OpenCV polylines throws assertion failure even when arg type is what it's supposed to be

Question

Several threads report this problem, but all of them show fixing the type of the point list (to int32, i.e. CV_32S) as the solution. The point list below has the appropriate type.

>>> ti = np.zeros((400,400,3), np.uint8) # Test image >>> mask = np.array([[213,60],[333,240],[93,240],[150,120]], np.int32) # Test point list >>> cv2.polylines(ti, mask, True, (255,0,0), 1) *** cv2.error: OpenCV(3.4.3) C:\projects\opencv-python\opencv\modules\imgproc\src\drawing.cpp:2437: error: (-215:Assertion failed) p.checkVector(2, CV_32S) >= 0 in function 'cv::polylines' >>> print(mask.dtype, mask.shape) int32 (4, 2)

I've tried changing the type (I get a different error, complains it's the wrong type) and re-casting it to int32. Can't make it play nice...

fillConvexPoly works fine (which got me what I needed for now), fillPoly exhibits the same problem as above.

I.Newton · Accepted Answer · 2018-10-04 09:34:53Z

4

Correction to the above is -

cv2.polylines(ti, [mask], True, (255,0,0), 1)

answered Oct 4, 2018 at 9:34

I.Newton

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2 Comments

Riker Over a year ago

Thanks! If mask is defined with np.array( [ [ [x1, y1], [x2, y2], [x3, y3] ] ], np.int32 ) it's all good. Not sure why it wants an array of an array of points, or how one would infer that from any of the docs I've seen -- great example of the value of SO!

Franva Over a year ago

the trick is to add [ ] around the mask. it works for me

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