Image types (grayscale, RGB) in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read an image file into a variable.

MATLAB

img = imread([1]);

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

Aimage.jpg

Bload image.jpg

Cread('image.jpg')

D'image.jpg'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the filename

Using functions other than imread to load images

The imread function requires the filename as a string, so it must be enclosed in quotes.

2fill in blank

medium

Complete the code to convert a color image to grayscale.

MATLAB

gray_img = rgb2gray([1]);

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

Aimage

Bimread

Cimg

Dgray_img

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the grayscale variable to rgb2gray instead of the original image

Passing the function name instead of a variable

The rgb2gray function converts a color image stored in img to grayscale.

3fill in blank

hard

Fix the error in the code to check if an image is RGB by checking its size.

MATLAB

is_rgb = (size([1], 3) == 3);

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

Aimg

Bgray_img

Crgb_img

Dimage

Attempts:

3 left

💡 Hint

Common Mistakes

Checking grayscale image variable instead of original image

Using incorrect variable names

The variable img holds the original image. Checking the third dimension size of img tells if it is RGB.

4fill in blank

hard

Fill both blanks to create a grayscale image of the same size as the original RGB image.

MATLAB

gray_img = zeros(size([1], [2]), size([1], 2));

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

Aimg

B3

C1

Dgray_img

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as dimension for grayscale image size

Using grayscale variable instead of original image

To create a 2D grayscale image matching the height and width of the original RGB image, use zeros(size(img, 1), size(img, 2)). The first blank is the image variable img and the second blank is the dimension 1 (rows).

5fill in blank

hard

Fill all three blanks to create an RGB image with all pixels set to red.

MATLAB

red_img = zeros(size([1]));
red_img(:, :, [2]) = [3];

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

Aimg

B1

C255

Dgray_img

Attempts:

3 left

💡 Hint

Common Mistakes

Using grayscale image size instead of original image

Setting the wrong channel for red

Using 0 instead of 255 for red intensity

We create a zero matrix with the same size as the original image img. The red channel is the first channel, so we set red_img(:, :, 1) to 255 to make all pixels red.

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