Sobel and Laplace edge detection in SciPy - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Sobel filter from scipy.ndimage.

SciPy

from scipy.ndimage import [1]

Drag options to blanks, or click blank then click option'

Alaplace

Bsobel

Cgaussian_filter

Dmedian_filter

Attempts:

3 left

💡 Hint

Common Mistakes

Importing laplace instead of sobel.

Using gaussian_filter which is for smoothing, not edge detection.

The Sobel filter is imported from scipy.ndimage as 'sobel'.

2fill in blank

medium

Complete the code to apply the Sobel filter on the image array along the x-axis.

SciPy

edge_x = sobel(image, axis=[1])

Drag options to blanks, or click blank then click option'

A0

B-1

C1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using axis=0 which is the y-axis (rows).

Using axis=2 which does not exist in 2D images.

Axis 1 corresponds to the x-axis (columns) in the image array for the Sobel filter.

3fill in blank

hard

Fix the error in the code to apply the Laplace filter correctly on the image.

SciPy

from scipy.ndimage import laplace

edges = laplace([1])

Drag options to blanks, or click blank then click option'

Aimage

Bedges

Claplace

Dsobel

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the output variable 'edges' instead of the image.

Passing the function name 'laplace' as argument.

The Laplace filter function takes the image array as input, so 'image' must be passed.

4fill in blank

hard

Fill both blanks to create a dictionary of edge images using Sobel and Laplace filters.

SciPy

edges = {
    'sobel_x': sobel(image, axis=[1]),
    'laplace': [2](image)
}

Drag options to blanks, or click blank then click option'

A0

Blaplace

Csobel

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using axis 0 for sobel_x which detects horizontal edges.

Using 'sobel' instead of 'laplace' for laplace key.

Sobel filter along axis 1 detects vertical edges; laplace is called as a function on image.

5fill in blank

hard

Fill all three blanks to compute the combined edge magnitude from Sobel edges along x and y axes.

SciPy

edge_x = sobel(image, axis=[1])
edge_y = sobel(image, axis=[2])
edge_magnitude = (edge_x[3]edge_y**2)**0.5

Drag options to blanks, or click blank then click option'

A0

B1

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition for combining edges.

Swapping axis 0 and 1 for edge_x and edge_y.

Sobel edges along axis 1 (x) and 0 (y) are combined by adding squares and taking square root for magnitude.

Practice

1. What is the main purpose of using the Sobel filter in image processing?

easy

A. To detect edges by highlighting horizontal and vertical changes

B. To blur the image and reduce noise

C. To increase the brightness of the image

D. To convert the image to grayscale

Solution

1. Step 1: Understand Sobel filter function

   The Sobel filter detects edges by calculating gradients in horizontal and vertical directions separately.

2. Step 2: Identify the purpose of edge detection

   Edges are found by highlighting where brightness changes sharply, which Sobel does by combining horizontal and vertical gradients.

3. Final Answer:

   To detect edges by highlighting horizontal and vertical changes -> Option A

4. Quick Check:

   Sobel = edge detection [OK]

Hint: Sobel finds edges by checking horizontal and vertical changes [OK]

Common Mistakes:

• Confusing Sobel with blurring filters
• Thinking Sobel changes brightness directly
• Mixing Sobel with color conversion

2. Which of the following is the correct way to apply the Sobel filter on a 2D image array named img using SciPy?

easy

A. scipy.ndimage.sobel(img, axis=2)

B. scipy.ndimage.laplace(img, axis=1)

C. scipy.ndimage.sobel(img, axis=0)

D. scipy.ndimage.gaussian_filter(img, sigma=1)

Solution

1. Step 1: Recall Sobel filter usage in SciPy

   The Sobel filter is applied using scipy.ndimage.sobel with the image and axis (0 for vertical, 1 for horizontal).

2. Step 2: Check axis validity

   For a 2D image, valid axes are 0 or 1. Axis=2 is invalid and will cause an error.

