Opencv polylines function in python throws exception

Question

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?

Answer 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)

Answer 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):

Answer 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.

Answer 4

Replace cv2.fillPoly( im, np.int32(points)) with cv2.fillPoly( im, np.int32([points])). It will work.

Answer 5

I also faced the same problem The solution is to make a array of 1 row, 2 columns and - 1 depth, - 1 means unknown dimensions so numpy will assign convenient depth to the array. If you make array of more than 1 row and 2 columns it will show error.

Also this error can come when the array you created is not of type int32

Vertices = np.array([[36,86] ,[73,73], [87,87]], dtype=np.int32)

Answer 6

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

Answer 7

import cv2
import numpy as np

sz, sh, of = 1000, 500, 100

# Create an Empty image with white background
im = 255 * np.ones(shape=[sz, sz, 3], dtype=np.uint8)

# Draw shapes
im = cv2.polylines(
    img=im,
    pts=[np.int32([
        [of, of], 
        [sh, of + of], 
        [sz - of, of],
        [sz-of-of,sh],
        [sz-of,sz-of],
        [sh,sz-of-of],
        [of,sz-of],
        [of+of,sh]])],
    isClosed=True,
    color=(128, 0, 200),
    thickness=30,
    lineType=cv2.LINE_AA,  # Anti-Aliased
)

cv2.imwrite("polylines.jpg", im)