QTKit how to record a movie to a file for a specific duration

Question

I'm having trouble with what I think should be an easy task.

I'm trying to record video from any attached webcam to a file for a specific duration. But the duration that actually gets recorded is always about 3/4 of a second off. E.g. 5 secs gets recorded as 4.28

Following some advice from Googling and stackoverflow-ing, i've seen that a simple way to do this is to use setMaximumRecordedDuration on the movie output file, and then stop capturing in the delegate captureOutput method.

For instance for capturing 5 secs of video, I'm using the following code right now;

    // set maximum duration (e.g. 5 secs at 30000 timescale) ??
    [mCaptureMovieFileOutput recordToOutputFileURL:[NSURL fileURLWithPath:path]];
    [mCaptureMovieFileOutput setMaximumRecordedDuration: QTMakeTime(5*30000, 30000)];
    [mCaptureSession startRunning];

    // start a run loop (for how long?)
    verbose("(recording for %.2lf seconds)\n", [recordSeconds doubleValue]);
    NSDate *now = [[NSDate alloc] init];
    [[NSRunLoop currentRunLoop] runUntilDate:[now dateByAddingTimeInterval: [recordSeconds doubleValue]]];
    verbose("(recording period ended)\n");

Then in my captureOutput delegate method I have;

- (BOOL)captureOutput:(QTCaptureFileOutput *)captureOutput shouldChangeOutputFileAtURL:(NSURL *)outputFileURL forConnections:(NSArray *)connections dueToError:(NSError *)error {
    QTTime durationRecorded = [mCaptureMovieFileOutput recordedDuration];
    NSString *outputDuration = QTStringFromTime(durationRecorded);
    verbose("(capture output reached %s !)\n", [outputDuration UTF8String]);

    [mCaptureMovieFileOutput recordToOutputFileURL:nil];

    return YES;
}

The full code for this is here; https://github.com/matthutchinson/videosnap/blob/master/videosnap/videosnap.m

This code is part of a new open source command line app i'm working on VideoSnap. You can see the full source code, download and open it in xCode from GitHub

3 questions;

• how can I set setMaximumRecordedDuration to make this work for 5 seconds
• the run loop I have is a hack right now, but when I use e.g. while(recordingFlag) the capturing never starts
• is this the best way to do this? should I count frames somehow or something else?

Below is the pasted output from the above code;

