Question

我遇到了我认为应该是 easy 任务的问题。

我正在尝试将视频从任何连接的网络摄像头录制到特定时间段的文件中。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Googling和stackoverflowing的一些建议，我已经看到一种简单的方法是在电影输出文件上使用setMaximumRecordedDuration，然后在委托captureOutput方法中停止捕获。

例如，为了拍摄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，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;
}

这里的完整代码在这里; https://github.com/matthutchinson/videosnap/blob/master/videosnap/videosnap.m

此代码是我正在使用VideoSnap的新开源命令行应用程序的一部分。您可以在GitHub

的xCode中查看完整的源代码，下载并打开它

3个问题;

如何设置setMaximumRecordedDuration以使其工作5秒
我现在的运行循环是一个hack，但是当我使用例如而（recordingFlag）捕捉永远不会开始
这是最好的方法吗？我应该以某种方式计算帧数吗？

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!)
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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

Answer 1

所以我基本上把它想出来了（在另一个iOS / Obj-C开发者的帮助下）;

我的代码存在2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，这样我就可以开始使用摄像头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对象来衡量我记录的时间比我需要处理的时间更麻烦，所以我选择了一个简单的NSDate对象（称为mRecordingStartedOnDate ）。同时查看我的输出很明显，会话/摄像头需要一些时间才能在捕获开始之前预热并将输出写入文件。所以我首先检查文件上的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，如此;

- (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];
    }
}

我可以重构一些事情，但是current state of this project is here的代码。

- 感谢Paul Tsochantaris的帮助！

QTKit如何在特定时间内将电影录制到文件中

1 个答案: