how cvCamShift works good - c++

Question

i am trying to use cvCamShift function in opencv. i have written a class for doing this.this is it's code:
#include <cv.h>
#include <highgui.h>
using namespace std;
class Tracker{
// File-level variables
IplImage * pHSVImg; // the input image converted to HSV color mode
IplImage * pHueImg; // the Hue channel of the HSV image
IplImage * pMask; // this image is used for masking pixels
IplImage * pProbImg; // the face probability estimates for each pixel
CvHistogram * pHist; // histogram of hue in the original face image
CvRect prevFaceRect; // location of face in previous frame
CvBox2D faceBox; // current face-location estimate
int vmin;
int vmax;
int smin;
void updateHueImage(IplImage* pImg);
public:
Tracker(IplImage * pImg, CvRect pFaceRect);
~Tracker();
CvBox2d track(IplImage* pImg);
};
Tracker::Tracker(IplImage * pImg, CvRect pFaceRect){
// File-level variables
int nHistBins = 30; // number of histogram bins
float rangesArr[] = {0,180}; // histogram range
vmin = 10;
vmax = 256;
smin = 55;
float * pRanges = rangesArr;
pHSVImg = cvCreateImage( cvGetSize(pImg), 8, 3 );
pHueImg = cvCreateImage( cvGetSize(pImg), 8, 1 );
pMask = cvCreateImage( cvGetSize(pImg), 8, 1 );
pProbImg = cvCreateImage( cvGetSize(pImg), 8, 1 );
pHist = cvCreateHist( 1, &nHistBins, CV_HIST_ARRAY, &pRanges, 1 );
float maxVal = 0.f;
// Create a new hue image
updateHueImage(pImg);
// Create a histogram representation for the face
cvSetImageROI( pHueImg, pFaceRect );
cvSetImageROI( pMask, pFaceRect );
cvCalcHist( &pHueImg, pHist, 0, pMask );
cvGetMinMaxHistValue( pHist, 0, &maxVal, 0, 0 );
cvConvertScale( pHist->bins, pHist->bins, maxVal? 255.0/maxVal : 0, 0 );
cvResetImageROI( pHueImg );
cvResetImageROI( pMask );
// Store the previous face location
prevFaceRect = pFaceRect;
}
Tracker::~Tracker(){
cvReleaseImage( &pHSVImg );
cvReleaseImage( &pHueImg );
cvReleaseImage( &pMask );
cvReleaseImage( &pProbImg );
cvReleaseHist( &pHist );
}
void Tracker::updateHueImage(IplImage * pImg)
{
// Convert to HSV color model
cvCvtColor( pImg, pHSVImg, CV_BGR2HSV );
// Mask out-of-range values
cvInRangeS( pHSVImg, cvScalar(0, smin, MIN(vmin,vmax), 0),
cvScalar(180, 256, MAX(vmin,vmax) ,0), pMask );
// Extract the hue channel
cvSplit( pHSVImg, pHueImg, 0, 0, 0 );
}
CvBox2D Tracker::track(IplImage * pImg)
{
CvConnectedComp components;
updateHueImage(pImg);
cvCalcBackProject( &pHueImg, pProbImg, pHist );
cvAnd( pProbImg, pMask, pProbImg, 0 );
cvCamShift( pProbImg, prevFaceRect,
cvTermCriteria( CV_TERMCRIT_EPS | CV_TERMCRIT_ITER, 10, 1 ),
&components, &faceBox );
prevFaceRect = components.rect;
faceBox.angle = -faceBox.angle;
return faceBox;
}
it works good but when tracked object goes out of the frame or somthing comes front of it, it makes a runtime error. one of my friends has a code that it's very similar to my code and it doesn't make any runtime error even in the worst situations. please guide me how can i debug this.

Answer 1

I think when tracked object goes out of the frame or something comes front of it, the parameter prevFaceRect will be null and may be because of that, it is showing error. So before calling this function
cvCamShift( pProbImg, prevFaceRect,
cvTermCriteria( CV_TERMCRIT_EPS | CV_TERMCRIT_ITER, 10, 1 ),
&components, &faceBox );
add a check, to know whether prevFaceRect is null or not