Image resizing and cropping in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of resizing an image with imresize

What is the size of the output image after running this code?

img = zeros(100, 200, 3); % black image 100x200 RGB
resized_img = imresize(img, 0.5);

MATLAB

img = zeros(100, 200, 3);
resized_img = imresize(img, 0.5);
size_resized = size(resized_img);

A [200 400 3]

B [50 100 3]

C [100 200 3]

D [50 50 3]

💡 Hint

Think about what scaling by 0.5 means for each dimension.

The original image is 100 rows by 200 columns. Scaling by 0.5 reduces each dimension by half, so the output size is 50 rows by 100 columns, with 3 color channels unchanged.

❓ Predict Output

intermediate

Result of cropping an image with imcrop

What is the size of the cropped image after running this code?

img = zeros(300, 400, 3);
cropped_img = imcrop(img, [50 100 149 99]);

MATLAB

img = zeros(300, 400, 3);
cropped_img = imcrop(img, [50 100 149 99]);
cropped_size = size(cropped_img);

A [99 149 3]

B [100 150 3]

C [150 100 3]

D [101 151 3]

💡 Hint

Remember that the rectangle vector is [xmin ymin width height].

imcrop returns an image of size height by width (rows x columns). Here width=149 and height=99, so size is 99 rows by 149 columns with 3 channels.

🔧 Debug

advanced

Identify the error in this image resizing code

What error will this code produce?

img = imread('peppers.png');
new_img = imresize(img, [200]);

MATLAB

img = imread('peppers.png');
new_img = imresize(img, [200]);

A No error, image resized to 200x200

B Error: Invalid scale factor

C Error: Input image must be grayscale

D Error: Size vector must have two elements

💡 Hint

Check the documentation for imresize size input format.

imresize expects the size argument as a two-element vector [numrows numcols]. Passing a single number causes an error.

🧠 Conceptual

advanced

Understanding aspect ratio preservation in resizing

Which option correctly resizes an image to have width 300 pixels while preserving the aspect ratio?

A imresize(img, [300 300])

B imresize(img, [300 NaN])

C imresize(img, [NaN 300])

D imresize(img, 300)

💡 Hint

NaN can be used to preserve one dimension automatically.

Using [NaN 300] tells imresize to set width to 300 and adjust height to preserve aspect ratio. Other options either fix height or distort the image.

❓ Predict Output

expert

Output pixel value after cropping and resizing

Given this code, what is the value of pixel (1,1) in the final image?

img = uint8(reshape(1:16, 4, 4));
cropped = imcrop(img, [2 2 1 1]);
resized = imresize(cropped, [2 2]);
pixel_val = resized(1,1);

MATLAB

img = uint8(reshape(1:16, 4, 4));
cropped = imcrop(img, [2 2 1 1]);
resized = imresize(cropped, [2 2]);
pixel_val = resized(1,1);

A 6

B Error: Index exceeds matrix dimensions

C 1

D 5

💡 Hint

Check which pixels are selected by cropping and how resizing interpolates values.

The crop [2 2 1 1] selects the pixel at (2,2) which is 6. Resizing the 1x1 image to 2x2 using bilinear interpolation fills all pixels with 6.

Practice

1. What does the imresize function do in MATLAB when working with images?

easy

A. Converts an image to grayscale

B. Extracts a rectangular part of an image

C. Changes the size of an image by scaling or specifying new dimensions

D. Rotates an image by a specified angle

Solution

1. Step 1: Understand the purpose of imresize

   The imresize function is used to change the size of an image either by scaling it up or down or by specifying exact new dimensions.

2. Step 2: Differentiate from cropping

   Unlike cropping, which extracts a part of the image, resizing changes the entire image size.

3. Final Answer:

   Changes the size of an image by scaling or specifying new dimensions -> Option C

4. Quick Check:

   imresize = Resize image size [OK]

Hint: Resize changes whole image size, crop extracts part [OK]

Common Mistakes:

• Confusing resize with crop
• Thinking resize changes image colors
• Assuming resize rotates image

2. Which of the following is the correct syntax to crop an image img to a rectangle starting at (50, 30) with width 100 and height 80 in MATLAB?

easy

A. cropped = imcrop(img, [50 30 100 80]);

B. cropped = imresize(img, [50 30 100 80]);

C. cropped = crop(img, 50, 30, 100, 80);

D. cropped = imcrop(img, [30 50 80 100]);

Solution

1. Step 1: Recall imcrop syntax

   The imcrop function takes the image and a rectangle vector [x y width height]. Here, x=50, y=30, width=100, height=80.

