title: OpenGL blend two FBOs
body: <p>In a game I'm writing, I have a level, which is properly rendered to the on-screen render buffer provided to me by the OS. I can also render this to a framebuffer, then render this framebuffer onto the output render buffer.</p>
  
  <p>To add a background, I want to render a different scene, an effect, or whatever to a second framebuffer, then have this "show through" wherever the framebuffer containing the level has no pixel set, i.e. the alpha value is 0. I think this is called alpha blending.</p>
  
  <p>How would I go about doing this with OpenGL? I think <code>glBlendFunc</code> could be used to achieve this, but I am not sure how I can couple this with the framebuffer drawing routines to properly achieve the result I want.</p>