too many values to unpack calling cv2.findContours
Question:
I am a python beginner . I was trying to run this code :
#applying closing function
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (7, 7))
closed = cv2.morphologyEx(th3, cv2.MORPH_CLOSE, kernel)
#finding_contours
(cnts, _) = cv2.findContours(closed.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
for c in cnts:
peri = cv2.arcLength(c, True)
approx = cv2.approxPolyDP(c, 0.02 * peri, True)
cv2.drawContours(frame, [approx], -1, (0, 255, 0), 2)
when I summon the mask.py I got this ValueError :
Traceback (most recent call last):
File "mask.py", line 22, in <module>
(cnts, _) = cv2.findContours(closed.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
ValueError: too many values to unpack
what is wrong with this code ?
Answers:
It appears that you’re using OpenCV version 3.x, while writing code intended for the 2.x branch. There were some API changes between those two branches. Since you’re using Python, you have a handy help available — make sure to use it, along with the documentation.
OpenCV 2.x:
>>> import cv2
>>> help(cv2.findContours)
Help on built-in function findContours in module cv2:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> contours, hierarchy
OpenCV 3.x:
>>> import cv2
>>> help(cv2.findContours)
Help on built-in function findContours:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> image, contours, hierarchy
This means that in your script the correct way to call findContours
when using OpenCV 3.x would be something like
(_, cnts, _) = cv2.findContours(closed.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
UPDATE (Dec 2018)
In OpenCV 4.x, findContours
returns 2 values only.
>>> help(cv2.findContours)
Help on built-in function findContours:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> contours, hierarchy
. @brief Finds contours in a binary image.
You can use cv2.findContours()
irrespective of the version with following code snippet:
import cv2 as cv
version = cv.__version__
version = version[0]
if version == '4' or version == '2':
contours, hierarchy = cv.findContours(thresh, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
elif version == '3':
im2, contours, hierarchy = cv.findContours(thresh, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
OpenCV 2.x
and 4.x
returns 2 variables, while 3.x
return 3 variables
I am a python beginner . I was trying to run this code :
#applying closing function
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (7, 7))
closed = cv2.morphologyEx(th3, cv2.MORPH_CLOSE, kernel)
#finding_contours
(cnts, _) = cv2.findContours(closed.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
for c in cnts:
peri = cv2.arcLength(c, True)
approx = cv2.approxPolyDP(c, 0.02 * peri, True)
cv2.drawContours(frame, [approx], -1, (0, 255, 0), 2)
when I summon the mask.py I got this ValueError :
Traceback (most recent call last):
File "mask.py", line 22, in <module>
(cnts, _) = cv2.findContours(closed.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
ValueError: too many values to unpack
what is wrong with this code ?
It appears that you’re using OpenCV version 3.x, while writing code intended for the 2.x branch. There were some API changes between those two branches. Since you’re using Python, you have a handy help available — make sure to use it, along with the documentation.
OpenCV 2.x:
>>> import cv2
>>> help(cv2.findContours)
Help on built-in function findContours in module cv2:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> contours, hierarchy
OpenCV 3.x:
>>> import cv2
>>> help(cv2.findContours)
Help on built-in function findContours:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> image, contours, hierarchy
This means that in your script the correct way to call findContours
when using OpenCV 3.x would be something like
(_, cnts, _) = cv2.findContours(closed.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
UPDATE (Dec 2018)
In OpenCV 4.x, findContours
returns 2 values only.
>>> help(cv2.findContours)
Help on built-in function findContours:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> contours, hierarchy
. @brief Finds contours in a binary image.
You can use cv2.findContours()
irrespective of the version with following code snippet:
import cv2 as cv
version = cv.__version__
version = version[0]
if version == '4' or version == '2':
contours, hierarchy = cv.findContours(thresh, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
elif version == '3':
im2, contours, hierarchy = cv.findContours(thresh, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
OpenCV 2.x
and 4.x
returns 2 variables, while 3.x
return 3 variables