OpenCV Replace a Part of Image Using Other Image or NumPy Array – Python OpenCV Tutorial

By | October 19, 2020

In this tutorial, we will introduce how to replace a part of big image using an small image or numpy array, which is very useful when you plan to  recognize objects from images.

Read a big image using python opencv

We will use python opencv to read an image data first.

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

img = cv2.imread('comment_source.jpg', cv2.IMREAD_COLOR)
img2 = img.copy()

Create a small image or numpy array

We will use a small image or numpy array to replace a part region of img2.

mask = np.zeros((100,300, 3))
print(mask.shape) #h, w, 3

In this code, mask is a small image with height=100 , width=300 and brg=(0,0,0)

Replace a part of big image

We will replace a part of big image from the coordinate (200, 200)

pos = (200, 200)

img2[200:(200+mask.shape[0]), 200:(200+mask.shape[1])] = mask

In order to understand how to replace numpy array with a small array, you can read:

NumPy Replace Value in Array Using a Small Array or Matrix – NumPy Tutorial

Show the replaced image

We will use matplotlib to show img2.

To know how to display image using matplotlib, you can view:

Understand matplotlib.pyplot.imshow(): Display Data as an Image – Matplotlib Tutorial

plt.subplot(121),
plt.imshow(img),
plt.title('Source Image'),
plt.axis('off')
    
    
plt.subplot(122),
plt.imshow(img2),
plt.title('Replaced Image'),
plt.axis('off')
 
plt.show()

Run this code, you will find:

OpenCV Replace a Part of Image Using Other Image or NumPy Array - Python OpenCV Tutorial

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