Checking contour area in opencv using python

Question

I try to use checkContour() function in new python api (cv2) and it do works if I create contours to be checked using findContours e.g.

contours, hierarchy = cv2.findContours(imgGray, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
cv2.contourArea(contours[0])

however when I create contour on my own the following code doesn't work

contour = numpy.array([[0,0], [10,0], [10,10], [5,4]])
area = cv2.contourArea(contour)

and returns "error: (-215) contour.checkVector(2) >= 0 && (contour.depth() == CV_32F || contour.depth() == CV_32S) in function contourArea"

when I change to

contour = numpy.array([[0,0], [10,0], [10,10], [5,4]], dtype=numpy.int32)

I got "error: (-210) The matrix can not be converted to point sequence because of inappropriate element type in function cvPointSeqFromMat"

How to make the following code in C++ from documentation

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);

work in latest python API (2.3)?

qik · Accepted Answer · 2012-01-24 14:17:34Z

35

This one should work:

contour = numpy.array([[[0,0]], [[10,0]], [[10,10]], [[5,4]]])
area = cv2.contourArea(contour)

answered Jan 24, 2012 at 14:17

qik

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1 Comment

Oliver Zendel Over a year ago

Thanks! This is the mad one-liner for calculating the area from a list of float tuples coordinates: area = cv2.contourArea(np.around(np.array([[pt] for pt in FLOAT_TUPLE_LIST])).astype(np.int32))

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