Get coordinates of contours in OpenCV - c++

Question

Let's say that I have the following output image:
Basically, I have video stream and I want to get coordinates of rectangle only in the output image. Here's my C++ code:
while(1)
{
capture >> frame;
if(frame.empty())
break;
cv::cvtColor( frame, gray, CV_BGR2GRAY ); // Grayscale image
Canny( gray, canny_output, thresh, thresh * 2, 3 );
// Find contours
findContours( canny_output, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
// Draw contours
Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
for( int i = 0; i< contours.size(); i++ )
{
Scalar color = Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
drawContours( drawing, contours, i, color, 2, 8, hierarchy, 0, Point() );
}
cv::imshow( "w", drawing );
waitKey(20); // waits to display frame
}
Thanks.

Answer 1

Look at the definition of the find contours function in the opencv documentation and see the parameters (link):
void findContours(InputOutputArray image, OutputArrayOfArrays contours, OutputArray hierarchy, int mode, int method, Point offset=Point())
Parameters: here
Look at contours, like Rafa said each contour is stored in a vector of points and each vector of points is stored in a vector, so, by walking in the outer vector and then walking in the inner vector you'll be finding the points you wish.
However, if you want to detect only the bigger contour you might want to use CV_RETR_EXTERNAL as the mode parameter, because it'll detect only most external contour (the big rectangle).
If you still wish to maintain the smaller contours then you might use the CV_RETR_TREE and work out with the hierarchy structure: Using hierarchy contours

Answer 2

Looking at the documentation, the OutputArrayOfArrays contours is the key.
contours – Detected contours. Each contour is stored as a vector of points.
so, you've got a vector< vector<Point> > contours. The vector<Point>(inside) is the coordinates of a contour, and every contour is stored in a vector.
So for instance, to know the 5-th vector, it's vector<Point> fifthcontour = contours.at(4);
and you have the coordinates in that vector.
You can access to those coordinates as:
for (int i = 0; i < fifthcontour.size(); i++) {
Point coordinate_i_ofcontour = fifthcontour[i];
cout << endl << "contour with coordinates: x = " << coordinate_i_ofcontour.x << " y = " << coordinate_i_ofcontour.y;
}