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

user_63621 0
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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):
    '''Create a mask of the same size as the image
       based on the interior of the contour.'''
    mask = np.zeros((img.shape[0], img.shape[1]), np.uint8)
    print("create_mask, cnt=%s" % cnt)
    cv2.drawContours(mask, [cnt], 0, (0, 255, 0), -1)
    return mask

print("Creating mask from contour %s, on raw shape %s" % (page_contour, raw.shape))
page_mask = create_mask(raw, page_contour)

Output (see bottom for error):

Creating mask from contour [[ 1626.   360.]
 [ 1776.  3108.]
 [  126.  3048.]
 [  330.   486.]], on raw shape (3840, 2160, 3)
create_mask, cnt=[[ 1626.   360.]
 [ 1776.  3108.]
 [  126.  3048.]
 [  330.   486.]]
OpenCV Error: Assertion failed (npoints > 0) in drawContours, file /tmp/opencv320160309-92782-1efch74/opencv-3.1.0/modules/imgproc/src/drawing.cpp, line 2380
Traceback (most recent call last):
  File "./books.py", line 209, in <module>
    page_mask = create_mask(raw, page_contour)
  File "./books.py", line 123, in create_mask
    cv2.drawContours(mask, [cnt], 0, (0, 255, 0), -1)
cv2.error: /tmp/opencv320160309-92782-1efch74/opencv-3.1.0/modules/imgproc/src/drawing.cpp:2380: error: (-215) npoints > 0 in function drawContours

The docs say it should get an array of arrays and this is seemingly what I'm giving it. So what's wrong?

Code is ported from OpenCV 2.x.

opencv python c++
7 Answers

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.

user_3317320

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.

user_74467700

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)
user_71306130

Its a numpy array so it will not work this way. Make sure you add this to your code np.array(loop variable).reshape((-1,1,2)).astype(np.int32) This works well.

user_168107760

This error will also occur if the data type of the numpy array passed into the drwaContours function is not int64. This data type error might occur if you apply a transformation to the points in the contour, which changes their dtype. To correct for this error, be sure to convert the contour data type to int64.

new_contour = old_contour_wrong_dtype.dtype('int64')
user_78717100

its hierarchy, contours so:

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

user_123603110
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