contourArea

Calculates a contour area.

double contourArea(InputArray contour, bool oriented=false )

Parameters:
contour – Input vector of 2D points (contour vertices), stored in std::vector or Mat.
oriented – Oriented area flag. If it is true, the function returns a signed area value, depending on the contour orientation (clockwise or counter-clockwise). Using this feature you can determine orientation of a contour by taking the sign of an area. By default, the parameter is false, which means that the absolute value is returned.会有方向的话，就会返回有正负，否则就是绝对值。
例子

vector<Point> 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<Point> approx;
approxPolyDP(contour, approx, 5, true);
double area1 = contourArea(approx);

cout << "area0 =" << area0 << endl <<
        "area1 =" << area1 << endl <<
        "approx poly vertices" << approx.size() << endl;

可以看出拟合的多边形有误差，面积有5的差距，就是

void approxPolyDP(InputArray curve, OutputArray approxCurve, double epsilon, bool closed)

中的epslion就是拟合的误差。可以看出，正好误差是5.

所以可以检测轮廓后，然后用contourArea算出面积去掉那些面积很小的轮廓，就可以实现一些差不多的目标检测的目的。