小贝子编程

QTKit如何将电影录制到特定持续时间的文件中



我遇到了问题，我认为这应该是一个简单的任务。

我正在尝试将任何连接的网络摄像头的视频录制到一个文件中，持续特定的时间。但实际记录的持续时间总是大约3/4秒。例如，5秒被记录为4.28

根据谷歌搜索和stackoverflow的一些建议，我发现一个简单的方法是在电影输出文件上使用setMaximumRecordedDuration，然后在delegate captureOutput方法中停止捕获。

例如，为了拍摄5秒的视频，我现在使用以下代码；

    // 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");

然后在我的captureOutput委托方法中，我有；

- (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;
}

这里有完整的代码；https://github.com/matthutchinson/videosnap/blob/master/videosnap/videosnap.m

这段代码是我正在VideoSnap上开发的一个新的开源命令行应用程序的一部分。你可以看到完整的源代码，从GitHub 下载并在xCode中打开

3个问题；

  • 如何设置setMaximumRecordedDuration使其工作5秒
  • 我现在的运行循环是一个破解，但当我使用while（recordingFlag）时，捕获永远不会开始
  • 这是最好的方法吗？我应该以某种方式数帧还是其他什么

下面是上面代码的粘贴输出；

(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!)
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(recorded 0:00:00:00.00/1  - frameCount 24!)
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(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

所以我基本上解决了（在另一位iOS/Obj-C开发人员的帮助下）；

我的代码有2个问题；

  • 我需要一个适当的运行循环（在主线程之外）来运行，这样我就可以用相机开始我的捕获会话，而不是立即退出（或者使用while循环）。为此，我在processArgs()方法的末尾添加了以下内容

    [[NSRunLoop currentRunLoop] run];

  • 我发现使用QTTime对象来测量我记录的时间比我需要处理的更麻烦，所以我选择了一个简单的NSDate对象（称为mRecordingStartedOnDate）。此外，从我的输出中可以清楚地看到，在拍摄开始之前，会话/相机需要一些时间来预热，并且输出会写入文件。所以我先检查文件上的recordedFileSize，然后开始计算时间。所有这些都发生在我的captureOutput委托方法中，就像这样；

    - (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];
    }
}

我可能会重构一些东西，但这个项目当前状态的代码就在这里。

--感谢Paul Tsochantaris的帮助！

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