How can I draw two contours on an image given a contour distance of 100 pixels between them using python and opencv

Question

What I have tried is drawing the external contour using the following lines

cnts, hierarchy = cv2.findContours(edged.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

cv2.drawContours(orig, cnts, -1, (0, 255, 0), 3) # this draws the external contour

Referring the below [![enter image description here][1]][1]

How can I arrive at the answer below?

[![enter image description here][2]][2]

Answer 1

I don't know how it is solved in the links but you can use a blank mask on which you can draw your contour and then use cv2.dilate to expand it using a kernel size of the number of pixels you require between them. Once that is done find contours on the mask and draw the second one onto your original image.

import cv2
import numpy as np

img = cv2.imread('contour.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
_, thresh = cv2.threshold(gray, 240, 255, cv2.THRESH_BINARY_INV + cv2.THRESH_OTSU)
cnts, hierarchy = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cv2.drawContours(img, cnts, -1, (0, 255, 0), 3)
mask = np.zeros(img.shape[:2], dtype=np.uint8)
cv2.drawContours(mask, cnts, -1, 255, 1)
kernel = np.ones((100, 100), np.uint8)
mask = cv2.dilate(mask, kernel, iterations = 1)
cnts, hierarchy = cv2.findContours(mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
cv2.drawContours(img, cnts, 1, (255, 0, 0), 3)
cv2.imshow("img", img)
cv2.waitKey(0)
cv2.destroyAllWindows()