3. Final Answer:

   scipy.ndimage.sobel(img, axis=0) -> Option C

4. Quick Check:

   Sobel syntax = scipy.ndimage.sobel(img, axis) [OK]

Hint: Use axis 0 or 1 for 2D images with sobel [OK]

Common Mistakes:

• Using axis=2 on 2D images causes errors
• Confusing laplace and sobel functions
• Using gaussian_filter instead of sobel for edges

3. Given the following code, what is the shape of the output array edges?

import numpy as np
from scipy import ndimage
img = np.array([[1, 2, 1], [0, 1, 0], [2, 1, 2]])
sobel_x = ndimage.sobel(img, axis=0)
sobel_y = ndimage.sobel(img, axis=1)
edges = np.hypot(sobel_x, sobel_y)

medium

A. (3, 3)

B. (2, 2)

C. (3, 2)

D. (1, 3)

Solution

1. Step 1: Check input image shape

   The input image img is a 3x3 array, so shape is (3, 3).

2. Step 2: Understand Sobel output shape

   Sobel filter preserves the input shape, so sobel_x and sobel_y are also (3, 3).

3. Step 3: Calculate edges shape

   Using np.hypot combines two (3, 3) arrays element-wise, resulting in (3, 3).

4. Final Answer:

   (3, 3) -> Option A

5. Quick Check:

   Input shape = output shape for sobel [OK]

Hint: Sobel keeps input shape; hypot combines same shapes [OK]

Common Mistakes:

• Assuming output shape shrinks after sobel
• Confusing axis with shape dimensions
• Thinking hypot changes array shape

4. The following code attempts to apply the Laplace filter but raises an error. What is the mistake?

from scipy import ndimage
img = [[1, 2, 3], [4, 5, 6], [7, 8]]
laplace_img = ndimage.laplace(img)

medium

A. ndimage.laplace does not exist in SciPy

B. Laplace filter requires axis parameter

C. The image must be grayscale

D. The input image must be a NumPy array, not a list

Solution

1. Step 1: Check input type for ndimage filters

   ndimage functions require NumPy arrays, not Python lists, for image input.

2. Step 2: Identify error cause

   Passing a list causes a TypeError because ndimage.laplace expects an array.

3. Final Answer:

   The input image must be a NumPy array, not a list -> Option D

4. Quick Check:

   ndimage needs np.array input [OK]

Hint: Convert lists to np.array before filtering [OK]

Common Mistakes:

• Passing lists instead of arrays
• Thinking laplace needs axis parameter
• Believing laplace function is missing

5. You want to detect edges in a noisy grayscale image using SciPy. Which approach best combines Sobel and Laplace filters to improve edge detection?

hard

A. Apply Gaussian blur, then Sobel filter on axis=0 only

B. Apply Sobel filters on both axes, combine results, then apply Laplace to sharpen edges

C. Use only Laplace filter because Sobel does not work on noisy images

D. Apply Laplace filter first, then Sobel on axis=2

Solution

1. Step 1: Understand Sobel and Laplace roles

   Sobel detects edges by gradients on horizontal and vertical axes; combining both gives full edge info.

2. Step 2: Use Laplace to enhance edges

   Applying Laplace after Sobel can sharpen edges by detecting second-order changes.

3. Step 3: Evaluate options

   Apply Sobel filters on both axes, combine results, then apply Laplace to sharpen edges correctly combines Sobel on both axes and Laplace for sharpening; others misuse axis or filters.

4. Final Answer:

   Apply Sobel filters on both axes, combine results, then apply Laplace to sharpen edges -> Option B

5. Quick Check:

   Sobel + Laplace = better edge detection [OK]

Hint: Combine Sobel axes then Laplace for sharper edges [OK]

Common Mistakes:

• Applying Sobel on invalid axis
• Skipping combination of horizontal and vertical Sobel
• Using Laplace alone without Sobel for noisy images