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.

Kinght 金 · Accepted Answer · 2018-12-07T03:00:35.997

3

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)

Related similar questions:

(1) Python Opencv drawContour error

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

edited Dec 07 '18 at 03:00

answered Feb 23 '18 at 13:27

Kinght 金

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Thank you ! It seems that I used version > 3. – Mihai Alexandru-Ionut Feb 23 '18 at 13:46
1

@MihaiAlexandru-Ionut Notice, OpenCV 4.0 change the `cv2.findContours` back, so I modify the code example. – Kinght 金 Dec 07 '18 at 03:00

score 1 · Answer 2 · answered Feb 23 '18 at 16:10

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.

How to draw contours using opencv in Python?

2 Answers2