Blend FBO onto default framebuffer

Question

To clarify, when I say 'default framebuffer' I mean the one provided by the windowing system and what ends up on your monitor.

To improve my rendering speeds for a CAD app, I've managed to separate out the 3D elements from the Qt-handled 2D ones, and they now each render into their own FBO. When the time comes to get them onto the screen, I blit the 3D FBO onto the default FB, and then I want to blend my 2D FBO on top of it.

I've gotten to the blitting part fine, but I can't see how to blend my 2D FBO onto it? Both FBOs are identical in size and format, and they are both the same as the default FB.

I'm sure it's a simple operation, but I can't find anything on the net - presumably I'm missing the right term for what I am trying to do. Although I'm using Qt, I can use native OpenGL commands without issue.

Answer 1

A blit operation is ultimately a pixel copy operation. If you want to layer one image on top of another, you can't blit it. You must instead render a full-screen quad as a texture and use the proper blending parameters for your blending operation.

Answer 2

You can use GL_EXT_framebuffer_blit to blit contents of the framebuffer object to the application framebuffer (or to any other). Although, as the spec states, it is not possible to use blending:

The pixel copy bypasses the fragment pipeline. The only fragment operations which affect the blit are the pixel ownership test and the scissor test.

So any blending means to use fragment shader as suggested. One fullscreen pass with blending should be pretty cheap, I believe there is nothing to worry about.