光流检测（optical flow）

 cvGoodFeaturesToTrack函数中count参数，虽然是输出参数，表示特征点的数目，但是这个初值设置至关重要。设置为0时，代码运行会错误。设置为1时，没有特征点输出，一般设置为最大特征点数目。（原因不明白）

 

cvGoodFeaturesToTrack，http://blog.csdn.net/moc062066/article/details/6634120，和

cvFindCornerSubPix，http://blog.csdn.net/moc062066/article/details/6634961

两个函数的基础，接下来就是在视频中检测光流（optical flow），经常用的函数是cvCalcOpticalFlowPyrLK，函数说明如下；

Calculates the optical flow for a sparse feature set using the iterative Lucas-Kanade method with
pyramids.

//通过Lucas-Kanade方法与图像金字塔的结合，计算稀疏特征集合的光流

<span style="font-size: 18px;">void cvCalcOpticalFlowPyrLK(
                            const CvArr* prev,
                            const CvArr* curr,
                            CvArr* prevPyr,
                            CvArr* currPyr,
                            const CvPoint2D32f* prevFeatures,
                            CvPoint2D32f* currFeatures,
                            int count,
                            CvSize winSize,
                            int level,
                            char* status,
                            float* track error,
                            CvTermCriteria criteria,
                            int flags );</span>

void cvCalcOpticalFlowPyrLK(
							const CvArr* prev,
							const CvArr* curr,
							CvArr* prevPyr,
							CvArr* currPyr,
							const CvPoint2D32f* prevFeatures,
							CvPoint2D32f* currFeatures,
							int count,
							CvSize winSize,
							int level,
							char* status,
							float* track error,
							CvTermCriteria criteria,
							int flags );

prev First frame, at time t  //取t时刻为第一帧

curr Second frame, at time t + dt //第二帧出现在 t + dt时刻

prevPyr Buffer for the pyramid for the first frame. If the pointer is not NULL , the buffer must
have a sufficient size to store the pyramid from level 1 to level level ; the total size of
(image width+8)*image height/3 bytes is sufficient

//第一帧的图像金字塔的缓存之处。如果该指针不为空，该buffer必须有足够的空间来存储从第1层到第level 层的图像金字塔；prevPyr指针所指的图像/矩阵的大小为(image width+8)* （image height/3） 就足够了。

currPyr Similar to prevPyr, used for the second frame //同上

prevFeatures Array of points for which the flow needs to be found //在数组中定义（当前帧中的）那些点是要在（下一帧）检测的

currFeatures Array of 2D points containing the calculated new positions of the input features
in the second image

//一个二维的点数组，用于存放输入的特征（就是prevFeatures）在第二帧中的新位置

count Number of feature points//特征点的数目

winSize Size of the search window of each pyramid level //每一层金字塔所有的搜索窗口的大小

level Maximal pyramid level number. If 0 , pyramids are not used (single level), if 1 , two levels
are used, etc

//最多有多少层金字塔。如果是0，就不用图像金字塔，如果是1，就有两层，以此类推。

status Array. Every element of the array is set to 1 if the flow for the corresponding feature has
been found, 0 otherwise

//是一个数组，对应点在第二帧中找到，那该位置就值为1，找不到就值为0.

track error Array of double numbers containing the difference between patches around the
original and moved points. Optional parameter; can be NULL
criteria Specifies when the iteration process of finding the flow for each point on each pyramid
level should be stopped

flags Miscellaneous flags:

CV LKFLOWPyr A READY pyramid for the first frame is precalculated before the call
CV LKFLOWPyr B READY pyramid for the second frame is precalculated before the call
CV LKFLOW INITIAL GUESSES array B contains initial coordinates of features before the
function call

cvCalcOpticalFlowPyrLK函数中最后一个标志位，FLAG在视频处理中很重要。要提高速度，不重复计算金字塔，这里需要设置FLAG.即保存前一帧的金字塔数据和特征点，做为下一帧的初始帧。

 

//cvCaclOpticalFlowPyrLk_demo
//mochen
//2011年7月26日20:23:42

#include <stdio.h>
#include "cv.h"
#include "cxcore.h"
#include "highgui.h"

#pragma comment(lib, "opencv_core220d.lib")
#pragma comment(lib, "opencv_highgui220d.lib")
#pragma comment(lib, "opencv_imgproc220d.lib")
#pragma comment(lib, "opencv_calib3d220d.lib")
#pragma comment(lib, "opencv_features2d220d.lib")
#pragma comment(lib, "opencv_contrib220d.lib")
#pragma comment(lib, "opencv_ml220d.lib")
#pragma comment(lib, "opencv_video220d.lib")


