Reading and displaying images in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function in MATLAB is used to read an image file into the workspace?

The imread function reads an image file and stores it as a matrix in MATLAB.

beginner

How do you display an image stored in a variable in MATLAB?

Use the imshow function to display the image stored in a variable.

intermediate

What type of data does imread return when reading a color image?

imread returns a 3D matrix where the third dimension represents the Red, Green, and Blue color channels.

intermediate

Why is it important to use imshow instead of image for displaying images?

imshow automatically scales and displays the image correctly, while image may require manual axis adjustments.

beginner

What happens if you try to read a non-existent image file with imread?

MATLAB throws an error indicating the file cannot be found or read.

Which MATLAB function reads an image file into a matrix?

A imshow

B image

C imread

D imwrite

imread reads image files into MATLAB as matrices.

What does imshow do in MATLAB?

A Reads an image file

B Displays an image stored in a variable

C Saves an image to a file

D Converts an image to grayscale

imshow displays images stored in variables.

If you read a color image with imread, what is the size of the returned matrix?

A 3D matrix with RGB channels

B Scalar value

C 1D vector

D 2D matrix

Color images are stored as 3D matrices with separate Red, Green, and Blue layers.

What will happen if imread tries to read a file that does not exist?

A Creates a blank image

B Displays a warning but continues

C Returns an empty matrix

D Throws an error

MATLAB throws an error if the file cannot be found.

Why might you prefer imshow over image to display images?

A <code>imshow</code> automatically scales and displays images correctly

B <code>image</code> is faster

C <code>imshow</code> saves images

D <code>image</code> converts images to grayscale

imshow handles image display scaling automatically.

Explain how to read and display an image in MATLAB step-by-step.

Think about how you open a photo on your computer and then show it on screen.

What are the differences between the outputs of imread for grayscale and color images?

Consider how many layers or channels the image data has.

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