Calculates a contour area.
The function computes a contour area. Similarly to "moments", the area is
computed using the Green formula. Thus, the returned area and the number of
non-zero pixels, if you draw the contour using "drawContours" or "fillPoly",
can be different.
Also, the function will most certainly give a wrong results for contours with
self-intersections.
Example:
// C++ code:
vector contour;
contour.push_back(Point2f(0, 0));
contour.push_back(Point2f(10, 0));
contour.push_back(Point2f(10, 10));
contour.push_back(Point2f(5, 4));
double area0 = contourArea(contour);
vector approx;
approxPolyDP(contour, approx, 5, true);
double area1 = contourArea(approx);
cout << "area0 =" << area0 << endl <<
"area1 =" << area1 << endl <<
"approx poly vertices" << approx.size() << endl;