OpenCV 3.1 drawContours '(-215) npoints > 0'

Question

I\'m trying to create a mask from a contour, but am getting a C++ error.

Using OS X Yosemite, Python 2.7.10, OpenCV 3.1.0.

def create_mask(img, cnt):
            


        

Answer 1

For me this worked. But I'm not sure why.

cv2.drawContours(mask, [cnt.astype(int)], 0, (0, 255, 0), -1)

When you get an array of rounded floats from findContours, drawContours doesn't complain. But when I construct a similar (4,2) array of floats myself, it complains.

Answer 2

I think you are adding extra [] around cnt it should be

cv2.drawContours(mask, cnt, 0, (0, 255, 0), -1)

as cnt is already array of array but [cnt] is array of array of arrays which won't work

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Update to the above code

you should convert your contour to numpy array first

ctr = numpy.array(cnt).reshape((-1,1,2)).astype(numpy.int32)
cv2.drawContours(mask, [ctr], 0, (0, 255, 0), -1)

check documentation here

contours is a Python list of all the contours in the image. Each individual contour is a Numpy array of (x,y) coordinates of boundary points of the object.

Answer 3

You might have commited mistake while finding the contours. Contour is the second value returned by findContours() function as the docs say

im2, contours, hierarchy = cv.findContours(thresh, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

So the following code will not work

cnt = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

This might solve your problem.

Answer 4

If you just use this, it will work...

ctr = np.array(cnt).reshape((-1,1,2)).astype(np.int32)
cv2.drawContours(mask, [ctr], -1, 255, -1)

Answer 5

its hierarchy, contours so:

contours, hierarchy = cv.findContours(thresh, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)