Color space conversion in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert an RGB image to grayscale using built-in function.

MATLAB

grayImage = rgb2[1](rgbImage);

Drag options to blanks, or click blank then click option'

Alab

Bhsv

Cgray

Dxyz

Attempts:

3 left

💡 Hint

Common Mistakes

Using rgb2hsv or rgb2lab instead of rgb2gray.

The function rgb2gray converts an RGB image to grayscale.

2fill in blank

medium

Complete the code to convert an RGB image to HSV color space.

MATLAB

hsvImage = rgb2[1](rgbImage);

Drag options to blanks, or click blank then click option'

Ahsv

Bgray

Clab

Dxyz

Attempts:

3 left

💡 Hint

Common Mistakes

Using rgb2gray or rgb2lab instead of rgb2hsv.

The function rgb2hsv converts an RGB image to HSV color space.

3fill in blank

hard

Fix the error in the code to convert an RGB image to LAB color space.

MATLAB

labImage = rgb2[1](rgbImage);

Drag options to blanks, or click blank then click option'

Agray

Blab

Chsv

Dxyz

Attempts:

3 left

💡 Hint

Common Mistakes

Using rgb2hsv or rgb2gray instead of rgb2lab.

The correct function to convert RGB to LAB is rgb2lab.

4fill in blank

hard

Fill both blanks to convert an RGB image to XYZ color space and then back to RGB.

MATLAB

xyzImage = rgb2[1](rgbImage);
rgbImage2 = [2]2rgb(xyzImage);

Drag options to blanks, or click blank then click option'

Axyz

Blab

Chsv

Dgray

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up LAB or HSV functions instead of XYZ.

Use rgb2xyz to convert RGB to XYZ, and xyz2rgb to convert back.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping color spaces to their conversion functions from RGB.

MATLAB

colorConversions = struct([1], @rgb2[2], [3], @rgb2hsv);

Drag options to blanks, or click blank then click option'

A'gray'

Bgray

C'hsv'

Dlab

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names without quotes as keys or missing '@' for function handles.

The struct keys are strings like 'gray' and 'hsv'. The function names are rgb2gray and rgb2hsv.

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