iphone - Why is the AVCaptureVideoDataOutput callback dependent on the OpenGL draw framerate?

Question

When looking at the GLCameraRipple example, the AVCaptureVideoDataOutput is setup in such a way that a callback is called (captureOutput) whenever a new frame arrives from the iphone camera.

However, putting a "sleep(1)" at the beginning of the "drawInRect" function (that is used for OpenGL drawing), this callback gets called only 1 time per second, instead of 30 times per second.

Can anyone tell me why the framerate of the iphone camera is linked with the framerate of the OpenGL draw call?

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Update: Steps to reproduce

• Download the GLCameraRipple sample from here: http://developer.apple.com/library/ios/#samplecode/GLCameraRipple/Introduction/Intro.html
• In RippleViewController.m => captureOutput, add a NSLog(@"Got Frame");. Running it will generate a lot of "Got Frame" messages (about 30 per second)
• In RippleViewController.m => drawInRect, add a sleep(1); at the very beginning of the function. Only one message per second appears now.

Answer 1

当 AVCaptureVideoDataOutput 调用委托方法 captureOutput:didOutputSampleBuffer:fromConnection: 以使程序员能够编辑或记录来自相机的图像时，该方法从主线程调用。而且，通常我们应该编写直接通过主线程与用户界面交互的代码，这就是为什么 OpenGL 喜欢使用 AVCaptureVideoDataOutput 因为从相机和绘图到屏幕的方法都在主线程中运行。

如果 iPhone 无法处理 captureOutput:didOutputSampleBuffer:fromConnection: 和 AVCaptureVideoDataOutput 类可以丢弃图像，如处理时间超过 1/30 秒下一帧将被忽略，您可以使用 captureOutput: didDropSampleBuffer: fromConnection: 方法收集数据