Python code Error "name 'screenCnt' is not defined"

Question

I am new in python and I created anaconda environment and running given code but it's showing error

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

        # our approximated contour should have four points
        if len(approx) == 4:
            screenCnt = approx
            break

# show the contour (outline) of the piece of paper
#print("STEP 2: Find contours of paper")
    cv2.drawContours(orig, [screenCnt], -1, (0, 255, 0), 2)

#     apply the four point transform to obtain a top-down
#     view of the original image
    warped = four_point_transform(orig, screenCnt.reshape(4, 2) * ratio)

Error is

NameError Traceback (most recent call last) in 122 # show the contour (outline) of the piece of paper 123 #print("STEP 2: Find contours of paper") --> 124 cv2.drawContours(orig, [screenCnt], -1, (0, 255, 0), 2) 125 126 # apply the four point transform to obtain a top-down

NameError: name 'screenCnt' is not defined

but in code I already defined screenCnt. initially, this code was running properly but now it's not working or it's show error after every alternate execution. (it run properly the first time and when restart kernel giving same error).

or if i set screenCnt = 0 or screenCnt = None it giving error

error Traceback (most recent call last) in 123 # show the contour (outline) of the piece of paper 124 #print("STEP 2: Find contours of paper") --> 125 cv2.drawContours(orig, [screenCnt], -1, (0, 255, 0), 2) 126 127 # apply the four point transform to obtain a top-down

error: OpenCV(4.2.0) C:\projects\opencv-python\opencv\modules\imgproc\src\drawing.cpp:2509: error: (-215:Assertion failed) npoints > 0 in function 'cv::drawContours'

Answer 1

looking at your code, probably the statement screenCnt = approx is never hit during the execution. Try initializing screenCnt to some default value.

Answer 2

Looks like the screenCnt variables in lines 81 and 83 are not of the same. Or either you are trying to access the screenCnt which is defined locally within the scope of the if statement.

Answer 3

Obviously, neither 0 nor none meets the data type required by the second parameter of cv2.drawcontours, The specific details of the function can be seen here https://docs.opencv.org/4.x/d4/d73/tutorial_py_contours_begin.html

Analyze the data type. cnts is a set containing multiple contours. Each contour taken out during the cycle is defined as cnt . cnt is a point set defined by multiple two-tuples, so we can initialize it like this screenCnt = [[0,0], [255,0], [255,255], [0,255]] to initialize with a contour.

Answer 4

It is showing not defined because the program could not detect 4 definite points in the image. Try to place the paper on a dark background.