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It is possible to bind multiple framebuffers and renderbuffers in OpenGL ES? I'm rendering into an offscreen framebuffer/renderbuffer and would prefer to just use my existing render code.

Here's what I'm currently doing:

// create/bind framebuffer and renderbuffer (for screen display)

// render all content

// create/bind framebuffer2 and renderbuffer2 (for off-screen rendering)

// render all content again (would like to skip this)

Here's what I'd like to do:

// create/bind framebuffer and renderbuffer (for screen display)

// create/bind framebuffer2 and renderbuffer2 (for off-screen rendering)

// render all content (only once)

Is this possible?

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You cannot render into multiple framebuffers at once. You might be able to use MRTs to render into multiple render targets (textures/renderbuffers) that belong the same FBO by putting out multiple colors in the fragment shader, but not into multiple FBOs, like an offscreen FBO and the default framebuffer. But if I'm informed correctly, ES doesn't support MRTs at the moment, anyway.

But in your case you still don't need to render the scene twice. If you need it in an offscreen renderbuffer anyway, why don't you just use a texture instead of a renderbuffer to hold the offscreen data (shouldn't make a difference). This way you can just render the scene once into the offscreen buffer (texture) and then display this texture to the screen framebuffer by drawing a simple textured quad with a simple pass-through fragment shader.

Though, in OpenGL ES it may make a difference if you use a renderbuffer or a texture to hold the offscreen data, as ES doesn't have a glGetTexImage. So if you need to copy the offscreen data to the CPU you won't get around glReadPixels and therefore need a renderbuffer. But in this case you still don't need to render the scene twice. You just have to introduce another FBO with a texture attached. So you render the scene once into the texture using this FBO and then render this texture into both the offsrceen FBO and the screen framebuffer. This might still be faster than drawing the whole scene twice, though only evaluation can tell you.

But if you need to copy the data to the CPU for processing, you can also just copy it from the screen framebuffer directly and don't need an offscreen FBO. And if you need the offscreen data for GPU-based processing only, then a texture is better than a renderbuffer anyway. So it might be usefull to reason if you actually need an additional offscreen buffer anyway, if it only contains the same data as the screen framebuffer. This might render the whole problem obsolete.

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9 Comments

@MrDatabase Do you have FBOs in 1.1 (if not, the whole question would be invalid, anyway)? If yes, then this will just work. Instead of the simple pass-through fragment shader, you just render a standard textured quad, disabling anything unneccessary (like lighting) to speed things up.
@MrDatabase And by the way, you shouldn't create the FBO and renderbuffer anew each frame. Create them at the start and just bind them each frame.
@MrDatabase What do you want to do with the offscreen data?
@MrDatabase - Yes, it should work on iOS with 1.1, because FBOs are used even for rendering to the screen in the OS. I don't think you'll even need glReadPixels() for anything here, as you could just bind the render target texture as an input for the onscreen rendering and go. I've done this myself, and it worked great. The only way I'd think you'd need that would be if you wanted the rendered scene for something else on the CPU side (like saving out to disk as an image).
@MrDatabase In this case you won't get around the solution in the third paragraph (be sure to make the texture full-resolution), but you should evaluate if it really gives a speed-up over rendering twice or even dropping the additional FBO and directly reading from the full-res screen (though the latter might really be a worse idea, performance-wise).
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