cvFindContours always returns 0 - OpenCV - visual-c++

Question

I'm calling the cvFindContours function inside a separate thread that I've created to handle all OpenCV work while another is kept for OpenGL stuff.
I noticed that my cvFindContours function always returns 0 when this code is executed inside a separate thread. It worked fine before, when executed in the main thread itself. I used breakpoints and Watches to evaluate value changes. everything else (variables) gets values except for contourCount (value: 0).
Any clue?
// header includes goes here
CvCapture* capture = NULL;
IplImage* frame = NULL;
IplImage* image;
IplImage* gray;
IplImage* grayContour;
CvMemStorage *storage;
CvSeq *firstcontour=NULL;
CvSeq *polycontour=NULL;
int contourCount = 0;
DWORD WINAPI startOCV(LPVOID vpParam){
capture = cvCaptureFromCAM(0); // NOTE 1
capture = cvCaptureFromCAM(0);
frame = cvQueryFrame(capture);
image = cvCreateImage(cvGetSize(frame), IPL_DEPTH_8U,3);
gray = cvCreateImage(cvGetSize(image), IPL_DEPTH_8U,1);
grayContour = cvCreateImage(cvGetSize(image), IPL_DEPTH_8U,1);
storage = cvCreateMemStorage (0);
firstcontour=NULL;
while(1){
frame = cvQueryFrame(capture);
cvCopy(frame,image);
cvCvtColor(image,gray,CV_BGR2GRAY);
cvSmooth(gray,gray,CV_GAUSSIAN,3);
cvThreshold (gray, gray, 0, 255, CV_THRESH_BINARY | CV_THRESH_OTSU);
cvNot(gray,gray);
cvCopy(gray,grayContour);
contourCount=cvFindContours (grayContour, storage, &firstcontour, sizeof (CvContour),
CV_RETR_CCOMP);
polycontour=cvApproxPoly(firstcontour,sizeof(CvContour),storagepoly,CV_POLY_APPROX_DP,3,1); // Error starts here (Pls refer to stack trace)
}
// goes on...
}
int main(int argc, char** argv){
DWORD qThreadID;
HANDLE ocvThread = CreateThread(0,0,startOCV, NULL,0, &qThreadID);
initGL(argc, argv); //some GL intitialization functions
glutMainLoop(); // draw some 3D objects
CloseHandle(ocvThread);
return 0;
}
NOTE1: these lines had to be duplicated due to the error mentioned at How to avoid "Video Source -> Capture source" selection in OpenCV 2.3.0 - Visual C++ 2008
Environment:
OpenCV 2.3.0
Visual C++ 2008
EDIT
Traces
opencv_core230d.dll!cv::error(const cv::Exception & exc={...}) Line 431 C++
opencv_imgproc230d.dll!cvPointSeqFromMat(int seq_kind=20480, const void * arr=0x00000000, CvContour * contour_header=0x01a6f514, CvSeqBlock * block=0x01a6f4f4) Line 47 + 0xbd bytes C++
opencv_imgproc230d.dll!cvApproxPoly(const void * array=0x00000000, int header_size=88, CvMemStorage * storage=0x017e7b40, int method=0, double parameter=3.0000000000000000, int parameter2=1) Line 703 + 0x28 bytes C++
Project.exe!startOCV(void * vpParam=0x00000000) Line 267 + 0x24 bytes C++
All this stuff boils down to the function CV_Assert( arr != 0 && contour_header != 0 && block != 0 ) in cvPointSeqFromMat and it fails since arr it requires is empty.

Answer 1

Your variable contourCount is not doing what you think it's doing. From the contours.cpp source file:
/*F///////////////////////////////////////////////////////////////////////////////////////
// Name: cvFindContours
// Purpose:
// Finds all the contours on the bi-level image.
// Context:
// Parameters:
// img - source image.
// Non-zero pixels are considered as 1-pixels
// and zero pixels as 0-pixels.
// step - full width of source image in bytes.
// size - width and height of the image in pixels
// storage - pointer to storage where will the output contours be placed.
// header_size - header size of resulting contours
// mode - mode of contour retrieval.
// method - method of approximation that is applied to contours
// first_contour - pointer to first contour pointer
// Returns:
// CV_OK or error code
// Notes:
//F*/
You are getting CV_OK == 0, which means it successfully ran. cvFindContours does not return the number of contours found to you. It merely lets you known if it failed or not. You should use the CvSeq* first_contour to figure out the number of contours detected.
Hope that helps!