OpenCV Python: cv2.findContours – ValueError: too many values to unpack

I’m writing an opencv program and I found a script on another stackoverflow question: Computer Vision: Masking a human hand

When I run the scripted answer, I get the following error:

Traceback (most recent call last):
    File "skinimagecontour.py", line 13, in <module>
    contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
ValueError: too many values to unpack

The code:

import sys
import numpy
import cv2

im = cv2.imread('Photos/test.jpg')
im_ycrcb = cv2.cvtColor(im, cv2.COLOR_BGR2YCR_CB)

skin_ycrcb_mint = numpy.array((0, 133, 77))
skin_ycrcb_maxt = numpy.array((255, 173, 127))
skin_ycrcb = cv2.inRange(im_ycrcb, skin_ycrcb_mint, skin_ycrcb_maxt)
cv2.imwrite('Photos/output2.jpg', skin_ycrcb) # Second image

contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
for i, c in enumerate(contours):
    area = cv2.contourArea(c)
    if area > 1000:
        cv2.drawContours(im, contours, i, (255, 0, 0), 3)
cv2.imwrite('Photos/output3.jpg', im)

Any help is appreciated!

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

I got the answer from the OpenCV Stack Exchange site. Answer

THE ANSWER:

I bet you are using the current OpenCV’s master branch: here the return statements have changed, see http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=findcontours.

Thus, change the corresponding line to read:

_, contours, _= cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

Or: since the current trunk is still not stable and you probably will run in some more problems, you may want to use OpenCV’s current stable version 2.4.9.

Method 2

This works in all cv2 versions:

contours, hierarchy = cv2.findContours(
    skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)[-2:]

Explanation: By using [-2:], we are basically taking the last two values from the tuple returned by cv2.findContours. Since in some versions, it returns (image, contours, hierarchy) and in other versions, it returns (contours, hierarchy), contours, hierarchy are always the last two values.

Method 3

You have to change this line;

image, contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

Method 4

I’m using python3.x and opencv 4.1.0
i was getting error in the following code :

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

ERROR : too many values to Unpack

then i came to know that above code is used in python2.x
SO i just replaced above code with below one(IN python3.x) by adding one more ‘_’ in the left most side
have a look

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

Method 5

The thing you need to do is just add ‘_’ where you are not using the required var , originally given by:

im2, contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

to

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

Here the original doc is given:
https://docs.opencv.org/3.1.0/d4/d73/tutorial_py_contours_begin.html

Method 6

python is right.
you cannot unpack 3 values from the turple and place them in a turple of two

contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)


use

img, contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)


Method 7

@Safwan has provided a simple hack by choosing the last two tuples returned by cv2.findContours() function. But what if you want all the values returned from every tuple of the function independent of the version of cv2?

The following snippet will work irrespective of the OpenCV version (2, 3 or 4) installed in your system/environment and will also store all the tuples in individual variables.

• OpenCV version 2 & 4, return 2 variables: contours and its hierarchy
• OpenCV version 3, returns 3 variables: input image, contours and its hierarchy

First, get the version of OpenCV installed (we don’t want the entire version just the major number either 2, 3 or 4) :

import cv2
major_version = cv2.__version__[0]

Based on the version either of the following two statements will be executed and the corresponding variables will be populated:

if major_version == '4' | major_version == '2':
    contours, hierarchy = cv2.findContours(img_binary, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

elif major_version == '3':
    im2, contours, hierarchy = cv2.findContours(img_binary, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

The contours can be used for further processing.

Method 8

No big deal, just you might be using open-cv 3.something, which returns 3 values at the point of error and you must be catching only 2, just add any random variable before the contours variable –

_,contours,_ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

Method 9

If the error is:

not enough values to unpack (expected 3, got 2)

then use:

ctrs,hier=cv2.findContours(im_th.copy(),cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

instead of

_,ctrs,hier=cv2.findContours(im_th.copy(),cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0