为什么 OpenCV 计算的视频 FPS 是错的

/**
  * This is the fundamental unit of time (in seconds) in terms
  * of which frame timestamps are represented. For fixed-fps content,
  * timebase should be 1/framerate and timestamp increments should be
  * identically 1.
  * This often, but not always is the inverse of the frame rate or field rate
  * for video. 1/time_base is not the average frame rate if the frame rate is not
  * constant.
  *
  * Like containers, elementary streams also can store timestamps, 1/time_base
  * is the unit in which these timestamps are specified.
  * As example of such codec time base see ISO/IEC 14496-2:2001(E)
  * vop_time_increment_resolution and fixed_vop_rate
  * (fixed_vop_rate == 0 implies that it is different from the framerate)
  *
  * - encoding: MUST be set by user.
  * - decoding: the use of this field for decoding is deprecated.
  *             Use framerate instead.
  */
AVRational time_base;

* @section lavf_decoding_open Opening a media file
* The minimum information required to open a file is its URL, which
* is passed to avformat_open_input(), as in the following code:
* @code
* const char    *url = "file:in.mp3";
* AVFormatContext *s = NULL;
* int ret = avformat_open_input(&s, url, NULL, NULL);
* if (ret < 0)
*     abort();
* @endcode
* The above code attempts to allocate an AVFormatContext, open the
* specified file (autodetecting the format) and read the header, exporting the
* information stored there into s. Some formats do not have a header or do not
* store enough information there, so it is recommended that you call the
* avformat_find_stream_info() function which tries to read and decode a few
* frames to find missing information.

/**
 * Read packets of a media file to get stream information. This
 * is useful for file formats with no headers such as MPEG. This
 * function also computes the real framerate in case of MPEG-2 repeat
 * frame mode.
 * The logical file position is not changed by this function;
 * examined packets may be buffered for later processing.
 *
 * @param ic media file handle
 * @param options  If non-NULL, an ic.nb_streams long array of pointers to
 *                 dictionaries, where i-th member contains options for
 *                 codec corresponding to i-th stream.
 *                 On return each dictionary will be filled with options that were not found.
 * @return >=0 if OK, AVERROR_xxx on error
 *
 * @note this function isn't guaranteed to open all the codecs, so
 *       options being non-empty at return is a perfectly normal behavior.
 *
 * @todo Let the user decide somehow what information is needed so that
 *       we do not waste time getting stuff the user does not need.
 */
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

const AVCodec ff_h264_decoder = {
    .name                  = "h264",
    .type                  = AVMEDIA_TYPE_VIDEO,
    .id                    = AV_CODEC_ID_H264,
    .priv_data_size        = sizeof(H264Context),
    .init                  = h264_decode_init,
    .close                 = h264_decode_end,
    .decode                = h264_decode_frame,
    ......
};

ret = avctx->codec->decode(avctx, frame, &got_frame, pkt);