Image types (grayscale, RGB) in MATLAB - Mini Project: Build & Apply

Image types (grayscale, RGB)

📖 Scenario: You are working with images in MATLAB. Images can be grayscale (black and white) or RGB (color). You want to create simple examples of both types to understand how they are stored and displayed.

🎯 Goal: Create a grayscale image and an RGB image using matrices in MATLAB. Then display both images to see the difference.

📋 What You'll Learn

Create a 3x3 grayscale image matrix with specific values

Create a 3x3x3 RGB image matrix with specific color values

Use a variable to store the grayscale image

Use a variable to store the RGB image

Display both images using MATLAB's imshow function

💡 Why This Matters

🌍 Real World

Working with images is common in photography, medical imaging, and computer vision. Understanding how images are stored helps in editing and analyzing them.

💼 Career

Many jobs in data science, engineering, and research require manipulating images. Knowing how to handle grayscale and RGB images in MATLAB is a useful skill.

1

Create a grayscale image matrix

Create a variable called grayImage that is a 3x3 matrix with these exact values: [0 128 255; 64 192 32; 255 0 128].

MATLAB

% Create a variable called grayImage with the given 3x3 matrix
% Your code here

Hint

Use square brackets to create the matrix. Separate rows with semicolons. Use uint8 to specify the image data type.

2

Create an RGB image matrix

Create a variable called rgbImage that is a 3x3x3 matrix representing an RGB image. Use these exact values for the red channel: [255 0 0; 0 255 0; 0 0 255], green channel: [0 255 0; 255 0 255; 0 255 0], and blue channel: [0 0 255; 255 255 0; 255 0 0].

MATLAB

grayImage = uint8([0 128 255; 64 192 32; 255 0 128]);
% Create a 3x3x3 matrix called rgbImage with the given red, green, and blue channels
% Your code here

Hint

Use zeros(3,3,3,'uint8') to create an empty RGB image matrix. Assign each color channel separately.

3

Display the grayscale image

Use the imshow function to display the grayscale image stored in grayImage.

MATLAB

grayImage = uint8([0 128 255; 64 192 32; 255 0 128]);
rgbImage = zeros(3,3,3,'uint8');
rgbImage(:,:,1) = uint8([255 0 0; 0 255 0; 0 0 255]);
rgbImage(:,:,2) = uint8([0 255 0; 255 0 255; 0 255 0]);
rgbImage(:,:,3) = uint8([0 0 255; 255 255 0; 255 0 0]);
% Display the grayscale image
% Your code here

Hint

Simply call imshow with the grayscale image variable.

4

Display the RGB image

Use the imshow function to display the RGB image stored in rgbImage.

MATLAB

grayImage = uint8([0 128 255; 64 192 32; 255 0 128]);
rgbImage = zeros(3,3,3,'uint8');
rgbImage(:,:,1) = uint8([255 0 0; 0 255 0; 0 0 255]);
rgbImage(:,:,2) = uint8([0 255 0; 255 0 255; 0 255 0]);
rgbImage(:,:,3) = uint8([0 0 255; 255 255 0; 255 0 0]);
imshow(grayImage);
% Display the RGB image
% Your code here

Hint

Call imshow with the RGB image variable after displaying the grayscale image.

Practice

1. Which of the following correctly describes a grayscale image in MATLAB?

easy

A. It has one channel per pixel representing shades of gray.

B. It has three channels representing red, green, and blue colors.

C. It stores color information in four channels including alpha transparency.

D. It is always stored as a 3D matrix with size MxNx3.

Solution

1. Step 1: Understand grayscale image structure

   Grayscale images have only one channel per pixel, showing intensity from black to white.

2. Step 2: Compare with RGB image structure

   RGB images have three channels (red, green, blue), so grayscale images differ by having only one channel.

