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Errors with python and OpenCV ( error: (-215:Assertion failed) npoints > 0 in function 'cv::drawContours') ( too many values to unpack (expected 2)) [duplicate]

When I run this code:
import cv2
image = cv2.imread('screenshoot10.jpg')
cv2.imshow('input image', image)
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
edged = cv2.Canny(gray, 30, 200)
cv2.imshow('canny edges', edged)
_, contours = cv2.findContours(edged, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
cv2.imshow('canny edges after contouring', edged)
print(contours)
print('Numbers of contours found=', len(contours))
cv2.drawContours(image, contours, -1, (0, 255, 0), 3)
cv2.imshow('contours', image)
cv2.waitKey(0)
cv2.destroyAllWindows()
I am getting this error:
OpenCV(4.1.1)
C:\projects\opencv-python\opencv\modules\imgproc\src\drawing.cpp:2509:
error: (-215:Assertion failed) npoints > 0 in function
'cv::drawContours'
What am I doing wrong?

According to the documentation for findContours, the method returns (contours, hierarchy), so I think the code should be:
contours, _ = cv2.findContours(edged,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_NONE)
instead of
_, contours = cv2.findContours(edged,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_NONE)

Depending on the OpenCV version, cv2.findContours() has varying return signatures. In v3.4.X, three items are returned.
image, contours, hierarchy = cv2.findContours(image, mode, method[, contours[, hierarchy[, offset]]])
In v2.X and v4.1.X, two items are returned.
contours, hierarchy = cv2.findContours(image, mode, method[, contours[, hierarchy[, offset]]])
You can easily obtain the contours regardless of the version like this:
cnts = cv2.findContours(image, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]
for c in cnts:
...
Since the last two values are always the same, we can further condense it into a single line using [-2:] to extract the contours from the tuple returned by cv2.findContours()
cnts, _ = cv2.findContours(image, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)[-2:]

Because this question shows up on top when looking for Error: (-215:Assertion failed) npoints > 0 while working with contours using OpenCV, I want to point out another possible reason as to why one can get this error:
I was using the BoxPoints function after doing minAreaRect to get a rotated rectangle around a contour. I hadn't done np.int0 to its output (to convert the returned array to integer values) before passing it to drawContours. This fixed it:
rect = cv2.minAreaRect(cnt)
box = cv2.cv.BoxPoints(rect)
box = np.int0(box) # convert to integer values
cv2.drawContours(im,[box],0,(0,0,255),2)

All the solutions provided here are either fixed for a particular version or they do not store all the values returned from cv2.findContours(). You can store 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 installed in your system/environment and will also store all the tuples in individual variables.
First, get the version of OpenCV installed (we don't want the entire version just the major number either 3 or 4) :
import cv2
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 version == '4':
contours, hierarchy = cv2.findContours(binary_image, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
elif version == '3':
img, contours, hierarchy = cv2.findContours(binary_image, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
The contours returned from the function in either scenarios will be stored in contours.
Note: the above snippet is written assuming either version 3 or 4 of OpenCV is installed. For older versions, please refer to the documentation or update to the latest.
If you are using OpenCV version 4, and wondering what the first variable returned by cv2.findContours() of version 3 is; its just the same as the input image (in this case binary_image).

cv2.findContours doesn't find contours

i need to find contours of brown regions. but befıre that, i tried to draw all contours. but i can't see any contour on
i tried this:
contours, hierarchy = cv2.findContours(thresh_dummy, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
cv2.drawContours(thresh_dummy, contours, -1, (0, 255, 0), 3)

