How to solve an python openCV error "not enough values to unpack (expected 3, got 2)" produced while finding the contours?

Question

I am working on an openCV project where I need to find a contours of an image. The code is working with the educational video but it produces errors when I tried it myself.

_, contours,_= cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

This is the complete code:

import cv2
import numpy as np

cap = cv2.VideoCapture("eye_recording.flv")

while True:
    ret, frame = cap.read()
    if ret is False:
        break

    roi = frame[269: 795, 537: 1416]
    rows, cols, _ = roi.shape
    gray_roi = cv2.cvtColor(roi, cv2.COLOR_BGR2GRAY)
    gray_roi = cv2.GaussianBlur(gray_roi, (7, 7), 0)

    _, threshold = cv2.threshold(gray_roi, 3, 255, cv2.THRESH_BINARY_INV)
    _, contours,_= cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
    print( cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE))
    #_,_,_= cv2.findContours(image=threshold, mode= cv2.PETER_TREE, method=cv2.CHAIN_APPROX_SIMPLE)

    contours = sorted(contours, key=lambda x: cv2.contourArea(x), reverse=True)
    for cnt in contours:
        (x, y, w, h) = cv2.boundingRect(cnt)

        #cv2.drawContours(roi, [cnt], -1, (0, 0, 255), 3)
        cv2.rectangle(roi, (x, y), (x + w, y + h), (255, 0, 0), 2)
        cv2.line(roi, (x + int(w/2), 0), (x + int(w/2), rows), (0, 255, 0), 2)
        cv2.line(roi, (0, y + int(h/2)), (cols, y + int(h/2)), (0, 255, 0), 2)
        break

    cv2.imshow("Threshold", threshold)
    cv2.imshow("gray roi", gray_roi)
    cv2.imshow("Roi", roi)
    key = cv2.waitKey(30)
    if key == 27:
        break

cv2.destroyAllWindows()

I have tried some solutions online like downloading the openCV contribs as well as printing the returned value by the "findContours" function to see expected result yet I couldn't figure out what is the problem.

In OpenCV 2 findContours has two outputs, in OpenCV 3 findContours has three outputs and in OpenCV 4 findContours has two outputs. For compatibility you may use: cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)[-2] or use imutils: import imutils and contours = imutils.grab_contours(cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)) — Rotem
– Rotem, Commented Jun 23, 2021 at 19:47

Mohammed Radman · Accepted Answer · 2021-06-23 20:09:24Z

I have got some help from a friend, whom I owe the answer credit to Mr.Ahmed, the following code would be a replica of the code line that caused the error:

(cnts, _) = cv2.findContours(thresh_delta.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours, _ = cv2.findContours(thresh_delta.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = sorted(contours, key=lambda x: cv2.contourArea(x), reverse=True)

The mean reason were in the structure of the python OpenCV library and the way to call the function. The complete code is following:

import cv2
import numpy as np


cap = cv2.VideoCapture("eye_recording.flv")

while True:
    ret, frame = cap.read()
    if ret is False: 
        break

#roi = frame[0:800,0:800]
roi = frame
rows, cols, _ = roi.shape
gray_roi = cv2.cvtColor(roi, cv2.COLOR_BGR2GRAY)
gray_roi = cv2.GaussianBlur(gray_roi, (7, 7), 0)

_, thresh_delta = cv2.threshold(gray_roi, 3, 255, cv2.THRESH_BINARY_INV)
(cnts, _) = cv2.findContours(thresh_delta.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours, _ = cv2.findContours(thresh_delta.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = sorted(contours, key=lambda x: cv2.contourArea(x), reverse=True)

for cnt in contours:
    (x, y, w, h) = cv2.boundingRect(cnt)

    #cv2.drawContours(roi, [cnt], -1, (0, 0, 255), 3)
    cv2.rectangle(roi, (x, y), (x + w, y + h), (255, 0, 0), 2)
    cv2.line(roi, (x + int(w/2), 0), (x + int(w/2), rows), (0, 255, 0), 2)
    cv2.line(roi, (0, y + int(h/2)), (cols, y + int(h/2)), (0, 255, 0), 2)
    print("x="+str((x + int(w/2)))+"y="+str((y + int(h/2))))
    break

#cv2.imshow("Threshold", threshold)
cv2.imshow("gray roi", gray_roi)
cv2.imshow("Roi", roi)
key = cv2.waitKey(30)
if key == 27:
    break

cv2.destroyAllWindows()

The video is a Pysource website property. I would include a link to their site for those who are interested in getting it. Pysource.com Eye motion detection example video

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