3. Final Answer:

   It has one channel per pixel representing shades of gray. -> Option A

4. Quick Check:

   Grayscale = 1 channel [OK]

Hint: Check if image size third dimension equals 1 for grayscale [OK]

Common Mistakes:

• Confusing grayscale with RGB images having 3 channels
• Assuming grayscale images have multiple color channels
• Thinking grayscale images are always 3D matrices

2. Which MATLAB command correctly checks if an image img is RGB?

easy

A. size(img, 3) == 1

B. ndims(img) == 2

C. length(img) == 3

D. size(img, 3) == 3

Solution

1. Step 1: Recall RGB image dimension

   RGB images have three color channels, so the third dimension size is 3.

2. Step 2: Identify correct MATLAB syntax

   Using size(img, 3) == 3 checks if the third dimension equals 3, confirming RGB.

3. Final Answer:

   size(img, 3) == 3 -> Option D

4. Quick Check:

   Third dimension = 3 means RGB [OK]

Hint: Use size(img,3) to check color channels quickly [OK]

Common Mistakes:

• Using ndims instead of size for channel count
• Checking length instead of size for dimensions
• Confusing 2D grayscale with RGB dimensions

3. Given the MATLAB code:

img = imread('peppers.png');
dim = size(img);
channels = dim(3);

What is the value of channels for this image?

medium

A. 1

B. 2

C. 3

D. 4

Solution

1. Step 1: Understand the image type of 'peppers.png'

   'peppers.png' is a standard RGB image with three color channels.

2. Step 2: Analyze the code output

   The size(img) returns a vector where the third element is the number of channels, which is 3 for RGB.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   peppers.png is RGB, so channels = 3 [OK]

Hint: Standard color images usually have 3 channels [OK]

Common Mistakes:

• Assuming PNG images are grayscale by default
• Confusing number of dimensions with number of channels
• Expecting 4 channels for PNG without alpha

4. Consider this MATLAB code snippet:

img = imread('cameraman.tif');
if size(img, 3) == 3
    disp('RGB Image');
else
    disp('Grayscale Image');
end

What is the output when running this code?

medium

A. Grayscale Image

B. RGB Image

C. Error: Index exceeds matrix dimensions.

D. No output

Solution

1. Step 1: Identify image type of 'cameraman.tif'

   'cameraman.tif' is a grayscale image with only one channel.

2. Step 2: Evaluate the condition in the code

   Since size(img, 3) returns 1 (or may not exist), the else branch runs, printing 'Grayscale Image'.

3. Final Answer:

   Grayscale Image -> Option A

4. Quick Check:

   cameraman.tif is grayscale, so output = Grayscale Image [OK]

Hint: Grayscale images have no 3rd dimension or size 1 [OK]

Common Mistakes:

• Expecting 3 channels for all images
• Assuming error due to missing third dimension
• Ignoring else branch output

5. You have an image matrix img in MATLAB. Write a code snippet to convert an RGB image to grayscale only if it is RGB. Which of the following is correct?

hard

A. if ndims(img) == 2
img_gray = rgb2gray(img);
else
img_gray = img;
end

B. if size(img, 3) == 3
img_gray = rgb2gray(img);
else
img_gray = img;
end

C. if size(img, 3) == 1
img_gray = rgb2gray(img);
else
img_gray = img;
end

D. if size(img, 3) == 4
img_gray = rgb2gray(img);
else
img_gray = img;
end

Solution

1. Step 1: Check if image is RGB by third dimension size

   RGB images have 3 channels, so size(img, 3) == 3 confirms RGB.

2. Step 2: Convert RGB to grayscale only if condition is true

   Use rgb2gray(img) to convert RGB to grayscale; otherwise keep image as is.

3. Final Answer:

   if size(img, 3) == 3 img_gray = rgb2gray(img); else img_gray = img; end -> Option B

4. Quick Check:

   Convert only if 3 channels (RGB) [OK]

Hint: Check size(img,3)==3 before rgb2gray conversion [OK]

Common Mistakes:

• Checking for 1 channel instead of 3
• Using ndims instead of size for channels
• Assuming 4 channels means RGB