OpenCV findContours() just returning one external contour

Question

I'm trying to isolate letters in a captcha, I managed to filter a captcha and that result in this black and white image:

But when I tried to separate the letters with findContours method of OpenCV it just found a external contour that wraps my entire image, resulting in this image (black contour outside image).

I'm using this code with Python 3 and OpenCV 3.4.2.17:

img = threshold_image(img)
cv2.imwrite("images/threshold.png", img)

image, contours, _ = cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)

for i, contour in enumerate(contours):
    area = cv2.contourArea(contour)
    cv2.drawContours(img, contours, i, (0, 0, 0), 3)

cv2.imwrite('images/output3.png', img)

I just want my final result is 5 contours outside each character.

Amog Chandrashekar · Accepted Answer · 2020-01-08T15:00:37.970

The contour to be extracted should be in white, background being black. I have modified your code a bit, eliminated the lines which were not adding any value.

import cv2
img = cv2.imread('image_to_be_read',0)
backup = img.copy()   #taking backup of the input image
backup = 255-backup    #colour inversion

I am using RETR_TREE as the contour retrieval mode, which retrieves all the contours and creates full family hierarchy list. Please find the documentation for the same here

_, contours, _ = cv2.findContours(backup, cv2.RETR_TREE, cv2.CHAIN_APPROX_NONE)

In opencv4, the finContours method has been changed. Please use:

contours, _ = cv2.findContours(backup, cv2.RETR_TREE, cv2.CHAIN_APPROX_NONE)

Then iterate through contours and draw the rectangle around the contours

for i, contour in enumerate(contours):
     x, y, w, h = cv2.boundingRect(contour)
     cv2.rectangle(img, (x, y), (x + w, y + h), (0, 255, 0), 1)

Save the image

cv2.imwrite('output3.png', img)

I got result which looks like this -

In OpenCV 4, `cv2.findContours` returns different values; use `contours, _ = ...` for this code. Because this is StackOverflow, of course an edit to update the code sample was rejected. — rgov, Jan 04 '20 at 18:17

score 2 · Answer 2 · answered Jul 26 '18 at 08:01

You used flag RETR_EXTERNAL, which means it is looking only for outermost contours of objects, but not holes. In your case the white object that covers a whole image with few holes (letters/digits) is found. You have two choices:

Inverse colors in your image with "bitwise_not"
Collect all contours with RETR_LIST flag. Note that it will also collect holes inside digits.

OpenCV findContours() just returning one external contour

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