CODEWITHSUNDEEP
pythonopencvnumpyimage-processing
Edmund Carvalho
Edmund Carvalho

Reputation: 107

How can I check for a contour containing another contour using OpenCV in Python?

Let's say I have the following image and I'm looking to analyse it with OpenCV in Python with Numpy:

Checker board with dots

I mark all the white blocks as contours. I also mark contours around the red and green dots.

How do I check which dot is in which white block?

Here's what I have tried:

import numpy as np
import cv2
img = cv2.imread('crapypimg.bmp')
gray = cv2.imread('crapypimg.bmp',0)
ret, thresh = cv2.threshold(gray,127,255,1)
contours,h = cv2.findContours(thresh,1,2)
for cnt in contours:
    approx = cv2.approxPolyDP(cnt,0.01*cv2.arcLength(cnt,True),True)
    print len(approx)
    if len(approx)==4:
        print "square"
        cv2.drawContours(img,[cnt],0,(0,0,255),2)
    elif len(approx) == 9:
        print "half-circle"
        cv2.drawContours(img,[cnt],0,(255,255,0),3)
    elif len(approx) > 15:
        print "circle"
        cv2.drawContours(img,[cnt],0,(0,255,255),3)
cv2.imshow('img',img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Upvotes: 2

Views: 4302

Answers (2)

megamind
megamind

Reputation: 78

import cv2

img = cv2.imread('OFLCp.png')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
thresh = cv2.threshold(gray, 100, 255, cv2.THRESH_BINARY_INV)[1]  # ensure binary
contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

hierarchy is the answer to your question.If you will check hierarchy, it will look like this:

array([[[-1, -1,  1, -1],
        [ 3, -1,  2,  0],
        [-1, -1, -1,  1],
        [ 4,  1, -1,  0],
        [ 6,  3,  5,  0],
        [-1, -1, -1,  4],
        [ 7,  4, -1,  0],
        [ 9,  6,  8,  0],
        [-1, -1, -1,  7],
        [10,  7, -1,  0],
        [12,  9, 11,  0],
        [-1, -1, -1, 10],
        [-1, 10, -1,  0]]], dtype=int32)

where each row contains hierarchy information of the contour at that index.

row = [next_contour_index, previous_contour_index, first_child_contour_index, parent_contour_index]

so, 0 here is index of outermost square. contour at index 2 (circle) has parent as 1 (square). Plotting one pair of child(red), parent (yellow). enter image description here

Upvotes: 1

Uri
Uri

Reputation: 26966

If you are certain that the shapes are not overlapping, it is enough to check one point against a contour.

You can use cv2.pointPolygonTest for this.

A tutorial is available here.

Upvotes: 1

Related Questions