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I'd like to efficiently capture a "screenshot" of my OpenGL ES iPhone game. I put screenshot in quotes because I really want to move the screen contents into a pixel buffer that I can eventually use to create a mov file using AVFoundation.

Currently I'm taking the screenshot with this method from Apple. The resulting image is great... however capturing the image causes a small stutter in my game. Looks like the call to glReadPixels causes the stutter.

How can I capture the screen's contents more efficiently? Perhaps I can improve the efficiency of the call to glReadPixels? Or perhaps I can populate a CMSampleBuffer or CVPixelBuffer without calling glReadPixels?

Perhaps the OpenGL ES data could be accessed directly somehow w/o having to copy the data into a buffer via glReadPixels.

Edit #1: Perhaps a "Frame Buffer Object" could be useful? Or "rendering directly to a texture"?

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  • \$\begingroup\$ Look into pixel buffer objects. Alternatively, create a mode for your game which does not conflict with the additional load on the bus imposed by ReadPixels: Have the next frame rendered after be the frame that would have been rendered next. Your game will run slow but now you'll capture the right ones. \$\endgroup\$
    – Steven Lu
    Feb 11, 2012 at 11:52

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Frame Buffer Object allows to create a texture where you can draw. That is Render To Texture. It is very fast (no use of CPU) and useful for mirror effects or post 2D effects. But it is a gpu object so you can access to its content with CPU; you need glReadPixels if you want to use it on CPU. the main advantage is you can set your size. So you can create a small FBO for a good framerate.

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FBO is a lot faster than glReadPixels.

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