opencv中cvSetCaptureProperty定位不准的原因及解决

opencv中cvSetCaptureProperty定位不准的原因及解决

 

以前在opencv2.0里面用到cvSetCaptureProperty函数的时候总是发生定位不准确的问题，明明是让其跳到100帧，结果却总不是100帧，定位一段连续的视频，总是出现跳跃的现象。同样的代码在opencv1.0里面完全没错。可是这是为什么？这个问题一直困扰了我半年，终于在今天知道原因了。

经过差不多一晚上的探究，得出粗略的结论。原因在于opencv2.0以后，采用ffmpeg采集视频，而在opencv1.0采用vfw采集视频（具体的概念暂时还不清楚，有时间继续补上）。而opencv在定位时候，调用的ffmpeg的av_seek_frame()函数，此函数原型为：

int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp, int flags);

其中，最后一个参数有

AVSEEK_FLAG_BACKWARD = 1; ///< seek backward
AVSEEK_FLAG_BYTE     = 2; ///< seeking based on position in bytes
AVSEEK_FLAG_ANY      = 4; ///< seek to any frame, even non key-frames

ffmpeg默认的是选取关键帧（这个概念需要具体定义）。opencv里面这个函数的参数flag是0，

int ret = av_seek_frame(ic, video_stream, timestamp, 0);

也就是按照默认的读取关键帧。因此，视频跳跃就出现了。

解决这个问题需要将0改为 AVSEEK_FLAG_ANY ，即：

int ret = av_seek_frame(ic, video_stream, timestamp,AVSEEK_FLAG_ANY );

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

源代码包涵在OpenCV2.1\src\highgui\cvcap_ffmpeg.cpp文件中:

• bool CvCapture_FFMPEG::setProperty( int property_id, double value )
  {
  if( !video_st ) return false;
  
  switch( property_id )
  {
  case CV_CAP_PROP_POS_MSEC:
  case CV_CAP_PROP_POS_FRAMES:
  case CV_CAP_PROP_POS_AVI_RATIO:
  {
  int64_t timestamp = 0;
  AVRational time_base;
  switch( property_id )
  {
  case CV_CAP_PROP_POS_FRAMES:
  timestamp=(int64_t)value;
  if(ic->start_time != AV_NOPTS_VALUE_)
  timestamp += ic->start_time;
  break;
  
  case CV_CAP_PROP_POS_MSEC:
  time_base=ic->streams[video_stream]->time_base;
  timestamp=(int64_t)(value*(float(time_base.den)/float(time_base.num))/1000);
  if(ic->start_time != AV_NOPTS_VALUE_)
  timestamp += ic->start_time;
  break;
  
  case CV_CAP_PROP_POS_AVI_RATIO:
  timestamp=(int64_t)(value*ic->duration);
  if(ic->start_time != AV_NOPTS_VALUE_ && ic->duration != AV_NOPTS_VALUE_)
  timestamp += ic->start_time;
  break;
  }
  
  if ( filename )
  {
  // ffmpeg's seek doesn't work...
  if (!slowSeek((int)timestamp))
  {
  fprintf(stderr, "HIGHGUI ERROR: AVI: could not (slow) seek to position %0.3f\n",
  (double)timestamp / AV_TIME_BASE);
  return false;
  }
  }
  else
  {
  int ret = av_seek_frame(ic, video_stream, timestamp, AVSEEK_FLAG_ANY);
  if (ret < 0)
  {
  fprintf(stderr, "HIGHGUI ERROR: AVI: could not seek to position %0.3f\n",
  (double)timestamp / AV_TIME_BASE);
  return false;
  }
  }
  picture_pts=(int64_t)value;
  }
  break;
  
  default:
  return false;
  }
  
  return true;
  }

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

之后重新编译opencv库，就可以了。

P.S:测试的代码

#include "opencv/highgui.h"
#include <iostream>
using namespace std;
int main( int argc,char** argv )
{
    cvNamedWindow( "Example2", CV_WINDOW_AUTOSIZE );
    CvCapture* capture = cvCreateFileCapture(  "d://11.avi" );
    IplImage* frame;
    int pos=0;
    int pos1=0;
    while(1)
    {
        cvSetCaptureProperty(capture,CV_CAP_PROP_POS_FRAMES,pos);
        cout<<pos;
        frame = cvQueryFrame(capture);
        pos1=cvGetCaptureProperty(capture,CV_CAP_PROP_POS_FRAMES);
        cout<<"/t"<<pos1<<endl;
        if( !frame ) break;
        cvShowImage( "Example2", frame );
        char c = cvWaitKey(33);
        if( c == 27 )  break;
        pos++;
    }
    cvReleaseCapture( &capture );
    cvDestroyWindow( "Example2" );
}

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转载自：http://www.opencv.org.cn/forum/viewtopic.php?t=11276#p41262