Calculate the area of an object with OpenCV

Question

I need to calculate the area of a blob/an object in a grayscale picture (loading it as Mat, not as IplImage) using OpenCV. I thought it would be a good idea to get the coordinates of the edges (number of edges change form object to object) or to get all coordinates of the contour and then use contourArea() to calculate the area of my object.

I deleted all noise and got some nice and satisfying contours by using findContours() (programming in C++).

findContours(InputOutputArray image, OutputArrayOfArrays contours, OutputArray hierarchy,int mode, int method, Point offset=Point());

Now I got to understand that param contours already owns the coordinates of all contours of my object. Did I get that right?

If yes, it there a way to access them?

And if no, how do I get the coordinates of the contour anyway?

Answer 1

contours is actually defined as

vector<vector<Point> > contours;

And now I think it's clear how to access its points.

The contour area is calculated by a function nicely called contourArea():

for (unsigned int i = 0;  i < contours.size();  i++)
{
     std::cout << "# of contour points: " << contours[i].size() << std::endl;

     for (unsigned int j=0;  j<contours[i].size();  j++)
     {
         std::cout << "Point(x,y)=" << contours[i][j] << std::endl;
     }

     std::cout << " Area: " << contourArea(contours[i]) << std::endl;
}