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问题:
I'm trying to draw an arbitrary quadrilateral over an image using the polylines function in opencv. When I do I get the following error
OpenCV Error: Assertion failed (p.checkVector(2, CV_32S) >= 0) in
polylines, file
/tmp/buildd/ros-fuerte-opencv2-2.4.2-1precise-20130312-1306/modules/core/src/d
rawing.cpp, line 2065
I call the function as like so,
cv2.polylines(img, points, 1, (255,255,255))
Where points is as numpy array as shown below (The image size is 1280x960):
[[910 641]
[206 632]
[696 488]
[458 485]]
and img is just a normal image that I'm able to imshow. Currently I'm just drawing lines between these points myself, but I'm looking for a more elegant solution.
How should I correct this error?
回答1:
The problem in my case was that numpy.array
created int64
-bit numbers by default. So I had to explicitly convert it to int32
:
points = np.array([[910, 641], [206, 632], [696, 488], [458, 485]])
# points.dtype => 'int64'
cv2.polylines(img, np.int32([points]), 1, (255,255,255))
(Looks like a bug in cv2 python binding, it should've verified dtype
)
回答2:
This function is not enough well documented and the error are also not very useful. In any case, cv2.polylines
expects a list of points, just change your line to this:
import cv2
import numpy as np
img = np.zeros((768, 1024, 3), dtype='uint8')
points = np.array([[910, 641], [206, 632], [696, 488], [458, 485]])
cv2.polylines(img, [points], 1, (255,255,255))
winname = 'example'
cv2.namedWindow(winname)
cv2.imshow(winname, img)
cv2.waitKey()
cv2.destroyWindow(winname)
The example above will print the following image (rescaled):
回答3:
the error says your array should be of dimension 2. So reshape the array as follows:
points = points.reshape(-1,1,2)
Then it works fine.
Also, answer provided by jabaldonedo also works fine for me.
回答4:
pts = np.array([[40,300],[54,378],[60,420],[30,333]],np.int32)
pts = pts.reshape((-1,1,2))
img = cv2.polylines(img,pts,True,(125,215,145),1)
The official documentation to provide explanation,need reshape