The Image Processing Series Part 4: Image Transformation

We’re almost done with the basics, this will be the last article and we’ll talk about Image Transformation.

If you haven’t read through our previous articles in the image processing series, you can click the links below:

• Part 1 – Loading Images in OpenCV
• Part 2 – Setting Pixels with Python OpenCV
• Part 3 – Drawing with Python OpenCV

In this article, we’ll be discussing various methods to modify images using concepts such as Translation, Rotation, Resizing, Flipping, and Cropping.

Let’s not waste any time and get right into it!

Getting started with Image Transformation

All of the following programs consist of modules to be imported and will also be taking in image arguments.

This will, as such, be discussed here before we proceed to the actual concept.

Firstly, we shall import the modules used in transforming the image,

# importing the numpy module to work with pixels in images
import numpy as np

# importing argument parsers
import argparse

# importing the OpenCV module
import cv2

Next, we’ll set up the argument parsers so that we can take input from the user regarding the image file location.

# initializing an argument parser object
ap = argparse.ArgumentParser()

# adding the argument, providing the user an option
# to input the path of the image
ap.add_argument("-i", "--image", required=True, help="Path to the image")

# parsing the argument
args = vars(ap.parse_args())

Now, let’s get into the core of the transformations!

Translation in OpenCV

Working with translation in OpenCV, we’ll first define a function that takes in the input image, and the shift in both the X and Y axes.

The warpAffine method takes in both the input image, as well as the translation matrix and warps the image in the process of translation.

We then, finally, return the altered image back to the program.

# defining a function for translation
def translate(image, x, y):
    # defining the translation matrix
    M = np.float32([[1, 0, x], [0, 1, y]])

    # the cv2.warpAffine method does the actual translation
    # containing the input image and the translation matrix
    shifted = cv2.warpAffine(image, M, (image.shape[1], image.shape[0]))

    # we then return the image
    return shifted

Now, we have the code responsible for taking in input, and providing output. The translate function provides us with a simple direct call to the process.

# reads the image from image location
image = cv2.imread(args["image"])
cv2.imshow("Original", image)

# call the translation function to translate the image
shifted = translate(image, 0, 100)
cv2.imshow("Shifted Down", shifted)
cv2.waitKey(0)

You should be receiving a image shifted down by 100 pixels alongside the original one.

Image Rotation with Python OpenCV

Let’s first define a function for rotation, this allows us to use a single line of code to rotate our image later on.

The rotate function takes in the image, the angle we have to rotate the image, and we’ll also declare a few defaults for the center and scaling.

The cv2.getRotationMatrix2D allows us to create a matrix which upon warping, provides us with a rotated image.

Then, we return the rotated image.

# defining a function for rotation
def rotate(image, angle, center=None, scale=1.0):
    (h, w) = image.shape[:2]
    if center is None:
        center = (w / 2, h / 2)

    # the cv2.getRotationMatrix2D allows us to create a
    # Rotation matrix
    M = cv2.getRotationMatrix2D(center, angle, scale)

    # the warpAffine function allows us to rotate the image
    # using the rotation matrix
    rotated = cv2.warpAffine(image, M, (w, h))

    return rotated

Now, let’s test out the function by providing different angles for our images, ranging from 45 to 90 to 180!

# rotating the image by 45 degrees
rotated = rotate(image, 45)
cv2.imshow("Rotated by 45 Degrees", rotated)

# rotating the image by 90 degrees
rotated = rotate(image, 90)
cv2.imshow("Rotated by -90 Degrees", rotated)

# rotating the image by 180 degrees
rotated = rotate(image, 180)
cv2.imshow("Rotated by 180 degrees", rotated)

While we haven’t written a function which includes offsets into it, it’s much the same.

A more procedural approach to working with offsets, is given as below,

# working with offsets in images
# then, rotating 45 degrees
(h, w) = image.shape[:2]
(cX, cY) = (w / 2, h / 2)

M = cv2.getRotationMatrix2D((cX - 50, cY - 50), 45, 1.0)
rotated = cv2.warpAffine(image, M, (w, h))
cv2.imshow("Rotated by Offset & 45 Degrees", rotated)

# used to wait for user input before closing the images
cv2.waitKey(0)

The Upside Down, OpenCV and Flipping

Flipping in OpenCV is pretty simple, with a simple flip() function.

The cv2.flip function takes in two arguments, one being the image itself, and the other signifying how to flip the image.

Here’s the code to flip the image,

# flipping the image horizontally
flipped = cv2.flip(image, 1)
cv2.imshow("Flipped Horizontally", flipped)

# flipping the image vertically
flipped = cv2.flip(image, 0)
cv2.imshow("Flipped Vertically", flipped)

# flipping the image vertically and horizontally
flipped = cv2.flip(image, -1)
cv2.imshow("Flipped Vertically and Horizontally", flipped)

# wait for the user's key to proceed
cv2.waitKey(0)

A part of me you’ll never see: Cropping

Cropping the image in cv2 is as simple as accessing lists in Python.

There’s no function for it, as there doesn’t need to be one.

Code to crop,

# displaying the width and height of the image
print("Width", image.shape[1])
print("Height", image.shape[0])

# cropping the image manually
face = image[:400, :600]
cv2.imshow("Face", face)
cv2.waitKey(0)

# cropping the image manually
body = image[400:600, 120:600]
cv2.imshow("Body", body)
cv2.waitKey(0)

The imutils package

A good idea to resolve using a lot of code in your programs is to work with the imutils module, which inherently provides methods to transform images.

We can also include our own custom made transformations in the module and can use it anywhere with a simple import statement!

Conclusion

Image transformation marks the end of the basic operations that don’t alter the properties of the images by large.

We’ll be proceeding to work on the OpenCV code that delves into the fundamentals of Image Processing now.

The basics are over, but, that doesn’t mean we’re going into the advanced topics just yet.

Here’s a look at some future works if you wish to see what you might be dealing with 😉

Here’s OpenCV and Facial Recognition for further reading!

References

• Official OpenCV Website
• Introduction to starting out with OpenCV
• My GitHub Repository for Image Processing
• Getting started with Translation
• Working with Rotation
• Resizing the images
• Upside down with Flipping
• Hidden from everyone, Cropping