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OpenCV Drawing Contour Error Assertion Failed

So I am trying to follow guide on how to scan a document in https://www.pyimagesearch.com/2014/09/01/build-kick-ass-mobile-document-scanner-just-5-minutes/ exactly on the step 2 process where i am supposed to find the contour and draw it to the image, i got an “Assertion Failed” error on drawContour function

The guide did not have

screenCnt = None

so at first i got Error like screenCnt did not exist

after i add it, got Assertion Failed instead even though i used the same image as the guide and tried another image too

#find contour
cnts = cv2.findContours(edged.copy(), cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
cnts = imutils.grab_contours(cnts)
cnts = cnts[0] if imutils.is_cv2() else cnts[1]
cnts = sorted(cnts, key=cv2.contourArea, reverse=True)[:5]
screenCnt = None

# loop over the contours
for c in cnts:
    peri = cv2.arcLength(c, True)
    approx = cv2.approxPolyDP(c, 0.02 * peri, True)
    if len(approx) == 4:
        screenCnt = approx
        break

#draw contour
cv2.drawContours(image, [screenCnt], -1, (0, 255, 0), 2)

Here is what i got :

Traceback (most recent call last): File “C:/Users/User/PycharmProjects/learn/project/app.py”, line 218, in cv2.drawContours(image, [screenCnt], -1, (0, 255, 0), 2)

cv2.error: OpenCV(4.1.0) C:projectsopencv-pythonopencvmodulesimgprocsrcdrawing.cpp:2606: error: (-215:Assertion failed) reader.ptr != NULL in function ‘cvDrawContours’

Any solution for this ? Thanks before

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Answer

Try replacing screenCnt = 0 instead of None. and let me know. For your reference, I’m providing small code snippet:

(cnts, contours, heirarchy) = cv2.findContours(edged.copy(), 
cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

cnts = contours[0]
screenCnt = 0

for contour in contours:
    # get rectangle bounding contour
    [x,y,w,h] = cv2.boundingRect(contour)

# draw rectangle around contour on original image
#cv2.rectangle(image,(x,y),(x+w,y+h),(255,0,255),2)

##### code added..for thresholding
for c in cnts:
    # approximate the contour
    peri = cv2.arcLength(c, True)
    approx = cv2.approxPolyDP(c, 0.02 * peri, True)

    # if our approximated contour has four points, then
    # we can assume that we have found our screen
    if len(approx) == 4:
        screenCnt = approx
        break
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