Image rotation and zoom in SciPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function in scipy.ndimage is used to rotate an image?

The function scipy.ndimage.rotate is used to rotate an image by a specified angle.

beginner

How do you zoom an image using scipy?

You can zoom an image using scipy.ndimage.zoom, which scales the image by a given zoom factor.

intermediate

What does the parameter reshape control in scipy.ndimage.rotate?

The reshape parameter controls whether the output image size changes to fit the entire rotated image (True) or keeps the original size (False).

intermediate

Why might you want to use order parameter in scipy.ndimage.zoom?

The order parameter controls the spline interpolation order used for zooming. Higher order means smoother images but more computation.

intermediate

What is the effect of rotating an image by 90 degrees with reshape=False?

Rotating by 90 degrees with reshape=False keeps the original image shape, so parts of the rotated image may be cropped.

Which scipy function rotates an image by a given angle?

A scipy.ndimage.shift

B scipy.ndimage.zoom

C scipy.ndimage.flip

D scipy.ndimage.rotate

scipy.ndimage.rotate is used for image rotation.

What does scipy.ndimage.zoom do?

A Flips the image horizontally

B Rotates the image

C Scales the image size

D Changes image color

scipy.ndimage.zoom scales the image by a zoom factor.

If you want to keep the original image size after rotation, what should reshape be set to?

A True

B False

C None

D 0

Setting reshape=False keeps the original image size.

Which parameter controls the smoothness of zoomed images in scipy.ndimage.zoom?

A order

B reshape

C mode

D angle

The order parameter controls interpolation smoothness.

What happens if you rotate an image by 90 degrees with reshape=True?

A Image size adjusts to fit rotated image

B Image size stays the same

C Image is cropped

D Image is flipped

reshape=True adjusts the output size to fit the rotated image.

Explain how to rotate and zoom an image using scipy.ndimage functions.

Think about which functions handle rotation and zoom separately.

Describe the effect of the reshape parameter in image rotation and why it matters.

Consider what happens to image edges after rotation.

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