I think the reason why you are not seeing contours in the output image is because you are drawing contours with (0, 255, 0), which would not be visible on a grayscale image (thresh_dummy happens to be a grayscale image). What you could do is convert thresh_dummy to RGB before drawing the contours or you could just use a random color like (128, 255, 128) which would be visible on the specific example you have.
contours, hierarchy = cv2.findContours(thresh_dummy, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
thresh_color = cv2.cvtColor(thresh_dummy, cv2.COLOR_GRAY2RGB)
cv2.drawContours(thresh_color, contours, -1, (0, 255, 0), 3)
cv2.imwrite("thresh_color.jpg", thresh_color)

cv2.findContours function is not working in both versions

I am new to computer vision and haven't really went through any tutorials on thresholding or blurring or other filters.
I am using the below two piece of codes which finds out the contours in an image. On one hand the method is working but on the other it is not. I would need help in understanding the reason this is happening so as to convince myself what is happening in the background.
Working code snippet:
img=cv2.imread('path.jpg')
imgBlurred = cv2.GaussianBlur(img, (5, 5), 0)
gray = cv2.cvtColor(imgBlurred, cv2.COLOR_BGR2GRAY)
sobelx = cv2.Sobel(gray, cv2.CV_8U, 1, 0, ksize=3)
cv2.imshow("Sobel",sobelx)
cv2.waitKey(0)
ret2, threshold_img = cv2.threshold(sobelx, 120, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)
im2, contours, hierarchy = cv2.findContours(threshold_img, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
Not working code snippet
# read image
src = cv2.imread(file_path, 1)
# show source image
cv2.imshow("Source", src)
# convert image to gray scale
gray = cv2.cvtColor(src, cv2.COLOR_BGR2GRAY)
# blur the image
blur = cv2.blur(gray, (3, 3))
# binary thresholding of the image
ret, thresh = cv2.threshold(blur, 200, 255, cv2.THRESH_BINARY)
# find contours
im2, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
I would really appreciate if anyone can find out the reason for the wrong which is happening here.
The error that i am facing is:
Traceback (most recent call last): File "convexhull.py", line 27, in
im2, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE) ValueError: not enough values to unpack
(expected 3, got 2)
Let me know if any other information is also required.

This is due to a change in openCV. Since version 4.0 findContours returns only 2 values: the contours and the hierarchy. Before, in version 3.x, it returned 3 values. You can use the documentation to compare the different versions.
The second code snippet should work when you change your code to:
# find contours
contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
Why the first snippet picks a different openCV version can't be determined from the information given.

The following snippet will work irrespective of the OpenCV version installed in your system/environment and will also store all the tuples in individual variables that can be used later in the code.
Store the major version of OpenCV installed:
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':
contours, hierarchy = cv2.findContours(image_binary, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
elif major_version == '3':
image, contours, hierarchy = cv2.findContours(image_binary, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
The contours returned from the function in either scenarios will be stored in contours.

Segmenting two contours into two different images of same size using openCV-python

I have two contours in the first image. I need to segment out individual contours and make two images out of it like this: image1 and image2. The individual output image has to be of the same dimension as the input image. How can this be achieved using openCV-python ? My code for drawing contours:
image, contours, hier = cv2.findContours(im, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
for c in contours:
rect = cv2.minAreaRect(c)
box = cv2.boxPoints(rect)
# convert all coordinates floating point values to int
box = np.int0(box)
# draw a red 'nghien' rectangle
cv2.drawContours(im, [box], 0, (0, 0, 255))
cv2.drawContours(im, contours, -1, (255, 255, 0), 1)

You are using the cv2.drawContours in wrong way. Passing the -1 as contour index would draw all the contours and not the individual ones. To draw the individual contours you need to pass the corresponding index as:
_, cnt, hierarchy = cv2.findContours(canvas.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
for i in xrange(len(cnt)):
output_canvas = np.zeros(canvas.shape, dtype=np.uint8)
cv2.drawContours(output_canvas, cnt, i, np.array([255, 255, 255, 255]), -1)
cv2.imwrite("./contour{}.png".format(i), output_canvas)

python opencv - How to combine two contours

I want cut up the captcha image.
the origin picture is here:
captcha url
There are my results:
First,I remove noise and line from captcha.
Then find contours.
I draw contours in image using green line.
But some chars in image will separated in two piece.
How to determine two piece is belong to one char?
contours, hierarchy = cv2.findContours(img, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
cv2.drawContours(self.im, contours, -1, (0, 255, 0), 1)