Reading and displaying images in MATLAB - Step-by-Step Execution

Concept Flow - Reading and displaying images

Start

↓

Read Image File

↓

Store Image Data in Variable

↓

Display Image on Screen

↓

End

The program starts by reading an image file, stores the image data in a variable, then displays the image on the screen.

Execution Sample

MATLAB

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

This code reads the image 'peppers.png' and displays it in a figure window.

Execution Table

Step	Action	Variable	Value/Result	Output
1	Call imread('peppers.png')	img	Matrix of image pixel data	No output yet
2	Call imshow(img)	img	Matrix of image pixel data	Image displayed in figure window
3	End of script	-	-	Program stops

💡 Script ends after displaying the image.

Variable Tracker

Variable	Start	After imread	After imshow	Final
img	undefined	Matrix of size MxNx3 (RGB image)	Same matrix (unchanged)	Same matrix (unchanged)

Key Moments - 2 Insights

Why do we need to store the image in a variable before displaying it?

Because imshow needs the image data as input, so we first read the image into 'img' (see execution_table step 1), then pass 'img' to imshow (step 2).

What type of data does imread return?

imread returns a matrix representing the image pixels (see variable_tracker for 'img' after imread). This matrix can be 2D for grayscale or 3D for color images.

Visual Quiz - 3 Questions

Look at the execution table, what is stored in 'img' after step 1?

A A matrix containing image pixel data

B A string with the filename

C An empty variable

D A figure window handle

💡 Hint

Check the 'Variable' and 'Value/Result' columns in execution_table row 1.

At which step is the image actually shown on the screen?

A Step 3

B Step 1

C Step 2

D Before step 1

💡 Hint

Look at the 'Output' column in execution_table to see when the image is displayed.

If we skip storing the image in 'img' and call imshow('peppers.png'), what happens?

A Image is read twice

B Image displays correctly

C Error because imshow expects image data, not filename

D Nothing happens

💡 Hint

MATLAB's imshow accepts a filename directly, reads the image internally, and displays it correctly.

Concept Snapshot

imread('filename') reads an image file into a matrix.
imshow(matrix) displays the image matrix.
Store image data in a variable before displaying.
Image data is a matrix representing pixels.
Use these two functions to read and show images in MATLAB.

Full Transcript

This example shows how MATLAB reads and displays images. First, the imread function reads the image file 'peppers.png' and stores the pixel data in the variable 'img'. Then, imshow takes this matrix 'img' and displays the image in a figure window. The execution table traces these steps: reading the image, storing it, and displaying it. The variable tracker shows how 'img' changes from undefined to holding the image matrix. Key moments clarify why storing the image is necessary and what data type imread returns. The quiz tests understanding of these steps and common mistakes. This simple flow helps beginners see how image reading and displaying works in MATLAB.

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