iOS:将空/黑帧添加到AVMutableComposition

Question

从CAF录音开始,我需要创建一个视频文件,其中视频轨道只是空帧,音轨是这个录音文件.我知道这是一个奇怪的用例,这可能就是为什么我找不到有关如何操作的信息.我正在尝试使用AVMutableComposition,但我无法弄清楚如何让它工作.

这就是我现在所拥有的:

AVMutableComposition *composition = [AVMutableComposition composition];    

/* soundFilePath is the path to the CAF audio file */
AVURLAsset *audioAsset = [[AVURLAsset alloc] initWithURL:[NSURL fileURLWithPath:soundFilePath] options:nil];
CMTimeRange fullTime = CMTimeRangeMake(kCMTimeZero, [audioAsset duration]);
AVAssetTrack *sourceAudioTrack = [[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

/* Add audio track */
AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionAudioTrack insertTimeRange:fullTime ofTrack:sourceAudioTrack atTime:kCMTimeZero error:nil];

/* docsDir is defined previously as the path to where I want to save the file */
NSString *videoFilePath = [docsDir stringByAppendingPathComponent:@"video.mov"];
NSURL *videoFileURL = [NSURL fileURLWithPath:videoFilePath];

AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetHighestQuality];

exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.outputURL = videoFileURL;

[exporter exportAsynchronouslyWithCompletionHandler:nil];

这给我带来的是一个包含音频的.mov文件,但没有视频.我需要该文件有某种视频内容 - 只是空视频帧.我对所有这些iOS库都很陌生,所以上面的所有代码基本上都是从各种示例拼凑而成的,我当然不完全理解这一切应该如何工作.

我也尝试AVMutableVideoComposition在我AVMutableComposition的情况下添加一个正确的方法.为了做到这一点,我添加了一个空白视频轨道,AVMutableComposition就像我添加音轨一样:

/* Add video track */
AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionVideoTrack insertEmptyTimeRange:fullTime];

然后我创建并配置了AVMutableVideoComposition如下:

AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.frameDuration = [audioAsset duration];
videoComposition.renderScale = 1.0;
videoComposition.renderSize = CGSizeMake(320, 240);
AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTrack];
instruction.layerInstructions = [NSArray arrayWithObject:layerInstruction];
instruction.timeRange = compositionVideoTrack.timeRange;

videoComposition.instructions = [NSArray arrayWithObject:instruction];

最后,添加AVMutableVideoComposition到AVMutableComposition:

exporter.videoComposition = videoComposition;

但是,通过这种方法,我什么都没得到.没有创建视频文件.我在导出的完成处理程序中放了一个日志语句,它确实完成了,但没有创建视频文件.

任何有关这方面的帮助将不胜感激.谢谢!