(no filename specified, using default)
(no device specified, using default)
(options before recording)
  delay:    0.00s
  record:   10.00s
  filename: movie.mov
  device:   Built-in iSight
(no delay)
(starting device...)
(recording for 10.00 seconds)
(recorded (null)  - frameCount 0!)
(recorded (null)  - frameCount 1!)
(recorded (null)  - frameCount 2!)
(recorded (null)  - frameCount 3!)
(recorded (null)  - frameCount 4!)
(recorded (null)  - frameCount 5!)
(recorded (null)  - frameCount 6!)
(recorded (null)  - frameCount 7!)
(recorded (null)  - frameCount 8!)
(recorded (null)  - frameCount 9!)
(recorded (null)  - frameCount 10!)
(recorded (null)  - frameCount 11!)
(recorded (null)  - frameCount 12!)
(recorded (null)  - frameCount 13!)
(recorded (null)  - frameCount 14!)
(recorded (null)  - frameCount 15!)
(recorded (null)  - frameCount 16!)
(recorded (null)  - frameCount 17!)
(recorded (null)  - frameCount 18!)
(recorded (null)  - frameCount 19!)
(recorded (null)  - frameCount 20!)
(recorded (null)  - frameCount 21!)
(recorded (null)  - frameCount 22!)
(recorded (null)  - frameCount 23!)
(recorded 0:00:00:00.00/1  - frameCount 24!)
(recorded 0:00:00:00.00/1  - frameCount 25!)
(recorded 0:00:00:00.00/1  - frameCount 26!)
(recorded 0:00:00:00.18000/30000  - frameCount 27!)
(recorded 0:00:00:00.18000/30000  - frameCount 28!)
(recorded 0:00:00:00.18000/30000  - frameCount 29!)
(recorded 0:00:00:00.20000/30000  - frameCount 30!)
(recorded 0:00:00:00.20000/30000  - frameCount 31!)
(recorded 0:00:00:00.20000/30000  - frameCount 32!)
(recorded 0:00:00:00.20000/30000  - frameCount 33!)
(recorded 0:00:00:00.22000/30000  - frameCount 34!)
(recorded 0:00:00:00.22000/30000  - frameCount 35!)
(recorded 0:00:00:00.22000/30000  - frameCount 36!)
(recorded 0:00:00:00.22000/30000  - frameCount 37!)
(recorded 0:00:00:00.24000/30000  - frameCount 38!)
(recorded 0:00:00:00.24000/30000  - frameCount 39!)
(recorded 0:00:00:00.24000/30000  - frameCount 40!)
(recorded 0:00:00:00.24000/30000  - frameCount 41!)
(recorded 0:00:00:00.26000/30000  - frameCount 42!)
(recorded 0:00:00:00.26000/30000  - frameCount 43!)
(recorded 0:00:00:00.26000/30000  - frameCount 44!)
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(recorded 0:00:00:00.28000/30000  - frameCount 46!)
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(recorded 0:00:00:01.4000/30000  - frameCount 57!)
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(recorded 0:00:00:01.6000/30000  - frameCount 61!)
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(recorded 0:00:00:01.6000/30000  - frameCount 63!)
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(recorded 0:00:00:01.8000/30000  - frameCount 65!)
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(recorded 0:00:00:01.10000/30000  - frameCount 69!)
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(recorded 0:00:00:02.14000/30000  - frameCount 135!)
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(recorded 0:00:00:02.28000/30000  - frameCount 162!)
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(recorded 0:00:00:04.6000/30000  - frameCount 236!)
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(recorded 0:00:00:04.22000/30000  - frameCount 267!)
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(recorded 0:00:00:04.26000/30000  - frameCount 274!)
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(recorded 0:00:00:04.26000/30000  - frameCount 276!)
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(recorded 0:00:00:04.28000/30000  - frameCount 278!)
(recorded 0:00:00:04.28000/30000  - frameCount 279!)
(recorded 0:00:00:04.28000/30000  - frameCount 280!)
(recorded 0:00:00:04.28000/30000  - frameCount 281!)
(capture output reached 0:00:00:04.28000/30000 !)
(recorded 0:00:00:05.00/30000  - frameCount 282!)
(recorded 0:00:00:05.00/30000  - frameCount 283!)
(recorded 0:00:00:05.00/30000  - frameCount 284!)
(recorded 0:00:00:05.00/30000  - frameCount 285!)
(finished writing to movie file duration was 0:00:00:05.00/30000 !)
(stopping session)
(recording period ended)
DONE! movie.mov

Answer 1

So I basically figured it out (with some help from a another iOS/Obj-C developer);

There were 2 problems with my code;

• I needed a proper run loop (outside the main thread) to be running so that I can start my capture session with the camera and not exit right away (or use a while loop). To do this I added the following at the end of my processArgs() method

  [[NSRunLoop currentRunLoop] run];
  

• I discovered using QTTime objects to measure the amount of time I had recorded was more trouble than I needed to deal with, so instead I opted for a simple NSDate object (called mRecordingStartedOnDate). Also looking at my output it was clear that the session/camera take some time to warm up before capturing begins and the output is written to the file. So I check the recordedFileSize on the file first, and start counting time from then. All this happens in my captureOutput delegate method like so;

  - (void)captureOutput:(QTCaptureFileOutput *)captureOutput didOutputSampleBuffer:(QTSampleBuffer *)sampleBuffer fromConnection:(QTCaptureConnection *)connection {
      // check we have started to record some bytes (this allows us to wait for camera to warm up and start capturing)
      long recordedBytes = [mCaptureMovieFileOutput recordedFileSize];
  
      if(mRecordingStartedOnDate != nil) {
          // check if we have recorded enough video yet
          float recordedSeconds = [[NSDate date] timeIntervalSinceDate:mRecordingStartedOnDate];
          verbose("(recorded %f seconds)\n", recordedSeconds);
          if(recordedSeconds >= 5.0) {
              // stop recording to the file
              [mCaptureMovieFileOutput recordToOutputFileURL:nil];
          }
      } else if(recordedBytes > 0) {
          // set the start date when recording initiated
          mRecordingStartedOnDate = [NSDate date];
      }
  }
  

I may refactor things, but the code for the current state of this project is here.

-- Thanks to Paul Tsochantaris for the help!