xcode – AvMutableComposition在最后发出黑框

我在iOS应用程序中使用AVCaptureConnection捕获视频.之后,我在视频中添加一些图像作为CALayers.一切都运行正常,但在添加图像后,在结果视频的最后得到一个黑框.在此没有影响实际音频/视频的帧.对于音频我正在提取它并改变它的音高然后使用AVMutableComposition添加它.这是我正在使用的代码.请帮我解决我做错了什么,或者我需要添加其他东西.

cmp = [AVMutableComposition composition];

    AVMutableCompositionTrack *videoComposition = [cmp addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    AVMutableCompositionTrack *audioComposition = [cmp addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    AVAssetTrack *sourceVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    AVAssetTrack *sourceAudioTrack = [[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];


    [videoComposition insertTimeRange:CMTimeRangeMake(kCMTimeZero,[asset duration]) ofTrack:sourceVideoTrack atTime:kCMTimeZero error:nil] ;
    [audioComposition insertTimeRange:CMTimeRangeMake(kCMTimeZero,[asset duration]) ofTrack:sourceAudioTrack atTime:kCMTimeZero error:nil];

    animComp = [AVMutableVideoComposition videoComposition];
    animComp.renderSize = CGSizeMake(320,320);
    animComp.frameDuration = CMTimeMake(1,30);
    animComp.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer  inLayer:parentLayer];

    // to gather the audio part of the video
    NSArray *tracksToDuck = [cmp tracksWithMediaType:AVMediaTypeAudio];
    NSMutableArray *trackMixArray = [NSMutableArray array];
    for (NSInteger i = 0; i < [tracksToDuck count]; i++) {
        AVMutableAudioMixInputParameters *trackMix = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:[tracksToDuck objectAtIndex:i]];
        [trackMix setVolume:5 atTime:kCMTimeZero];
        [trackMixArray addObject:trackMix];
    }
    audioMix = [AVMutableAudioMix audioMix];
    audioMix.inputParameters = trackMixArray;

    AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    instruction.timeRange = CMTimeRangeMake(kCMTimeZero,[asset duration]);
    AVMutableVideoCompositionLayerInstruction *layerVideoInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoComposition];

    [layerVideoInstruction setOpacity:1.0 atTime:kCMTimeZero];
    instruction.layerInstructions = [NSArray arrayWithObject:layerVideoInstruction] ;
    animComp.instructions = [NSArray arrayWithObject:instruction];
    [self exportMovie:self];

这是我导出视频的方法

-(IBAction) exportMovie:(id)sender{

    //successCheck = NO;
    NSArray *docPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES);
    NSString *tempPath = [docPaths objectAtIndex:0];
    //NSLog(@"Temp Path: %@",tempPath);

    NSString *fileName = [NSString stringWithFormat:@"%@/Final.MP4",tempPath];
    NSFileManager *fileManager = [NSFileManager defaultManager] ;
    if([fileManager fileExistsAtPath:fileName ]){
        NSError *ferror = nil ;
        [fileManager removeItemAtPath:fileName error:&ferror];
    }

    NSURL *exportURL = [NSURL fileURLWithPath:fileName];

    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:cmp presetName:AVAssetExportPresetMediumQuality]  ;
    exporter.outputURL = exportURL;
    exporter.videoComposition = animComp;
    //exporter.audioMix = audioMix;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;

    [exporter exportAsynchronouslyWithCompletionHandler:^(void){
        switch (exporter.status) {
            case AVAssetExportSessionStatusFailed:{
                NSLog(@"Fail");
                break;
            }
            case AVAssetExportSessionStatusCompleted:{
                NSLog(@"Success video");
                                });
                break;
            }

            default:
                break;
        }
           }];
    NSLog(@"outside");
}