How to properly use cv2.findContours() on opencv version 4.4.0.?

Question

Im trying to use cv2.findContours() on opencv version 4.4.0. (Im using Python version 3.8.5) but it throws an error, I cant figure out. Im not sure whats wrong with the code. Here's some background:

According to OpenCV the syntax for cv2.findContours() is as follows: Python: contours, hierarchy = cv.findContours(image, mode, method[, contours[, hierarchy[, offset]]])

I looked for some examples to make sure how to properly implement it, heres what I found:

• example 1

  (_, contours, _) = cv2.findContours(binary_image, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)

• example 2

  (_, cnts, _) = cv2.findContours(thresholded.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

Those are from working projects I found online, there are plenty examples like those. So, Im trying to implement some code I got from a video to gain some understanding on the topic but it does not seem to work for me and I cant find why. Heres the code:

import cv2 
import numpy as np 

imagen =cv2.imread('lettuce.jpg')
gray = cv2.cvtColor(imagen,cv2.COLOR_BGR2GRAY)
_,binary = cv2.threshold(gray,100,255,cv2.THRESH_BINARY)

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

cv2.drawContours(image,contours, -1, (0,255,0),3)

Error:

Traceback (most recent call last):
  line 8, in <module>
    image,contours,hierarchy = cv2.findContours(binary,cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE)
ValueError: not enough values to unpack (expected 3, got 2)

Answer 1

In Python/OpenCV 4.4.0, findContours returns only 2 values, you list 3.

You show:

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

OpenCV 4.4.0, lists:

contours, hierarchy = cv.findContours(image, mode, method[, contours[, hierarchy[, offset]]])

Please always check the documentation. See

https://docs.opencv.org/4.4.0/d3/dc0/group__imgproc__shape.html#gadf1ad6a0b82947fa1fe3c3d497f260e0

One way to handle this in a version independent way, if all you want are the contours, is (credit to @nathancy):

contours = cv2.findContours(binary, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]

If you do not want all nested contours, then use RETR_EXTERNAL and not RETR_LIST.