ValueError when using cv2.findContours function in python

Question

import cv2
import numpy as np 

img = cv2.imread("img.jpg")
img_gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
ret, thresh = cv2.threshold(img_gray, 127, 255,0)
contours,hierarchy = cv2.findContours(thresh,2,1)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "test.py", line 7, in <module>
    contours,hierarchy = cv2.findContours(thresh,2,1)

When test went there, an error occuredValueError: too many values to unpack, can anyone tell me why it happened since my opencv is 3.0.0 and the findContours return two values as said in Documents and how to solve this problem

After print the result of threshold function, it returns this

(127.0, array([[255, 255, 255, ..., 255, 255, 255], [255, 255, 255, ..., 255, 255, 255], [255, 255, 255, ..., 255, 255, 255], ..., [255, 255, 255, ..., 255, 255, 255], [255, 255, 255, ..., 255, 255, 255], [255, 255, 255, ..., 255, 255, 255]], dtype=uint8))

Please print out and report back what cv2.threshold really returns, without your unpacking into (ret, thresh) — deets
– deets, Commented Mar 17, 2015 at 11:36

berak · Accepted Answer · 2015-03-17 11:45:25Z

5

"since my opencv is 3.0.0 and the findContours return two values" - you're wrong about this:

>>> help(cv2.findContours)
Help on built-in function findContours:

findContours(...)
    findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> image, contours, hierarchy

see, it returns 3 values, an additional image (that you should discard)

answered Mar 17, 2015 at 11:45

berak

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2 Comments

Louie Over a year ago

oh, yes, thank you. After I add a useless image, the test run well~~

berak Over a year ago

yes, indeed, the image is useless in this case. (but note, that a lot of 3.0 python functions (like all the drawing stuff) differ in the same way, and return an additional image [for chaining, i guess])

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