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pythonopencvcontour

Only find contour without child

Good day,

I have used cv2.findContours on an image. After that, I have extracted the contour and hierarchy information. From there, how do I filter and draw only contours without child (which from my understanding has a value of -1 in the 3rd column in the hierarchy array)?

below is my code:my image

from imutils import perspective
from imutils import contours
import numpy as np
import imutils
import cv2

img = cv2.imread('TESTING.png') 
imgs = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
edged = imgs

cnts = cv2.findContours(edged,cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
hierarchy = cnts[2]
ChildContour = hierarchy [0, :,2]
WithoutChildContour = (ChildContour==-1).nonzero()[0]

cntsA = cnts[0] if imutils.is_cv2() else cnts[1]
if not cntsA:
    print ("no contours")



(cntsB, _) = contours.sort_contours(cntsA)

orig = cv2.imread('TESTING.png')
for c in cntsB:


    if cv2.contourArea(c) < 100: 
        continue

    box = cv2.minAreaRect(c)
    box = cv2.boxPoints(box) if imutils.is_cv2() else cv2.boxPoints(box)
    box = np.array(box, dtype="int")
    box = perspective.order_points(box)
    cv2.drawContours(orig, [box.astype("int")], -1, (0, 255, 0), 2)

screen_res = 972, 648
scale_width = screen_res[0] / img.shape[1]
scale_height = screen_res[1] / img.shape[0]
scale = min(scale_width, scale_height)
window_width = int(img.shape[1] * scale)
window_height = int(img.shape[0] * scale)

cv2.namedWindow('Image', cv2.WINDOW_NORMAL)
cv2.resizeWindow('Image', window_width, window_height)

cv2.imshow("Image", orig)
cv2.waitKey(0)       
cv2.destroyAllWindows()
Solution

  • The hierarchy returned by findContours has four columns : [Next, Previous, First_Child, Parent]. As you pointed out, we are interested in index 2 i.e. First_Child. To filter and draw only contours without child, you can loop on indices present in WithoutChildContour.

    cntsA=[ cntsA[i] for i in WithoutChildContour]

    Here's the corresponding snippet:

    Note: Since opencv 4.0, findContours returns only 2 values (cnts and hierarchy).

    # ...
hierarchy = cnts[1] #changed index
ChildContour = hierarchy [0, :,2]
WithoutChildContour = (ChildContour==-1).nonzero()[0]

cntsA = cnts[0]
# get contours from indices
cntsA=[ cntsA[i] for i in WithoutChildContour]
# ...

    Running on your sample image:

    Inner contours