opencv中的视频数据

Question

Can anybody please tell me in what format the video data is saved in opencv, after decoding it by ffmpeg. I mean if it is saved as a matrix then is it 8bit/pixel kind of thing? If anybody can provide some documents or link explaining in details will be a great help. I also tried to google but couldn't find something related.

Thanks.

Answer 1

The definitive reference to examine is probably the code itself! Look at the function CvCapture_FFMPEG::open in the file modules/highgui/src/cap_ffmpeg_impl.hpp . This method is called by, for example, cvCaptureFromAVI() and cvCaptureFromFile() . The code excerpted below clearly gets the width and height from the file, but it ignores the input bit depth and always uses a packed 8:8:8 bit RGB representation of the video in memory (as indicated by the PIX_FMT_BGR24 constant):

#if LIBAVFORMAT_BUILD >= CALC_FFMPEG_VERSION(52, 111, 0)
    int err = avformat_open_input(&ic, _filename, NULL, NULL);
#else
    int err = av_open_input_file(&ic, _filename, NULL, 0, NULL);
#endif

    .......    

#if LIBAVFORMAT_BUILD >= CALC_FFMPEG_VERSION(53, 3, 0)
    avformat_find_stream_info(ic, NULL);
#else
    av_find_stream_info(ic);
#endif

    .......    

    avpicture_fill( (AVPicture*)&rgb_picture, rgb_picture.data[0],
                    PIX_FMT_BGR24, enc->width, enc->height );

Further evidence that this is the case is provided by the following code in CvCapture_FFMPEG::retrieveFrame :

img_convert_ctx = sws_getCachedContext(
                NULL,
                video_st->codec->width, video_st->codec->height,
                video_st->codec->pix_fmt,
                video_st->codec->width, video_st->codec->height,
                PIX_FMT_BGR24,
                SWS_BICUBIC,
                NULL, NULL, NULL
                );

Looking at the ffmpeg documentation, the PIX_FMT_BGR24 in this call specifies the destination pixel format, ie, what will be handed off to opencv.