OpenGL: Multipass Rendering using Frame Buffer Objects

Question

While developing a small 2D engine, I stumbled across a little design problem which I'm unfortunately with my level of knowledge unable to solve without the help of you.

Let me explain: The game I'm developing the 2D engine for should contain complex effects, which likely would require multiple render passes. The problem is, I'm not sure if I understood the concept of multipass rendering right.

At this level of knowledge, I would implement it this way:

Initially create, setup (attach it to a texture, ...) and bind a frame buffer object
Now render the stuff onto which the effect should be applied to the FBO's attached texture
Render that texture to the same FBO by using the shader for the first pass
Unbind the FBO and render the attached texture to the standard FBO by using the shader for the last pass

The question is now if this is the most efficent way of solving my problem, or if there are any better ways of doing this out there.

I don't understand what step 3 does. Could you clarify that? — Nicol Bolas, Jul 07 '12 at 17:16
I'm still unsure about Step 3. Are you saying that you want to render from the same texture that you're currently rendering *to* as well? — Nicol Bolas, Jul 08 '12 at 17:23
Hm... Initially, yes. But your answer made a problem in my concept clear: I obviously need two textures to switch between, while rendering the different passes, right? — cafaxo, Jul 08 '12 at 18:25
That's not an answer; that's a comment. Answers go down below. — Nicol Bolas, Jul 08 '12 at 18:27

score 2 · Accepted Answer · answered Jul 08 '12 at 18:29

Render that texture to the same FBO by using the shader for the first pass

You cannot simultaneously read from and write to the same image in the same texture. Well, you can, but you get undefined behavior, which probably isn't helpful. So you need to either use multiple textures or perhaps use NV_texture_barrier where available.

OpenGL: Multipass Rendering using Frame Buffer Objects

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