- (kxMovieError) openVideoStream: (NSInteger) videoStream
{
// get a pointer to the codec context for the video stream
AVCodecContext *codecCtx = _formatCtx->streams[videoStream]->codec;
// find the decoder for the video stream
AVCodec *codec = avcodec_find_decoder(codecCtx->codec_id);
[_Tool setupFilters:@"crop=1/3*in_w:1/3*in_h" videoCodecCtx:codecCtx];
add filter, but inoperative.