1

我在我认为应该是一件容易的事情上遇到了麻烦。

我正在尝试将任何附加网络摄像头的视频录制到文件中特定持续时间。但实际记录的持续时间总是大约 3/4 秒。例如5 秒记录为4.28

根据谷歌搜索和 stackoverflow-ing 的一些建议，我看到一个简单的方法是setMaximumRecordedDuration在电影输出文件上使用，然后在委托 captureOutput 方法中停止捕获。

例如，为了捕捉 5 秒的视频，我现在正在使用以下代码；

    // 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；

- (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;
}

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

此代码是我正在开发 VideoSnap 的新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 中打开它

3个问题；

  • 我怎样才能setMaximumRecordedDuration使这项工作持续 5 秒
  • 我现在拥有的运行循环是一个 hack，但是当我使用例如 while(recordingFlag) 时，捕获永远不会开始
  • 这是最好的方法吗？我应该以某种方式或其他方式计算帧数吗？

以下是上述代码的粘贴输出；

(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!)
(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!)
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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
4

1 回答 1

0

所以我基本上想通了（在另一个 iOS/Obj-C 开发人员的帮助下）；

我的代码有两个问题；

  • 我需要一个正确的运行循环（在主线程之外）才能运行，以便我可以使用相机开始我的捕获会话而不是立即退出（或使用 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量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）。还查看我的输出，很明显会话/相机在捕获开始之前需要一些时间来预热并将输出写入文件。所以我先检查recordedFileSize文件，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，但是这个项目当前状态的代码在这里。

-- 感谢Paul Tsochantaris的帮助！

于 2013-08-30T10:12:57.550 回答