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.會有方向的話，就會返回有正負，否則就是絕對值。
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;

結果

可以看出擬合的多邊形有誤差，面積有5的差距，就是

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

中的epslion就是擬合的誤差。可以看出，正好誤差是5.

所以可以檢測輪廓后，然后用contourArea算出面積去掉那些面積很小的輪廓，就可以實現一些差不多的目標檢測的目的。