Why Image resizing and cropping in MATLAB? - Purpose & Use Cases

The Big Idea

What if you could resize and crop hundreds of images perfectly with just a few lines of code?

The Scenario

Imagine you have hundreds of photos from a trip, and you want to make them all the same size to create a neat slideshow or fit them into a photo album.

Doing this by opening each photo in an editor and resizing or cropping manually would take forever.

The Problem

Manually resizing and cropping images one by one is slow and boring.

It's easy to make mistakes like cropping the wrong part or resizing unevenly.

You also waste a lot of time repeating the same steps for each image.

The Solution

Using image resizing and cropping commands in MATLAB lets you automate this process.

You can write a few lines of code to resize or crop many images quickly and consistently.

This saves time, reduces errors, and makes your images look uniform.

Before vs After

✗ Before

open image
select crop area
crop
resize
save
repeat for each image

✓ After

img = imread('photo.jpg');
cropped = imcrop(img, [x y width height]);
resized = imresize(cropped, [newHeight newWidth]);
imwrite(resized, 'photo_resized.jpg');

What It Enables

You can easily prepare large sets of images for presentations, websites, or printing with perfect size and framing.

Real Life Example

A photographer needs to prepare hundreds of portraits for a client, all with the same size and focus on the face.

Using MATLAB code to resize and crop images automatically makes this task fast and reliable.

Key Takeaways

Manual image editing is slow and error-prone.

MATLAB commands automate resizing and cropping.

This saves time and ensures consistent image sizes.

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