Image resizing and cropping in MATLAB - Mini Project: Build & Apply

Image resizing and cropping

📖 Scenario: You have a photo that you want to prepare for a website. The website needs the image to be smaller and focused on the center part.

🎯 Goal: You will write a MATLAB program that loads an image, resizes it to a smaller size, then crops the center part of the resized image.

📋 What You'll Learn

Load an image file named input.jpg

Resize the image to 200 pixels wide and 150 pixels tall

Crop a 100 by 100 pixel square from the center of the resized image

Display the final cropped image

💡 Why This Matters

🌍 Real World

Websites and apps often require images to be a certain size and focused on important parts. Resizing and cropping images helps fit them perfectly.

💼 Career

Image processing skills are useful in jobs like web development, graphic design, and computer vision engineering.

1

Load the image

Write a line of MATLAB code to load the image file named input.jpg into a variable called img.

MATLAB

% Load the image from 'input.jpg'
% Your code here

Hint

Use the imread function to load images in MATLAB.

2

Resize the image

Write a line of MATLAB code to resize the image stored in img to width 200 and height 150 pixels, and save it in a variable called img_resized.

MATLAB

img = imread('input.jpg');
% Resize the image to 200x150 pixels
% Your code here

Hint

Use imresize with the size as a two-element vector: [height width].

3

Crop the center of the resized image

Write MATLAB code to crop a 100 by 100 pixel square from the center of img_resized. Save the cropped image in a variable called img_cropped. Use the size of img_resized to find the center coordinates.

MATLAB

img = imread('input.jpg');
img_resized = imresize(img, [150 200]);
% Crop a 100x100 square from the center of img_resized
% Your code here

Hint

Use size to get image dimensions and imcrop with rectangle [x y width height]. Remember MATLAB indexing starts at 1.

4

Display the cropped image

Write a line of MATLAB code to display the cropped image stored in img_cropped using the imshow function.

MATLAB

img = imread('input.jpg');
img_resized = imresize(img, [150 200]);
[height, width, ~] = size(img_resized);
row_start = floor(height/2) - 49;
col_start = floor(width/2) - 49;
img_cropped = imcrop(img_resized, [col_start row_start 99 99]);
% Display the cropped image
% Your code here

Hint

Use imshow to show images in MATLAB.

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