Finding external and internal contours in a binary image

Having contours extracted from a binary image gives you an alternative image representation and allows you to apply contour-specific image analysis methods. In this recipe, you will learn how to find contours in a binary image.

Getting ready

For this recipe, ensure that you have installed the OpenCV 3.x Python API package and the matplotlib package.

How to do it...

1. Import the modules:

import cv2
import numpy as np
import matplotlib.pyplot as plt

2. Load the test binary image:

image = cv2.imread('../data/BnW.png', 0)

3. Find the external and internal contours. Organize them into a two-level hierarchy:

_, contours, hierarchy = cv2.findContours(image, cv2.RETR_CCOMP, cv2.CHAIN_APPROX_SIMPLE)

4. Prepare the external contour binary mask:

image_external = np.zeros(image.shape, image.dtype)
for i in range(len(contours)):
    if hierarchy[0][i][3] == -1:
        cv2.drawContours(image_external, contours, i, 
        255, -1)

5. Prepare the internal contour binary...