Reading an image file in OpenCV lib using Python

 

Reading an image in OpenCV using Python

• Difficulty Level : Basic
• Last Updated : 15 Dec, 2020
• Reference : Geeks for geek and Open-CV documentaions
  

Prerequisite: Basics of OpenCV

In this article, we’ll try to open an image by using OpenCV (Open Source Computer Vision). To use the OpenCV library in python, we need to install these libraries as a prerequisite:

1. Numpy Library (Necessary, because OpenCV uses it in the background).
2. OpenCV python
    
   
   

To install these libraries, we need to run these pip commands in cmd:

pip install opencv-python
pip install numpy
pip install matplotlib

 To read the images cv2.imread() method is used. This method loads an image from the specified file. If the image cannot be read (because of missing file, improper permissions, unsupported or invalid format) then this method returns an empty matrix.

Syntax: cv2.imread(path, flag)

Parameters:
path: A string representing the path of the image to be read.
flag: It specifies the way in which image should be read. It’s default value is cv2.IMREAD_COLOR

Return Value: This method returns an image that is loaded from the specified file.

Note: The image should be in the working directory or a full path of image should be given.

All three types of flags are described below:

cv2.IMREAD_COLOR: It specifies to load a color image. Any transparency of image will be neglected. It is the default flag. Alternatively, we can pass integer value 1 for this flag.
cv2.IMREAD_GRAYSCALE: It specifies to load an image in grayscale mode. Alternatively, we can pass integer value 0 for this flag.
cv2.IMREAD_UNCHANGED: It specifies to load an image as such including alpha channel. Alternatively, we can pass integer value -1 for this flag.

Below codes are implementations to read images and display images on the screen using OpenCV and matplotlib libraries functions.
 
Example #1 (Using OpenCV) : 

Image Used:

# Python code to read image import cv2   # To read image from disk, we use # cv2.imread function, in below method, img = cv2.imread("geeksforgeeks.png", cv2.IMREAD_COLOR)   # Creating GUI window to display an image on screen # first Parameter is windows title (should be in string format) # Second Parameter is image array cv2.imshow("Cute Kitens", img)   # To hold the window on screen, we use cv2.waitKey method # Once it detected the close input, it will release the control # To the next line # First Parameter is for holding screen for specified milliseconds # It should be positive integer. If 0 pass an parameter, then it will # hold the screen until user close it. cv2.waitKey(0)   # It is for removing/deleting created GUI window from screen # and memory cv2.destroyAllWindows()

Output:

Example #2: Opening in grascale mode

# Python program to explain cv2.imread() method   # importing cv2 import cv2   # path path = r'geeksforgeeks.png'   # Using cv2.imread() method # Using 0 to read image in grayscale mode img = cv2.imread(path, 0)   # Displaying the image cv2.imshow('image', img)

Output : 

Python OpenCV | cv2.imshow() method

• Difficulty Level : Easy
• Last Updated : 02 Sep, 2020

OpenCV-Python is a library of Python bindings designed to solve computer vision problems. cv2.imshow() method is used to display an image in a window. The window automatically fits to the image size.
 

Syntax: cv2.imshow(window_name, image)
Parameters: 
window_name: A string representing the name of the window in which image to be displayed. 
image: It is the image that is to be displayed.
Return Value: It doesn’t returns anything. 
 

Image used for all the below examples: 
 

Example #1: 
 

# Python program to explain cv2.imshow() method     # importing cv2  import cv2     # path  path = r'C:\Users\Rajnish\Desktop\geeksforgeeks.png'    # Reading an image in default mode image = cv2.imread(path)    # Window name in which image is displayed window_name = 'image'    # Using cv2.imshow() method  # Displaying the image  cv2.imshow(window_name, image)    #waits for user to press any key  #(this is necessary to avoid Python kernel form crashing) cv2.waitKey(0)     #closing all open windows  cv2.destroyAllWindows()

Output: 
 

Example #2: 
 

# Python program to explain cv2.imshow() method    # importing cv2 import cv2    # path path = r'C:\Users\Rajnish\Desktop\geeksforgeeks.png'    # Reading an image in grayscale mode image = cv2.imread(path, 0)    # Window name in which image is displayed window_name = 'image'    # Using cv2.imshow() method # Displaying the image cv2.imshow(window_name, image)    # waits for user to press any key # (this is necessary to avoid Python kernel form crashing) cv2.waitKey(0)    # closing all open windows cv2.destroyAllWindows()

Output: