I was trying to create an application in which the mouse can be used to draw a rectangle to set the roi of the image captured from webcam. However, the application crashes when I draw the rectangle. The issue is not with the drawing of the rectangle but the line of code which crops the roi because things work fine when I remove that line. Somebody please help me resolve the issue.

int drag = 0;
int mouse_x = 0;
int mouse_y = 0;
Point point(0,0);
Mat original;
//Mat roi(Scalar(0,0,255));
VideoCapture xps(0);
Mat img;

void set_roi(int event, int x,int y, int flags,void* param)

{
    original = img.clone();

    if (event == CV_EVENT_LBUTTONDOWN && !drag)
    {
        point = Point(x, y);
        drag = 1;
    }

    if (event == CV_EVENT_MOUSEMOVE && drag)
    {
        //cout << "x " << x << "y " << y << "" << endl;
        mouse_x = x;
        mouse_y = y;
    }

    if (event == CV_EVENT_LBUTTONUP && drag)
    {
        //
        drag = 0;
    }


    if (event == CV_EVENT_RBUTTONUP)
    {
        drag = 0;
    }

}

int _tmain(int argc, _TCHAR* argv[])
{
    Mat roi;
    namedWindow("select roi", 1);
    namedWindow("roi", 1);
    setMouseCallback("select roi", set_roi, NULL);

    while (1)
    {

        xps.read(img);
        original = img.clone();
        roi = img.clone();
        rectangle(original, point, Point(mouse_x, mouse_y), Scalar(255, 0, 0), 1, 8, 0);
        roi = roi(Rect(point.x, point.y, mouse_x - point.x, mouse_y - point.y));
        imshow("Read image", img);
        imshow("select roi",original);
        imshow("roi", roi);
        waitKey(1);
    }
    return 0;
}

Okay...it was a problem with drawing contours. The error occurs when attempting to call draw contours even when nothing was found.

I was using contours for object identification. The code worked well with images and I modified the code to identify objects in real time with camera input. Things work well with my laptop's integrated cam but crashes after a few seconds when using an external camera. The external camera worked fine with a few other applications I developed using opencv. The camera is a 20MP camera. Please look at the code and help me figure out what might be wrong. My processor is good enough to handle images with high resolutions. It seems that the app crashes when I introduce an object in front of the cam which was not there before when the app started up.

include <iostream>

include "opencv2/highgui/highgui.hpp"

include "opencv2/imgproc/imgproc.hpp"

using namespace cv; using namespace std; int main() { int largest_area = 0; int largest_contour_index = 0; Rect bounding_rect; int x = 0; int y = 0; VideoCapture xps(0); Mat src; while (1) {

    xps.read(src);


    vector<vector<Point>> contours; // Vector for storing contour
    vector<Vec4i> hierarchy;


    Mat thr(src.rows, src.cols, CV_8UC1);
    Mat dst(src.rows, src.cols, CV_8UC1, Scalar::all(0));
    cvtColor(src, thr, CV_BGR2GRAY); //Convert to gray
    threshold(thr, thr, 80, 255, THRESH_BINARY_INV);
    findContours(thr, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE); 

    for (int i = 0; i< contours.size(); i++) // iterate through each contour.
    {
        double a = contourArea(contours[i], false);  //  Find the area of contour
        if (a>largest_area)
        {
            largest_area = a;
            largest_contour_index = i;                
            bounding_rect = boundingRect(contours[i]); 
        }

    }


    Scalar color(255, 255, 255);
    drawContours(dst, contours, largest_contour_index, color, CV_FILLED, 8, hierarchy); 
    rectangle(src, bounding_rect, Scalar(0, 255, 0), 1, 8, 0);

    x = bounding_rect.x + bounding_rect.width / 2;
    y = bounding_rect.y + bounding_rect.height / 2;

    circle(src, Point(x, y), 1, Scalar(0, 0, 255));

    imshow("src", src);
    imshow("largest Contour", dst);
    waitKey(2);
}

}

Hello, I was trying to learn object tracking by looking at an existing code. I understand what erosion and dilation. From the following piece of code, I understand that an ellipse is used as structuring element:

erode(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));
dilate(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));

//morphological closing (removes small holes from the foreground)
dilate(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));
erode(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)));

In this case, we are tracking a circular object. Is it because of that an ellipse is used as structuring element? How do you choose the right structuring element?

I am new to Open CV and was trying to find the centroid of an object. I understnad the moments object. But what do these mean

double dM01 = oMoments.m01; double dM10 = oMoments.m10; double dArea = oMoments.m00;

Can someone explain how the x and y coordinates are obtained by doing the following computation: int posX = dM10 / dArea; int posY = dM01 / dArea;