OpenCV Image Operations

Reading an Image File

The cv2.imread() function is used to read an image file and takes two arguments.

• The first is the file name or full path of the file to be read.
• The second is how the file should be read; the second argument can take 3 values:
  • cv2.IMREAD_COLOR comes as the default and gives a color output
  • cv2.IMREAD_GRAYSCALE reads in grayscale mode
  • cv2.IMREAD_UNCHANGED reads with the alpha channel

Example code:

import cv2
import numpy as np

# we read the image in grayscale mode
img = cv2.imread('messi5.jpg',0)

Displaying an Image

cv2.imshow() is the function used to display an image; you can use it to see the changes you made after processing. It takes two arguments:

• The name of the window to be opened ( each window name must be different )
• The image to be displayed

cv2.imshow('image',img)
cv2.waitKey(0)
cv2.destroyAllWindows()

cv2.waitKey() is a function that listens for keyboard events; the argument it takes is in milliseconds. If any key is pressed at any time, it allows the program to continue from where it left off.

If 0 is passed, it waits indefinitely for a key press. It can also be set to detect specific key presses as we will discuss below.

cv2.destroyAllWindows() is the function that allows us to close the windows that appear. You can also close a specific window by providing its name.

cv2.namedWindow('image', cv2.WINDOW_NORMAL)
cv2.imshow('image',img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Saving an Image

With the cv2.imwrite() function, we can save the image files we have modified. It takes two arguments:

• The name of the image file to be saved ( if only a name is given, it is saved to the current directory; if a full path is given, it is saved to that path )
• The image to be saved

cv2.imwrite('messigray.png',img)
# saved in PNG format to the current directory

As a summary, the code snippet below loads the image in grayscale mode, displays it by opening a window, saves it if the ‘s’ key is pressed and exits, or exits without saving if Esc is pressed.

import cv2
import numpy as np

img = cv2.imread('messi5.jpg',0)
cv2.imshow('image',img)
k = cv2.waitKey(0)
if k == 27:         # exit if Esc is pressed
    cv2.destroyAllWindows()
elif k == ord('s'): # if 's' key is pressed
    cv2.imwrite('messigray.png',img) # save
    cv2.destroyAllWindows() # and close the window

Warning!

If you are using a 64-bit machine, you should change this:

k = cv2.waitKey(0) # this line

k = cv2.waitKey(0) & 0xFF # should be done this way

Using Matplotlib — Matplotlib is a plotting library for Python that provides a wide variety of plotting methods.

Here you will learn how to open an image file with Matplotlib; you can zoom into the image and save it using Matplotlib.

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

img = cv2.imread('messi5.jpg',0)
plt.imshow(img, cmap = 'gray', interpolation = 'bicubic')
plt.xticks([]), plt.yticks([])  # hides tick values on X and Y axis
plt.show()

It will look like this.