't seem like an option.</p>
  
  <p>One method I can think of is to capture the outside world application's pixels and stream them into my application, so I can draw the overlay on top in the same draw loop, but that seems very inefficient.</p>
  
  <p>Is there an efficient way to draw over the outside world application without flickering?</p>
  
  <p>Is it possible to draw something over the final graphics card output / at the monitor's refresh rate?</p>
  
  <p>P.s. It doesn't have to be OpenGL, but the HUD is already written in OpenGL so it would make it easier.</p>