Question

I have been working on an opencv project for iOS. I was given a simple project to start developing with that captured and displayed frames. I never payed much attention to how it worked until I started having memory issues and traced them back to the original project setup. I now plan to re-write the capturing/displaying code but I don’t understand why it worked in the first place. There was a play/pause button which called the method

- (IBAction)play_pause:(id)sender
{
  play = !play;
  while(play)
  {
    if (_videoCapture && _videoCapture->grab())
    {
        (*_videoCapture) >> _display_frame;
        //process frame
        self.imageView.image = [UIImage imageWithCVMat:_display_frame];
    }
  }
}

play is just a global bool that signifies whether the application is playing or paused. The strange thing is that the processing should be taking place inside an infinite loop, there is no way out. play is never modified within the loop. In spite of that, when the application is running the play/pause button remains responsive and is capable of flipping the play bool and pausing the execution. Not only that, other bools (use_greyscale for instance) can be flipped by other buttons and their values change inside the loop. I would have expected the application to freeze and never even draw new frames to the screen. The application should stay trapped inside that function for most of its lifetime, unable to perform other tasks such as drawing and UIControl. It seems as though the only way this is possible is if the IBAction call is running on its own thread. I cannot find any evidence of threading in the source code. Could someone explain how apple handles threading in its UI? I was under the impression that there was one main runloop thread and that extra threads were not created automatically. If that is true, how can this behavior be explained?

side note-

What finally made me investigate this was that [UIImage imageWithCVMat:_display_frame] returns an auto-released object. Since all this takes place inside a loop, the objects could not be released without the execution being paused which was causing crashes.

Answer 1

The reason is worked is because the implementation of the cv::VideoCapture::grab() method runs the current run loop to pause the thread until it gets a frame.

When you launch your application, the main function executes a function named UIApplicationMain, which executes CFRunLoopRun. When CFRunLoopRun is executed on the main thread, it runs the main run loop, which is the run loop that processes all the UI events received from the system and refresh the user interface. For information on run loops, you may read Apple Threading Programming Guide.

So, when you execute an infinite loop, your code never returns to the run loop and the waiting events cannot be processed. But in your case, the grab method runs the run loop again with an expiration delay. So the run loop may process incoming events (which may invoke your code again) until the delay expires, then return to your code that will run the run loop again.

If you look at the callstack when you touch the button to pause, you will see this:

main function → run loop → event handling → your code → OpenCV → run loop → event handling → your code

The run loop is running inside itself, which is perfectly fine because run loops are reentrant. Scroll views actually use that behavior: When you scroll a UIScrollView, it runs the run loop again in a different mode in order to ignore some events until you end scrolling.

But I’m not sure the developers of OpenCV had this in mind when they wrote their code. So I think it would be better to load your frames in a background thread/queue.