OpenCV findContours are in reverse order

Question

Why the contour is in reverse order? I use the RETR_EXTERNAL flag for contours hierarchy for detect only the external contours

_, contours, hierarchy = cv2.findContours(connected.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

For example my my output is r,e B, why in reverse order?

The only order that you can depend on in findContours is the hierarchy. You cannot depend on which blob will be detected first. — zindarod
But I use the RETR_EXTERNAL flag so all contours are on the same hierarchy. — Razvan
@zindarod The hierarchy doesn't determine the positional parameters of a contour, it specifies the parent-child relationship between contours. — Jeru Luke
@JeruLuke I didn't say that it does. I said that the hierarchy is the only ordering that the findContours function provides. The OP was assuming that just because the B character was positioned first, it would be detected first, which it may or may not. — zindarod

jedwards jedwards · Accepted Answer · 2018-06-15T10:19:07

You could use some heuristic to sort the contours. The following example uses the minimum x coordinate of each contour to sort them:

import cv2
import numpy as np

img = cv2.imread('BB7eu.png')
img = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
_, contours, hierarchy = cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

contours.sort(key=lambda c: np.min(c[:,:,0]))

for i in range(len(contours)):
    cur = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
    cv2.drawContours(cur, contours, i, (0,0,255), -1)
    cv2.putText(cur, "i=%d" % i, (140,90), cv2.FONT_HERSHEY_PLAIN, 1, (0,255,255), 1)
    cv2.imshow("img", cur)
    cv2.waitKey()

You'll see that the contours are highlighted "in order": B,e,r.

OpenCV findContours are in reverse order

1 Answers