Assertion failed (axes.width >= 0 && axes.height...)line 1772

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Hello everybody,

In following code I read in my webcam and want to detect face. Unfortunately it does not work. I commented some parts of the end out and I have NO problems only reading in the cam. But the whole code gives the error like below.

Here is the code:

#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

#include <iostream>
#include <stdio.h>

using namespace std;
using namespace cv;

Mat detectFace(Mat src);

int main()
{
    VideoCapture cap(0);
    namedWindow("window1", 1);

    while (1)
    {
        Mat frame;
        cap >> frame;
        frame = detectFace(frame);

        imshow("window1", frame);
        // Press 'c' to escape
        if (waitKey(1) == 'c') break;
    }

    waitKey(0);
    return 0;
}

Mat detectFace(Mat image)
{
    // Load Face cascade (.xml file)
    CascadeClassifier face_cascade("haarcascade_frontalface_alt2.xml");

    // Detect faces
    std::vector<Rect> faces;
    face_cascade.detectMultiScale(image, faces, 1.1, 2, 0 | CV_HAAR_SCALE_IMAGE, Size(30, 30));

    ////// Draw circles on the detected faces
    for (int i = 0; i < faces.size(); i++)
    {
        Point center(faces[i].x + faces[i].width*0.5, faces[i].y + faces[i].height*0.5);
        ellipse(image, center, Size(faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar(255, 0, 255), 4, 8, 0);
    }
    return image;
}

This is the error message:

 OpenCV Error: Assertion failed (axes.width >= 0 && axes.height >= 0 && thickness <= 255 && 0 <= shift && shift <= XY_SHIFT) in cv::ellipse, file C:\builds\master_PackSlave-win32
-vc12-shared\opencv\modules\imgproc\src\drawing.cpp, line 1772

I am looking forward for your support and help. Thank you!