C++ First-chance exception - visual-c++

Question

I have wrote a shape detection code with c++.
I am using Visual Studio 2013 Express Desktop Edition.
When I run the program it will give following error.
First-chance exception at 0x54EE3C77 (opencv_imgproc244d.dll) in Final.exe: 0xC0000005: Access violation reading location 0x05958000.
If there is a handler for this exception, the program may be safely continued.
Following is the code sample.
#include "stdafx.h"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <thread>
#include <cv.h>
#include <highgui.h>
#include <windows.h>
void shapeDetectionNew(IplImage* img)
{
try
{
//converting the original image into grayscale
IplImage* imgGrayScale = cvCreateImage(cvGetSize(img), 8, 1);
cvCvtColor(img, imgGrayScale, CV_BGR2GRAY);
//thresholding the grayscale image to get better results
cvThreshold(imgGrayScale, imgGrayScale, 128, 255, CV_THRESH_BINARY);
CvSeq* contour; //hold the pointer to a contour
CvSeq* result; //hold sequence of points of a contour
CvMemStorage *storage = cvCreateMemStorage(0); //storage area for all contours
//finding all contours in the image
cvFindContours(imgGrayScale, storage, &contour, sizeof(CvContour), CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE, cvPoint(0, 0));
//***********************************
cv::Mat img = cv::imread("F:\\My Works\\Opencv\\Shape_Detection_Images\\shape.jpg");
//convert IplImage to Mat
//cv::Mat img(iplImg);
cv::Mat gray;
cv::cvtColor(img, gray, CV_BGR2GRAY);
// Use Canny instead of threshold to catch squares with gradient shading
cv::Mat bw;
cv::Canny(gray, bw, 0, 50, 5);
// Find contours
std::vector<std::vector<cv::Point> > contours;
cv::findContours(bw.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
std::vector<cv::Point> approx;
cv::Mat dst = img.clone();
//***********************************
for (int i = 0; i < contours.size(); i++)
{
// Approximate contour with accuracy proportional
// to the contour perimeter
cv::approxPolyDP(cv::Mat(contours[i]), approx, cv::arcLength(cv::Mat(contours[i]), true)*0.02, true);
// Skip small or non-convex objects
if (std::fabs(cv::contourArea(contours[i])) < 100 || !cv::isContourConvex(approx))
continue;
if (approx.size() == 3)
{
setLabel(dst, "TRI", contours[i]); // Triangles
}
else if (approx.size() >= 4 && approx.size() <= 6)
{
// Number of vertices of polygonal curve
int vtc = approx.size();
// Get the cosines of all corners
std::vector<double> cos;
for (int j = 2; j < vtc + 1; j++)
cos.push_back(angle(approx[j%vtc], approx[j - 2], approx[j - 1]));
// Sort ascending the cosine values
std::sort(cos.begin(), cos.end());
// Get the lowest and the highest cosine
double mincos = cos.front();
double maxcos = cos.back();
// Use the degrees obtained above and the number of vertices
// to determine the shape of the contour
if (vtc == 4 && mincos >= -0.1 && maxcos <= 0.3)
setLabel(dst, "RECT", contours[i]);
else if (vtc == 5 && mincos >= -0.34 && maxcos <= -0.27)
setLabel(dst, "PENTA", contours[i]);
else if (vtc == 6 && mincos >= -0.55 && maxcos <= -0.45)
setLabel(dst, "HEXA", contours[i]);
}
else
{
// Detect and label circles
double area = cv::contourArea(contours[i]);
cv::Rect r = cv::boundingRect(contours[i]);
int radius = r.width / 2;
if (std::abs(1 - ((double)r.width / r.height)) <= 0.2 &&
std::abs(1 - (area / (CV_PI * std::pow((double)radius, 2)))) <= 0.2)
setLabel(dst, "CIR", contours[i]);
}
}
cv::imshow("Shape_Detection", dst);
}
catch (int e)
{
throw e;
}
}
int main()
{
try
{
// Create CvCapture object to grab data from the webcam
CvCapture* pCapture;
// Start capturing data from the webcam
pCapture = cvCaptureFromCAM(1);
// Define the IplImage pointers we're going to use as globals
IplImage* pFrame;
IplImage* pProcessedFrame;
IplImage* tempFrame;
pFrame = cvQueryFrame(pCapture);
//Create the low threshold slider
// Format: Slider name, window name, reference to variable for slider, max value of slider, callback function
cvCreateTrackbar("Low Threshold", "Edge_Detection", &lowSliderPosition, maxLowThreshold, onLowThresholdSlide);
// Create the high threshold slider
cvCreateTrackbar("High Threshold", "Edge_Detection", &highSliderPosition, maxHighThreshold, onHighThresholdSlide);
// Create a greyscale image which is the size of our captured image
pProcessedFrame = cvCreateImage(cvSize(pFrame->width, pFrame->height), IPL_DEPTH_8U, 1);
// Create a frame to use as our temporary copy of the current frame but in grayscale mode
tempFrame = cvCreateImage(cvSize(pFrame->width, pFrame->height), IPL_DEPTH_8U, 1);
char keypress;
bool quit = false;
int counterCheck = 1;
while (quit == false)
{
// Make an image from the raw capture data
// Note: cvQueryFrame is a combination of cvGrabFrame and cvRetrieveFrame
pFrame = cvQueryFrame(pCapture);
// Draw the original frame in our window
cvShowImage("Live_Cam", pFrame);
shapeDetectionNew(pFrame);
} // End of while loop
cvDestroyAllWindows();
}
catch (Exception ex)
{
}
return 1;
}

Answer 1

You are not checking that the result of
pFrame = cvQueryFrame(pCapture);
is not null. If it is, eventuallu this will get passed to cvCvtColor() and hence the exception.
Completely separate problem:imshow() will not display anything without a subsequent call to waitKey()