Reading and displaying images in MATLAB - Mini Project: Build & Apply

Reading and displaying images

📖 Scenario: You have a photo saved on your computer and want to see it using MATLAB. This is useful when you want to check images before processing them.

🎯 Goal: Learn how to read an image file into MATLAB and display it on the screen.

📋 What You'll Learn

Use the imread function to load an image file.

Store the image data in a variable called img.

Use the imshow function to display the image.

Use the exact variable names and function calls as specified.

💡 Why This Matters

🌍 Real World

Viewing images is the first step in many image processing tasks like editing photos, analyzing medical images, or preparing pictures for machine learning.

💼 Career

Jobs in data science, computer vision, and digital media often require loading and displaying images to understand and manipulate visual data.

1

Create a variable to read the image

Write a line of code to read the image file named 'peppers.png' using imread and store it in a variable called img.

MATLAB

% Your code here to read the image 'peppers.png' into variable img

Hint

Use imread('filename') to load an image file.

2

Prepare to display the image

Add a line to create a new figure window using the figure function before displaying the image.

MATLAB

img = imread('peppers.png');
% Add a line to create a new figure window

Hint

Use figure; to open a new window for the image.

3

Display the image

Write a line of code to display the image stored in img using the imshow function.

MATLAB

img = imread('peppers.png');
figure;
% Your code here to display the image stored in img

Hint

Use imshow(img) to show the image on screen.

4

Show the image on screen

Run the code to display the image. The output should be the image shown in a figure window.

MATLAB

img = imread('peppers.png');
figure;
imshow(img);
% Run the code to display the image

Hint

Run the script. You should see the 'peppers.png' image in a new window.

Practice

1. What is the primary function of imread in MATLAB when working with images?

easy

A. To display an image on the screen

B. To save an image to a file

C. To load an image file into MATLAB as a matrix

D. To convert an image to grayscale

Solution

1. Step 1: Understand the purpose of imread

   imread reads image files and loads them into MATLAB as arrays (matrices) representing pixel data.

2. Step 2: Differentiate from other functions

   imshow displays images, imwrite saves images, and conversion functions change image formats. Only imread loads images.

3. Final Answer:

   To load an image file into MATLAB as a matrix -> Option C

4. Quick Check:

   imread = load image [OK]

Hint: Remember: 'read' means load data into MATLAB [OK]

Common Mistakes:

• Confusing imread with imshow
• Thinking imread displays images
• Assuming imread saves images

2. Which of the following is the correct syntax to display an image stored in variable img in MATLAB?

easy

A. imshow(img);

B. imread(img);

C. display(img);

D. show(img);

Solution

1. Step 1: Identify the function to display images

   imshow is the MATLAB function designed to show images stored in variables.

2. Step 2: Check syntax correctness

   The syntax imshow(img); correctly calls the function with the image variable.

3. Final Answer:

   imshow(img); -> Option A

4. Quick Check:

   Display image = imshow [OK]

Hint: Use imshow to show images, not imread [OK]

Common Mistakes:

• Using imread to display images
• Using non-existent functions like show
• Using display which does not render images

3. What will be the output of the following MATLAB code?

img = imread('peppers.png');
imshow(img);
size_img = size(img);

What does size_img contain?

medium

A. [384 512 3]

B. [512 384 3]

C. [384 512]

D. [512 384]

Solution

1. Step 1: Understand image dimensions

   The image 'peppers.png' is a color image with height 384 pixels, width 512 pixels, and 3 color channels (RGB).

2. Step 2: Interpret size output

   size(img) returns a vector with [rows, columns, color channels], so it is [384 512 3].

3. Final Answer:

   [384 512 3] -> Option A

4. Quick Check:

   Image size = [height width channels] = [384 512 3] [OK]

Hint: Remember size returns [rows columns channels] for images [OK]

Common Mistakes:

• Swapping width and height values
• Ignoring the third dimension for color channels
• Assuming grayscale image size format

4. Identify the error in the following MATLAB code snippet:

img = imread('flower.jpg')
imshow(img)

What is the likely cause if MATLAB throws an error?

medium

A. Missing semicolon after imread line

B. Variable img is not defined

C. Using imshow without parentheses

D. Incorrect file extension or file not found

Solution

1. Step 1: Check syntax correctness

   Missing semicolons do not cause errors, only suppress output. Parentheses are correctly used.

2. Step 2: Consider file-related errors

   If MATLAB cannot find 'flower.jpg' or the extension is wrong, imread throws an error.

3. Final Answer:

   Incorrect file extension or file not found -> Option D

4. Quick Check:

   File path/name errors cause imread failure [OK]

Hint: Check file name and path carefully if imread errors [OK]

Common Mistakes:

• Thinking missing semicolon causes error
• Assuming imshow syntax is wrong
• Ignoring file existence and path issues

5. You want to read an image and display only its red color channel in MATLAB. Which code snippet correctly achieves this?

hard

A. img = imread('image.png'); imshow(img(:,:,3));

B. img = imread('image.png'); imshow(img(:,:,1));

C. img = imread('image.png'); imshow(img(1,:,:));

D. img = imread('image.png'); imshow(img(:,1,:));

Solution

1. Step 1: Understand image color channels

   In MATLAB, the third dimension of an RGB image is color channels: 1=Red, 2=Green, 3=Blue.

2. Step 2: Extract and display red channel

   Using img(:,:,1) extracts the red channel matrix. Displaying it with imshow shows only red intensity.

3. Final Answer:

   img = imread('image.png'); imshow(img(:,:,1)); -> Option B

4. Quick Check:

   Red channel = third dimension index 1 [OK]

Hint: Use (:,:,1) to get red channel from RGB image [OK]

Common Mistakes:

• Using wrong channel index for red
• Trying to slice rows or columns incorrectly
• Confusing color channel order