c++ - 使用 capture.get(CV_CAP_PROP_FPS) 时，OpenCV 报告 TBR 而不是 FPS

Question

我尝试在 Mac OS 10.6.8 (Snow Leopard) 上使用 OpenCV 和 Qt 4.7.4 处理几个视频。如果我创建一个 cv::VideoCapture 对象，然后查询与此类视频相关的帧速率，我得到的是 TBR 而不是 FPS。

例如，如果使用 ffprobe Video1.mp4，我得到的是:

>> ffprobe Video1.mp4      
ffprobe version 0.7.8, Copyright (c) 2007-2011 the FFmpeg developers
built on Nov 24 2011 14:31:00 with gcc 4.2.1 (Apple Inc. build 5666) (dot 3)
configuration: --prefix=/opt/local --enable-gpl --enable-postproc --enable-swscale --   
enable-avfilter --enable-libmp3lame --enable-libvorbis --enable-libtheora --enable-
libdirac --enable-libschroedinger --enable-libopenjpeg --enable-libxvid --enable-libx264 
--enable-libvpx --enable-libspeex --mandir=/opt/local/share/man --enable-shared --
enable-pthreads --cc=/usr/bin/gcc-4.2 --arch=x86_64 --enable-yasm
libavutil    50. 43. 0 / 50. 43. 0
libavcodec   52.123. 0 / 52.123. 0
libavformat  52.111. 0 / 52.111. 0
libavdevice  52.  5. 0 / 52.  5. 0
libavfilter   1. 80. 0 /  1. 80. 0
libswscale    0. 14. 1 /  0. 14. 1
libpostproc  51.  2. 0 / 51.  2. 0

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Video1.mp4':
Metadata:
major_brand     : isom
minor_version   : 0
compatible_brands: mp41avc1qt  
creation_time   : 2012-01-09 23:09:43
encoder         : vlc 1.1.3 stream output
encoder-eng     : vlc 1.1.3 stream output
Duration: 00:10:10.22, start: 0.000000, bitrate: 800 kb/s
Stream #0.0(eng): Video: h264 (Baseline), yuvj420p, 704x480 [PAR 10:11 DAR 4:3], 798 
kb/s, 27.71 fps, 1001 tbr, 1001 tbn, 2002 tbc
Metadata:
  creation_time   : 2012-01-09 23:09:43

正确报告 FPS = 27.71 和 TBR = 1001。不过，如果我使用以下 OpenCV 代码查询 FPS:

QString filename = QFileDialog::getOpenFileName(this,
                                        "Open Video",
                                        "Video Files (*.mp4, *.mpg)");

capture.release();
capture.open(filename.toAscii().data());

if (!capture.isOpened()){
    qDebug() <<"Error when opening the video!";
    return;
}


qDebug() << "Frame Rate:" << capture.get(CV_CAP_PROP_FPS);
qDebug() << "Num of Frames:" << capture.get(CV_CAP_PROP_FRAME_COUNT);
qDebug() << "OpenCV Version" << CV_VERSION;

我得到的输出是:

Frame Rate: 1001 
Num of Frames: 610832 
OpenCV Version 2.3.1 

报告 TBR 而不是 FPS。当我尝试打开不同的视频时，此行为是一致的。

我检查了 OpenCV 的 bug tracker我还找到了this堆栈溢出问题是一个类似但不完全相同的问题，所以我不知道下一步该怎么做。任何提示或想法都是最受欢迎的，因为我已经尝试了很多东西，但似乎一无所获。

Answer 1

我想他们选择报告 TBR 是因为那是 ffmpeg's best guess至于帧率实际上是多少。在许多容器上，fps 字段(更具体地说是 AVStream.avg_frame_rate)不可用，因此不能真正依赖它。

以下是 ffmpeg 来源对 tbr、tbc 和 tbn 字段的注释:

TBR(ffmpeg 的最佳猜测):

struct AVStream {
...
/**
 * Real base framerate of the stream.
 * This is the lowest framerate with which all timestamps can be
 * represented accurately (it is the least common multiple of all
 * framerates in the stream). Note, this value is just a guess!
 * For example, if the time base is 1/90000 and all frames have either
 * approximately 3600 or 1800 timer ticks, then r_frame_rate will be 50/1.
 */
AVRational r_frame_rate;

TBC(编解码器时基):

struct AVCodecContext {
...
/**
 * This is the fundamental unit of time (in seconds) in terms
 * of which frame timestamps are represented. For fixed-fps content,
 * time base should be 1/framerate and timestamp increments should be 1.
 * decoding: set by libavformat
 * encoding: set by libavformat in av_write_header
 */
AVRational time_base;

TBN(流(容器)时基):

struct AVStream {
...
/**
 * 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.
 * - encoding: MUST be set by user.
 * - decoding: Set by libavcodec.
 */
 AVRational time_base;

希望这能解释为什么报告 TBR 而不是 FPS。 ffmpeg 似乎难以确定视频流的时基，而容器(例如 AVI、MP4、DivX、XviD 等...)提供帧率，因此 ffmpeg 显示它，即使它无法通过分析确定它。是否可以正确地重新编码视频？