OpenGL blend two FBOs

Question

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.

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.

How would I go about doing this with OpenGL? I think glBlendFunc 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.

Answer 1

glBlendFunc allows the application to blend (merge) the output of all your current draw operations (say, X) with the current "display" framebuffer (say, Y) that already exists.

ie,

New display output = X (blend) Y

You can control the blend function by gl as below snippet shows for example:

glEnable(GL_BLEND);
glBlendEquation(GL_FUNC_ADD);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Full usage shown here

https://github.com/prabindh/sgxperf/blob/master/sgxperf_test4.cpp

Note that the concepts of "showing through" and "blending" are a little different, you might just want to stick with "per pixel alpha blending" terminology.

Answer 2

FBOs are just a containers and are not storage. What you need to do is attach a texture target for each FBO and render your output to that texture, once you have done this. You can use your output textures on a fullscreen quad and do whatever you want with your blending.