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Opencv show image python

In this tutorial you will learn how to:

Read an image from file (using cv::imread)
Display an image in an OpenCV window (using cv::imshow)
Write an image to a file (using cv::imwrite)

Source Code

Explanation

In OpenCV 3 we have multiple modules. Each one takes care of a different area or approach towards image processing. You could already observe this in the structure of the user guide of these tutorials itself. Before you use any of them you first need to include the header files where the content of each individual module is declared.

You’ll almost always end up using the:

core section, as here are defined the basic building blocks of the library
imgcodecs module, which provides functions for reading and writing
highgui module, as this contains the functions to show an image in a window

We also include the iostream to facilitate console line output and input.

By declaring using namespace cv; , in the following, the library functions can be accessed without explicitly stating the namespace.

As a first step, the OpenCV python library is imported. The proper way to do this is to additionally assign it the name cv, which is used in the following to reference the library.

Now, let’s analyze the main code. As a first step, we read the image «starry_night.jpg» from the OpenCV samples. In order to do so, a call to the cv::imread function loads the image using the file path specified by the first argument. The second argument is optional and specifies the format in which we want the image. This may be:

IMREAD_COLOR loads the image in the BGR 8-bit format. This is the default that is used here.
IMREAD_UNCHANGED loads the image as is (including the alpha channel if present)
IMREAD_GRAYSCALE loads the image as an intensity one

OpenCV – Show Image – imshow()

You can display an image to the user during the execution of your Python OpenCV application.

To display an image using opencv cv2 library, you can use cv2.imshow() function.

The syntax of imshow() function is given below.

cv2.imshow(window_name, image)

where window_name is the title of the window in which the image numpy.ndarray will be shown. If a window is not created already, a new window will be created to fit the image.

In this tutorial, we will go through different scenarios where we shall display an image in a window.

Examples

1. Show image using cv2.imshow()

In this example, we read an image from storage and display the image to the user in a window.

Python Program

import cv2 #read image img = cv2.imread('D:/my-image.png') #show image cv2.imshow('Example - Show image in window',img) cv2.waitKey(0) # waits until a key is pressed cv2.destroyAllWindows() # destroys the window showing image

cv2.waitKey(0) is important for holding the execution of the python program at this statement, so that the image window stays visible. If you do not provide this statement, cv2.imshow() executes in fraction of a second and the program closes all the windows it opened, which makes it almost impossible to see the image on the window.

A window is opened to show the image. Also, the window size is such that it fits the image. If the image size is greater than your screen resolution, then a window is opened with the scaled down version of the image.

2. Show numpy.ndarray as image using OpenCV

In this example, we try to show an ndarray as image using imshow(). We initialize a numpy array of shape (300, 300, 3) such that it represents 300×300 image with three color channels. 125 is the initial value, so that we get a mid grey color.

Python Program

import cv2 import numpy as np #numpy array ndarray = np.full((300,300,3), 125, dtype=np.uint8) #show image cv2.imshow('Example - Show image in window', ndarray) cv2.waitKey(0) # waits until a key is pressed cv2.destroyAllWindows() # destroys the window showing image

Summary

In this tutorial of Python Examples, we learned how to show or display an image in a window to user using cv2.imshow().

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