#if 0
void cvCalcOpticalFlowPyrLK(
                            const CvArr* prev,
                            const CvArr* curr,
                            CvArr* prevPyr,
                            CvArr* currPyr,
                            const CvPoint2D32f* prevFeatures,
                            CvPoint2D32f* currFeatures,
                            int count,
                            CvSize winSize,
                            int level,
                            char* status,
                            float* track error,
                            CvTermCriteria criteria,
                            int flags );
#endif

const int MAX_CORNERS = 1000 ;

int main(int argc,char** argv)
{
    while ( 1 )
    {
        //use webcam
        CvCapture* cam = cvCaptureFromCAM( CV_CAP_ANY ) ;
        assert( NULL != cam ) ;

        //get a color image
        IplImage* frame = cvQueryFrame(cam) ;

        CvSize img_sz = cvGetSize(frame);
        const int win_size = 10 ;

        //convert the image to grey image
        IplImage* frame_prev = cvQueryFrame(cam) ;
        IplImage* img_prev = cvCreateImage(img_sz,IPL_DEPTH_8U,1) ;
        cvCvtColor( frame_prev,img_prev ,CV_BGR2GRAY);

        //convert the image to grey image
        IplImage* frame_cur = cvQueryFrame(cam) ;
        IplImage* img_curr = cvCreateImage(img_sz,IPL_DEPTH_8U,1) ;
        cvCvtColor( frame_cur,img_curr ,CV_BGR2GRAY);

        //create a imge to display result
        IplImage* img_res = cvCreateImage(img_sz,IPL_DEPTH_8U,1) ;
        for ( int y = 0 ; y < img_sz.height ; ++y )
        {
            uchar* ptr = (uchar*)( img_res->imageData + y * img_res->widthStep ) ;
            for ( int x = 0 ; x <img_res->width; ++x )
            {
                ptr[x] = 255 ;
            }
        }

        //get good features
        IplImage* img_eig = cvCreateImage(img_sz,IPL_DEPTH_32F,1) ;
        IplImage* img_temp = cvCreateImage(img_sz,IPL_DEPTH_32F,1) ;
        int corner_count = MAX_CORNERS ;
        CvPoint2D32f*  features_prev = new CvPoint2D32f[MAX_CORNERS] ;

        cvGoodFeaturesToTrack(
            img_prev,
            img_eig,
            img_temp,
            features_prev,
            &corner_count,
            0.01,
            5.0,
            0,
            3,
            0,
            0.4
            );

        cvFindCornerSubPix(
            img_prev,
            features_prev,
            corner_count,
            cvSize(win_size,win_size),
            cvSize(-1,-1),
            cvTermCriteria(CV_TERMCRIT_EPS | CV_TERMCRIT_ITER,20,0.03)
            );

        // L-K
        char feature_found[ MAX_CORNERS ] ;
        float feature_errors[ MAX_CORNERS ] ;

        CvSize pyr_sz = cvSize( frame->width + 8 ,frame->height / 3 ) ;

        IplImage* pyr_prev = cvCreateImage(img_sz,IPL_DEPTH_32F,1) ;
        IplImage* pyr_cur = cvCreateImage(img_sz,IPL_DEPTH_32F,1) ;
        CvPoint2D32f*  features_cur =  new CvPoint2D32f[ MAX_CORNERS ] ;

        cvCalcOpticalFlowPyrLK(
            img_prev,
            img_curr,
            pyr_prev,
            pyr_cur,
            features_prev,
            features_cur,
            corner_count,
            cvSize(win_size,win_size),
            5,
            feature_found,
            feature_errors,
            cvTermCriteria(CV_TERMCRIT_EPS | CV_TERMCRIT_ITER,20,0.3),
            0
            );

        for ( int i = 0 ; i < corner_count ; i++)
        {
            if ( 0 == feature_found[i] || feature_errors[i] > 550 )
            {
                printf("error is %f \n" , feature_errors[i] ) ;
                continue ;
            }

            printf("find it !\n") ;

