CMBufferAVScreenShot.m

- (CVImageBufferRef)getImageBufferFromURL:(NSURL *)url {
    AVAsset *asset = [AVAsset assetWithURL:url];
    AVAssetTrack *track = [[asset 
                             tracksWithMediaType:AVMediaTypeVideo] 
                                   objectAtIndex:0];
    AVAssetReaderTrackOutput 
    *readerTrack = [AVAssetReaderTrackOutput 
                     assetReaderTrackOutputWithTrack:track
                                      outputSettings:nil];
    AVAssetReader *reader = [AVAssetReader assetReaderWithAsset:asset
                                                          error:nil];
    [reader addOutput:readerTrack];

    CMSampleBuffer sample = [readerTrack copyNextSampleBuffer];
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sample); 
    return imageBuffer;
}