Checking whether a point is within a contour

In this recipe, we will discover a way of checking whether a point is inside of a contour, or if it belongs to the contour's border.

Getting ready

You need to have OpenCV 3.x installed, with Python API support.

How to do it...

1. Import all of the necessary modules, open an image, and display it on the screen:

import cv2, random
import numpy as np
img = cv2.imread('bw.png', cv2.IMREAD_GRAYSCALE)

2. Find the contours of the image and display them:

im2, contours, hierarchy = cv2.findContours(img, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

color = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
cv2.drawContours(color, contours, -1, (0,255,0), 3)

cv2.imshow('contours', color)
cv2.waitKey()
cv2.destroyAllWindows()

3. Define a callback function to handle a user click on the image. This function draws a small circle where the click has happened, and the color of the circle is determined by whether the click was inside or outside of the contour:

contour = contours[0]
image_to_show...