            CvPoint pt_prev = cvPoint( features_prev[i].x , features_prev[i].y ) ;
            CvPoint pt_cur = cvPoint( features_cur[i].x , features_cur[i].y ) ;

            cvLine( img_res,pt_prev,pt_cur,CV_RGB( 255,0,0),2 );
        }

        const char* window_prev ="img_prev" ;
        const char* window_curr ="img_curr" ;
        const char* window_res ="result" ;
        cvNamedWindow(window_prev,CV_WINDOW_AUTOSIZE);
        cvNamedWindow(window_curr,CV_WINDOW_AUTOSIZE);
        cvNamedWindow(window_res,CV_WINDOW_AUTOSIZE);

        cvShowImage( window_prev,img_prev );
        cvShowImage( window_curr,img_curr );
        cvShowImage( window_res,img_res );

        char opt = cvWaitKey( 10000 ) ;
        if ( 27 == opt )
        {
            break ;
        }

        cvReleaseCapture( &cam );
        cvReleaseImage( &img_curr );
        cvReleaseImage( &img_eig );
        cvReleaseImage( &img_prev );
        cvReleaseImage( &img_res );
        cvReleaseImage( &img_temp );
        cvDestroyAllWindows() ;
    }


    return 0 ;
}

 

下面一段代码为视频处理，设置FLAG

int CMotionOptic::OpticalFlow()
{ 
  CvPoint2D32f *m_pPreConners1 = new CvPoint2D32f[MAX_CONNERS];
  CvPoint2D32f *m_pCurConners1 = new CvPoint2D32f[MAX_CONNERS];

 char aFeatureFound[MAX_CONNERS];
 float aTrackErrors[MAX_CONNERS];

  IplImage* m_pPyrCurImg1 = cvCreateImage(cvSize(m_iWidth,m_iHeight), IPL_DEPTH_32F,1); 
  IplImage *m_pPyrPreImg1 = cvCreateImage(cvSize(m_iWidth,m_iHeight), IPL_DEPTH_32F,1);

 //int m_iConnersCnt1 = MAX_CONNERS;

 if(1 == m_iStartFrameCnt)
 {
  
  cvGoodFeaturesToTrack(m_pPreImg,m_pEigImg,m_pTmpImg,m_pPreConners,&m_iConnersCnt,0.01,5.0,0,3,0,0.04);

   cvFindCornerSubPix(m_pPreImg,m_pPreConners,m_iConnersCnt,cvSize(10,10),cvSize(-1,-1),
    cvTermCriteria(CV_TERMCRIT_EPS|CV_TERMCRIT_ITER, 200,0.03));

   cvCalcOpticalFlowPyrLK(m_pPreImg, m_pCurImg, m_pPyrPreImg, m_pPyrCurImg, m_pPreConners,
    m_pCurConners,m_iConnersCnt,cvSize(10,10),5,aFeatureFound,aTrackErrors,
    cvTermCriteria(CV_TERMCRIT_EPS|CV_TERMCRIT_ITER, 200, 0.03),0); 
 }
 else
 {
  cvCalcOpticalFlowPyrLK(m_pPreImg, m_pCurImg, m_pPyrPreImg, m_pPyrCurImg, m_pPreConners,
   m_pCurConners,m_iConnersCnt,cvSize(10,10),5,aFeatureFound,aTrackErrors,
   cvTermCriteria(CV_TERMCRIT_EPS|CV_TERMCRIT_ITER, 200, 0.03),CV_LKFLOW_PYR_A_READY);  
 } 

 
 for(int iLoop = 0;iLoop < m_iConnersCnt; iLoop++)
 {
  if((0 == aFeatureFound[iLoop])||(aTrackErrors[iLoop] > 550))
  {
   printf("Error is %f",aTrackErrors[iLoop]);
   continue;
  }

  CvPoint pt0 = cvPoint(cvRound(m_pPreConners[iLoop].x), cvRound(m_pPreConners[iLoop].y));
  CvPoint pt1 = cvPoint(cvRound(m_pCurConners[iLoop].x), cvRound(m_pCurConners[iLoop].y));

  if((abs(pt0.x - pt1.x) >= 1)||(abs(pt0.y - pt1.y) >= 1))
  {
   cvLine(m_pCurImg,pt0,pt1,cvScalar(255,0,0),2);
   cvLine(m_pPreImg,pt0,pt1,cvScalar(255,0,0),2);
  }
 }

 cvShowImage("PreOptica", m_pPreImg);
 cvShowImage("CurOptica", m_pCurImg);

 memcpy(m_pPyrPreImg->imageData, m_pPyrCurImg->imageData, (m_iWidth+8)*(m_iHeight/3)*4);
 memcpy(m_pPreConners, m_pCurConners, m_iConnersCnt*sizeof(CvPoint2D32f)); 

 return 0;
}