Error using cv2.findContours(), with python

Question

I've recently started learning OpenCV on Python.

I'm referring to this tutorial here, to get some help on getting the contours of an image.

My code is -

import cv2
import numpy as np

img = cv2.imread('shapes.jpg', 0)
img = cv2.medianBlur(img, 5)
thresh =     cv2.adaptiveThreshold(img,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C,\
cv2.THRESH_BINARY,11,2)

cv2.imshow('Thresh', thresh)
cv2.waitKey(0)
cv2.destroyAllWindows()

image, contours, hierarchy =   cv2.findContours(thresh.copy(),cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE)

cv2.drawContours(image, countours, -1, (0,255,0), 3)
cv2.imshow('Contours', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

The first thresholded image is appearing, but after that I get an error message as

Traceback (most recent call last):
  File "contours.py", line 21, in <module>
    image, contours, hierarchy =     cv2.findContours(thresh.copy(),cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE)
ValueError: need more than 2 values to unpack

Any help to resolve this issue will be appreciated.

Answer 1

Look at this example.

cv2.findContours(...)

only returns two objects, you're trying to unpack it into three.

change that line to this:

contours, hierarchy =   cv2.findContours(thresh.copy(),cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE)

and it should work.

Answer 2

The tutorial you have linked is for OpenCV version 3. cv2.findContours does return 3 objects in that version.

So either update opencv or use the solution by @will .

Answer 3

In version 4.1.2-dev it returns only two values. You have to unpack it with two values and then use cv2.drawContours() to see them. This is the link to the documentation: https://docs.opencv.org/master/d4/d73/tutorial_py_contours_begin.html