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

Question:

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!

Asked By: ahmadux

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Source

Answers:

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.

Answered By: ahmadux

You have to change this line;

image, contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
Answered By: Volkan Altıntaş

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)

Answered By: kishea

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

Answered By: Edwardabk

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.

Answered By: Safwan

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)
Answered By: Priyansh gupta

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)
Answered By: Rushil Goomer

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)
Answered By: Sweety Hissariya

@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.

Answered By: Jeru Luke