iOS开发中GUPImage抠除绿幕视频

最近接到一个需求，需要将一个绿幕视频的绿幕背景抠成透明色。并将其加到摄像机镜头上。

NSString *path = [DownLoadTool getFileWithName:[DataManage getObjectForKey:[self.uuid stringByAppendingString:@"-ar"]]];

    NSURL *mediaURL = [NSURL fileURLWithPath:path];
    self.item = [AVPlayerItem playerItemWithURL:mediaURL];
    self.myPlayer = [AVPlayer playerWithPlayerItem:self.item];
    [self.myPlayer seekToTime:kCMTimeZero];
    self.movieGreen = [[GPUImageMovie alloc] initWithURL:mediaURL];

    self.movieGreen.playAtActualSpeed = YES;
    self.movieGreen.shouldRepeat = YES;
    NSString*resFilename =@"1.png";
    UIImage*inputImage = [UIImage imageNamed:resFilename];
    self.movieNormal = [[GPUImagePicture alloc]initWithImage:inputImage smoothlyScaleOutput:YES];
    
    NSURL *videoURl = [NSURL fileURLWithPath:path];
    AVAsset *asset = [AVAsset assetWithURL:videoURl];
    NSArray *array = asset.tracks;
    CGSize videoSize = CGSizeZero;
    for (AVAssetTrack *track in array) {
        if ([track.mediaType isEqualToString:AVMediaTypeVideo]) {
            videoSize = track.naturalSize;
        }
    }
    self.gpuImageView = [[GPUImageView alloc] initWithFrame:CGRectMake(0, 0, kWholeScreenWidth, videoSize.height*kWholeScreenWidth/videoSize.width)];
    self.gpuImageView.center = self.center;
    self.gpuImageView.fillMode = kGPUImageFillModePreserveAspectRatio;

    self.gpuImageView.backgroundColor = [UIColor clearColor];
    [filteredVideoView addSubview:self.gpuImageView];
    


    [self.movieGreen addTarget:chromaKeyBlendFilter];
    [self.movieNormal addTarget:chromaKeyBlendFilter];
    [chromaKeyBlendFilter addTarget:self.gpuImageView];

解释一下AVPlayerItem和AVPlayer是用来播放视频声音的
GPUImageMovie是播放绿幕视频的
GPUImagePicture是背景图层放透明的图片1.png
GPUImageView是父组件
chromaKeyBlendFilter是GPUImageChromaKeyBlendFilter滤镜用来过滤绿色。

运行发现GPUImageMovie播放音画不同步。
通过搜索有大神已经贴出了相关解决办法。
就是修改GPUImageMovie.m中的readNextVideoFrameFromOutput方法。
具体链接