How to draw contours using opencv in Python?

Question

I have following image preprocessed:

Now I want to use findContours method in order to find cells from image and then use drawContours in order draw contour for every cell from image. I do this but nothing is shown.

contours =  cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cv2.drawContours(img, contours, -1, (0, 255, 0), 3)

I receive the following error.

contours is not a numpy array, neither a scalar

Another answers didn't helped me.

I also use following solution.

contours =  cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)[0]
ctr = numpy.array(contours).reshape((-1,1,2)).astype(numpy.int32)
cv2.drawContours(img, ctr, -1, (0, 255, 0), 3)

But this solution didn't draw anything on the given image.

Answer 1

Update , OpenCV 4.0 change the cv2.findContours . So I modify the code example. And a more general way:

cnts, hiers  = cv2.findContours(...)[:-2]

---

cv2.findContours returns a tuple, not just the contours. You can do like this:

## Find contours
if cv2.__version__.startswith("3."):
    _, contours, _ = cv2.findContours(bimg.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
else:
    contours, _ = cv2.findContours(bimg, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

## Draw contours 
cv2.drawContours(img, contours, -1, (0, 255, 0), 3)

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Related similar questions:

(1) Python Opencv drawContour error

(2) How to use `cv2.findContours` in different OpenCV versions?

Answer 2

The tuple returned by cv2.findContours has the form (im, contours, hierarchy) where im is the image with contours visualized, contours is a tuple of two numpy arrays in the form (x_values, y_values) and heirarchy depends on the retrieval mode. Since you are using CV_RETR_EXTERNAL, heirarchy doesn't have much significance.