Color space conversion in MATLAB - Mini Project: Build & Apply

Color space conversion

📖 Scenario: You are working with digital images and need to convert colors from one format to another. This is common in image processing tasks like filtering, enhancement, or analysis.

🎯 Goal: Build a simple MATLAB program that converts an RGB color to grayscale using a weighted sum formula.

📋 What You'll Learn

Create a variable with RGB color values

Define weights for red, green, and blue channels

Calculate the grayscale value using the weights

Display the grayscale value

💡 Why This Matters

🌍 Real World

Color space conversion is used in photo editing, computer vision, and graphics to simplify images or prepare them for analysis.

💼 Career

Understanding color spaces and conversions is important for roles in image processing, computer graphics, and machine learning involving visual data.

1

Create RGB color vector

Create a variable called rgb_color as a 1x3 vector with these exact values: [120, 200, 150] representing red, green, and blue intensities.

MATLAB

% Create the RGB color vector
% Your code here

Hint

Use square brackets to create a vector: [red, green, blue].

2

Define grayscale weights

Create three variables called weight_r, weight_g, and weight_b with values 0.2989, 0.5870, and 0.1140 respectively. These weights represent how much each color contributes to grayscale.

MATLAB

rgb_color = [120, 200, 150];
% Define weights for red, green, and blue
% Your code here

Hint

Assign each weight to its variable using the equals sign.

3

Calculate grayscale value

Calculate the grayscale value by multiplying each color in rgb_color by its weight and summing the results. Store the result in a variable called grayscale.

MATLAB

rgb_color = [120, 200, 150];
weight_r = 0.2989;
weight_g = 0.5870;
weight_b = 0.1140;
% Calculate grayscale value
% Your code here

Hint

Use parentheses to access vector elements: rgb_color(1) for red, etc.

4

Display grayscale value

Use disp to print the text 'Grayscale value:' followed by the value stored in grayscale.

MATLAB

rgb_color = [120, 200, 150];
weight_r = 0.2989;
weight_g = 0.5870;
weight_b = 0.1140;
grayscale = rgb_color(1)*weight_r + rgb_color(2)*weight_g + rgb_color(3)*weight_b;
% Display the grayscale value
% Your code here

Hint

Use num2str to convert the number to text for display.

Practice

1. What does the MATLAB function rgb2gray do?

easy

A. Converts an image from CMYK to RGB

B. Converts a grayscale image to RGB

C. Converts an image from HSV to RGB

D. Converts a color image from RGB to grayscale

Solution

1. Step 1: Understand the function name

   The function name rgb2gray suggests conversion from RGB to gray scale.

2. Step 2: Recall MATLAB image processing basics

   In MATLAB, rgb2gray converts a color image (RGB) to a grayscale image by removing color information.

3. Final Answer:

   Converts a color image from RGB to grayscale -> Option D

4. Quick Check:

   rgb2gray = RGB to grayscale [OK]

Hint: Look for 'rgb2gray' meaning RGB to gray conversion [OK]

Common Mistakes:

• Confusing rgb2gray with gray2rgb
• Thinking it converts HSV images
• Assuming it changes image size

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

easy

A. hsvImage = rgb2hsv(img);

B. hsvImage = hsv2rgb(img);

C. hsvImage = rgb2gray(img);

D. hsvImage = gray2hsv(img);

Solution

1. Step 1: Identify the function for RGB to HSV conversion

   MATLAB uses rgb2hsv to convert RGB images to HSV color space.

2. Step 2: Check the syntax correctness

   The syntax hsvImage = rgb2hsv(img); correctly calls the function with the image variable.

3. Final Answer:

   hsvImage = rgb2hsv(img); -> Option A

4. Quick Check:

   rgb2hsv syntax = correct [OK]

Hint: Remember rgb2hsv converts RGB to HSV [OK]

Common Mistakes:

• Using hsv2rgb instead of rgb2hsv
• Using rgb2gray for color conversion
• Using non-existent gray2hsv function

3. What will be the output size of the variable grayImage after running this code?

rgbImage = imread('peppers.png');
grayImage = rgb2gray(rgbImage);

medium

A. Same height, width, and channels as rgbImage

B. Same height and width as rgbImage but single channel

C. Half the height and width of rgbImage

D. Double the height and width of rgbImage

Solution

1. Step 1: Understand input image dimensions

   RGB images have height x width x 3 channels (for red, green, blue).

2. Step 2: Effect of rgb2gray on image size

   rgb2gray converts to grayscale, keeping height and width but reducing channels to 1.

3. Final Answer:

   Same height and width as rgbImage but single channel -> Option B

4. Quick Check:

   rgb2gray output = height x width x 1 [OK]

Hint: Grayscale keeps size but reduces channels to 1 [OK]

Common Mistakes:

• Assuming rgb2gray changes image dimensions
• Thinking output has 3 channels
• Confusing with resizing functions

4. Identify the error in this MATLAB code snippet for converting an RGB image to grayscale:

img = imread('image.jpg');
grayImg = rgb2gray(img, 'double');

medium

A. imread cannot read 'image.jpg'

B. Variable names are invalid in MATLAB

C. rgb2gray does not accept a second argument like 'double'

D. Missing semicolon after imread

Solution

1. Step 1: Check rgb2gray function usage

   rgb2gray only takes one input argument: the RGB image.

2. Step 2: Analyze the second argument 'double'

   Passing 'double' as a second argument causes an error because rgb2gray does not support it.

3. Final Answer:

   rgb2gray does not accept a second argument like 'double' -> Option C

4. Quick Check:

   rgb2gray single input only [OK]

Hint: Check function docs for allowed arguments [OK]

Common Mistakes:

• Adding unsupported extra arguments
• Assuming imread fails without reason
• Thinking semicolon is mandatory for correctness

5. You want to convert an RGB image to HSV and then extract only the Hue channel for further processing. Which MATLAB code snippet correctly does this?

hard

A. hsvImage = rgb2hsv(rgbImage); hueChannel = hsvImage(:,:,1);

B. hsvImage = rgb2gray(rgbImage); hueChannel = hsvImage(:,:,1);

C. hsvImage = hsv2rgb(rgbImage); hueChannel = hsvImage(:,:,2);

D. hsvImage = rgb2hsv(rgbImage); hueChannel = hsvImage(:,:,3);

Solution

1. Step 1: Convert RGB to HSV correctly

   Use rgb2hsv to convert the RGB image to HSV color space.

2. Step 2: Extract the Hue channel

   In HSV, the first channel (:,:,1) is the Hue component.

3. Final Answer:

   hsvImage = rgb2hsv(rgbImage); hueChannel = hsvImage(:,:,1); -> Option A

4. Quick Check:

   Hue channel is first in HSV [OK]

Hint: Hue is channel 1 after rgb2hsv conversion [OK]

Common Mistakes:

• Using rgb2gray instead of rgb2hsv
• Extracting wrong channel index
• Using hsv2rgb instead of rgb2hsv