Can't execute approxPolyDp function in OpenCV - c++

Question

I am trying to convert contour to set of polygonal curves, but I am stuck when I try to use approxPolyDP function. First I tested if findContours works properly and try to draw contours in my image - It works for contourIdx = 0. Then I try to use approxPolyDp as shown in example: http://docs.opencv.org/doc/tutorials/imgproc/shapedescriptors/bounding_rects_circles/bounding_rects_circles.html
But during execution I have error "Acces violation" connected with vector class and funciton size(). Here is my code:
IplImage* image = cvLoadImage("F:\\triangle.png");
waitKey(5000);
//Mat img = imread("triangle.png");
Mat img(image,true);
if(!img.data)
{
cout <<"image file not found";
cv::waitKey(5000);
return -1;
}
//namedWindow( "window", 0 );
//imshow( "window", img );
cvNamedWindow("window");
cvShowImage("window",image);
Mat imgGray;
Mat imgEdges;
cvtColor(img,imgGray,CV_BGR2GRAY);
blur(imgGray,imgGray,Size(3,3));
threshold(imgGray,imgEdges,128,255,CV_THRESH_BINARY);
Mat canny_output;
vector<vector<Point>> contours;
vector<Vec4i> hierarchy;
/// Detect edges using canny
Canny( imgGray, canny_output,100, 100*2, 3 );
/// Find contours
findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
/// Draw contours
RNG rng(12345);
Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
if (drawing.type() != CV_8UC3)
{
cout << "Error: image type different then CV_8UC3";
}
Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
drawContours( drawing, contours, 0, color, 2, 8, hierarchy, 0, Point() );
IplImage img3 = drawing;
cvNamedWindow( "Contours", CV_WINDOW_AUTOSIZE );
cvShowImage( "Contours", &img3 );
vector<vector<Point>> contoursOUT/*(contours.size())*/;
approxPolyDP(Mat(contours[0]),contoursOUT,3,true );
waitKey(0);
return 0;
Has anyone any idea what's wrong here?

Answer 1

The OpenCV-doc says about approxPolyDP:
void approxPolyDP (InputArray curve,
OutputArray approxCurve,
double epsilon,
bool closed)
approxCurve - ... The type should match the type of the input curve. ...
So the last part of your implementation should be:
vector<Point> contoursOUT;
approxPolyDP( Mat(contours[0]), contoursOUT, 3, true );
I teste your code with this small change, and it compiles and outputs resonable results.

Answer 2

I would recommend you to first study the documentation of approxPolyDP in opencv properly.
Now talking about your code, you have done a wrong declaration.
vector<vector<Point>> contoursOUT;
Use this one which is correct,
vector<Point> contoursOUT;
Also, in case of multiple object use for loop.