Image rotation and zoom in SciPy

Introduction

We rotate and zoom images to change their angle or size. This helps us see images from different views or focus on details.

You want to rotate a photo to fix its angle.

You need to zoom in on a part of an image to see details better.

You want to prepare images for machine learning by standardizing their size and orientation.

You want to create effects by rotating or zooming images in a project.

Syntax

SciPy

from scipy.ndimage import rotate, zoom

rotated_image = rotate(image, angle, reshape=True)
zoomed_image = zoom(image, zoom_factor)

image is a NumPy array representing the image.

angle is in degrees. Positive values rotate counter-clockwise.

Examples

Rotate image 45 degrees and zoom to double size.

SciPy

rotated = rotate(image, 45)
zoomed = zoom(image, 2)

Rotate image 90 degrees clockwise without changing shape, zoom to half size.

SciPy

rotated = rotate(image, -90, reshape=False)
zoomed = zoom(image, 0.5)

Sample Program

This code creates a simple black and white image, rotates it 45 degrees, and zooms it by 2 times. Then it shows all three images side by side.

SciPy

import numpy as np
import matplotlib.pyplot as plt
from scipy.ndimage import rotate, zoom

# Create a simple 5x5 image with a white square in the center
image = np.zeros((5,5))
image[1:4, 1:4] = 1

# Rotate the image 45 degrees
rotated_image = rotate(image, 45, reshape=True)

# Zoom the image by 2 times
zoomed_image = zoom(image, 2)

# Show original, rotated, and zoomed images
fig, axs = plt.subplots(1, 3, figsize=(9,3))
axs[0].imshow(image, cmap='gray')
axs[0].set_title('Original')
axs[0].axis('off')

axs[1].imshow(rotated_image, cmap='gray')
axs[1].set_title('Rotated 45°')
axs[1].axis('off')

axs[2].imshow(zoomed_image, cmap='gray')
axs[2].set_title('Zoomed x2')
axs[2].axis('off')

plt.tight_layout()
plt.show()

Important Notes

Rotation can change the image size if reshape=True. Set it to False to keep original size but parts may be cut off.

Zooming can make images bigger or smaller. Zoom factors >1 enlarge, <1 shrink.

Images are NumPy arrays, so you can use other NumPy tools to prepare or analyze them.

Summary

Use scipy.ndimage.rotate to turn images by any angle.

Use scipy.ndimage.zoom to resize images smoothly.

These tools help change how images look for analysis or display.

Practice

1. What does the function scipy.ndimage.rotate do to an image?

easy

A. It adds noise to the image.

B. It turns the image by a specified angle.

C. It crops the image to a smaller size.

D. It changes the image colors.

Solution

1. Step 1: Understand the function purpose

   scipy.ndimage.rotate is designed to rotate images by a given angle in degrees.

2. Step 2: Compare options with function behavior

   Only turning or rotating the image matches the function's purpose; other options describe different image operations.

3. Final Answer:

   It turns the image by a specified angle. -> Option B

4. Quick Check:

   Rotate = Turn image [OK]

Hint: Rotate means turn image by angle [OK]

Common Mistakes:

• Confusing rotate with crop or color change
• Thinking rotate changes image size
• Assuming rotate adds effects like noise

2. Which of the following is the correct way to rotate an image array img by 45 degrees using scipy?

easy

A. scipy.ndimage.rotate(img, 45)

B. scipy.ndimage.zoom(img, 45)

C. scipy.ndimage.rotate(45, img)

D. scipy.ndimage.zoom(45, img)

Solution

1. Step 1: Check function signatures

   scipy.ndimage.rotate takes the image first, then the angle as the second argument.

2. Step 2: Validate correct argument order

   scipy.ndimage.rotate(img, 45) correctly uses rotate(img, 45). Options C and D swap arguments incorrectly, and B uses zoom instead of rotate.

3. Final Answer:

   scipy.ndimage.rotate(img, 45) -> Option A

4. Quick Check:

   rotate(image, angle) correct syntax [OK]

Hint: Image first, angle second in rotate() [OK]

Common Mistakes:

• Swapping argument order
• Using zoom instead of rotate
• Passing angle before image

3. What will be the shape of the output image after applying scipy.ndimage.zoom(img, 2) if img.shape is (100, 100)?

medium

A. (50, 50)

B. (100, 100)

C. (200, 200)

D. (100, 200)

Solution

1. Step 1: Understand zoom factor effect

   A zoom factor of 2 doubles the size of each dimension of the image.

2. Step 2: Calculate new shape

   Original shape is (100, 100). Doubling each dimension gives (200, 200).

3. Final Answer:

   (200, 200) -> Option C

4. Quick Check:

   Zoom 2x doubles shape [OK]

Hint: Zoom factor multiplies each dimension [OK]

Common Mistakes:

• Confusing zoom with cropping
• Thinking zoom keeps shape same
• Mixing width and height dimensions

4. You run this code but get an error:

scipy.ndimage.rotate(45, img)

What is the problem?

medium

A. The image should be the first argument.

B. The angle should be the first argument.

C. The rotate function does not accept two arguments.

D. The angle must be in radians, not degrees.

Solution

1. Step 1: Check argument order for rotate()

   scipy.ndimage.rotate expects the image array as the first argument, then the angle.

2. Step 2: Identify error cause

   Passing angle first and image second causes a type error because the function tries to treat the number as an array.

3. Final Answer:

   The image should be the first argument. -> Option A

4. Quick Check:

   Image first, angle second in rotate() [OK]

Hint: Image must come before angle in rotate() [OK]

Common Mistakes:

• Swapping argument order
• Assuming angle must be radians
• Thinking rotate takes only one argument

5. You want to rotate an image by 90 degrees and then zoom it to half its size. Which code sequence correctly does this?

hard

A. rotated = scipy.ndimage.rotate(img, 0.5)
zoomed = scipy.ndimage.zoom(rotated, 90)

B. zoomed = scipy.ndimage.zoom(img, 0.5)
rotated = scipy.ndimage.rotate(zoomed, 90)

C. zoomed = scipy.ndimage.zoom(img, 90)
rotated = scipy.ndimage.rotate(zoomed, 0.5)

D. rotated = scipy.ndimage.rotate(img, 90)
zoomed = scipy.ndimage.zoom(rotated, 0.5)

Solution

1. Step 1: Understand operation order

   First rotate by 90 degrees, then zoom by 0.5 to reduce size by half.

2. Step 2: Check code correctness

   rotated = scipy.ndimage.rotate(img, 90)
   zoomed = scipy.ndimage.zoom(rotated, 0.5)

   correctly rotates img by 90, then zooms the rotated image by 0.5. Other options mix argument order or use wrong values.

3. Final Answer:

   rotated = scipy.ndimage.rotate(img, 90) zoomed = scipy.ndimage.zoom(rotated, 0.5) -> Option D

4. Quick Check:

   Rotate 90°, then zoom 0.5 [OK]

Hint: Rotate first, then zoom with correct factors [OK]

Common Mistakes:

• Swapping zoom and rotate order
• Using zoom factor as angle
• Passing wrong argument order