2. Step 2: Check option correctness

   cropped = imcrop(img, [50 30 100 80]); matches the correct syntax. cropped = imresize(img, [50 30 100 80]); uses imresize which is wrong. cropped = crop(img, 50, 30, 100, 80); uses a non-existent function. cropped = imcrop(img, [30 50 80 100]); swaps x and y and width and height incorrectly.

3. Final Answer:

   cropped = imcrop(img, [50 30 100 80]); -> Option A

4. Quick Check:

   imcrop(img, [x y width height]) = correct crop syntax [OK]

Hint: imcrop uses [x y width height] rectangle vector [OK]

Common Mistakes:

• Swapping x and y coordinates
• Using imresize instead of imcrop
• Incorrect rectangle vector order

3. What will be the size of the output image resized_img after running the following MATLAB code?

img = imread('peppers.png');
resized_img = imresize(img, 0.5);

medium

A. Half the width and half the height of the original image

B. Double the width and double the height of the original image

C. Same size as the original image

D. Width halved but height unchanged

Solution

1. Step 1: Understand imresize with scale factor

   Using imresize(img, 0.5) scales the image to 50% of its original size in both width and height.

2. Step 2: Determine output size

   The output image will have half the width and half the height of the original image.

3. Final Answer:

   Half the width and half the height of the original image -> Option A

4. Quick Check:

   imresize with 0.5 scales both dimensions by 0.5 [OK]

Hint: Scale 0.5 halves both width and height [OK]

Common Mistakes:

• Thinking only width or height changes
• Assuming scale factor doubles size
• Confusing imresize with imcrop

4. Identify the error in this MATLAB code snippet intended to crop an image img to a 100x100 square starting at (20, 20):

cropped_img = imcrop(img, [20 20 100]);

medium

A. The image variable name is incorrect

B. The rectangle vector is missing the height value

C. imcrop cannot crop square images

D. The starting coordinates should be (100, 100)

Solution

1. Step 1: Check imcrop rectangle vector format

   The rectangle vector must have four values: [x y width height]. Here only three values are given.

2. Step 2: Identify missing parameter

   The height value is missing, causing a syntax error or unexpected behavior.

3. Final Answer:

   The rectangle vector is missing the height value -> Option B

4. Quick Check:

   imcrop needs [x y width height] vector [OK]

Hint: Rectangle vector must have 4 values [OK]

Common Mistakes:

• Providing only 3 values instead of 4
• Confusing coordinates with size
• Assuming imcrop auto-fills missing values

5. You want to resize an image img to exactly 200x150 pixels and then crop a 100x100 square from the center. Which MATLAB code sequence correctly achieves this?

hard

A. cropped = imcrop(img, [75 50 100 100]); resized = imresize(cropped, [200 150]);

B. resized = imresize(img, [200 150]); cropped = imcrop(resized, [50 25 100 100]);

C. cropped = imcrop(img, [50 25 100 100]); resized = imresize(cropped, [200 150]);

D. resized = imresize(img, [150 200]); cropped = imcrop(resized, [50 25 100 100]);

Solution

1. Step 1: Resize image to 200x150 pixels

   Use imresize(img, [150 200]) to set width=200 and height=150.

2. Step 2: Crop 100x100 square from center

   The center of resized image is at (100, 75). To crop 100x100, start at (50, 25) because 50 = 100 - 100/2 and 25 = 75 - 100/2.

3. Step 3: Verify option correctness

   resized = imresize(img, [150 200]); cropped = imcrop(resized, [50 25 100 100]); matches the correct order and coordinates. resized = imresize(img, [200 150]); cropped = imcrop(resized, [50 25 100 100]); swaps width and height. Options C and D crop before resizing, which is not what is asked.

4. Final Answer:

   resized = imresize(img, [150 200]); cropped = imcrop(resized, [50 25 100 100]); -> Option D

5. Quick Check:

   Resize first, then crop center rectangle [OK]

Hint: Resize first, then crop center using half-dimensions [OK]

Common Mistakes:

• Swapping width and height in resize
• Cropping before resizing
• Incorrect crop start coordinates