Color spaces (RGB, BGR, grayscale, HSV) in Computer Vision - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert an image from BGR to grayscale using OpenCV.

Computer Vision

gray_image = cv2.cvtColor(image, [1])

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

Acv2.COLOR_BGR2RGB

Bcv2.COLOR_RGB2GRAY

Ccv2.COLOR_GRAY2BGR

Dcv2.COLOR_BGR2GRAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using cv2.COLOR_RGB2GRAY instead of cv2.COLOR_BGR2GRAY

Using a conversion code that changes color order instead of converting to grayscale

OpenCV loads images in BGR format by default. To convert to grayscale, use cv2.COLOR_BGR2GRAY.

2fill in blank

medium

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

Computer Vision

hsv_image = cv2.cvtColor(image, [1])

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

Acv2.COLOR_BGR2HSV

Bcv2.COLOR_RGB2HSV

Ccv2.COLOR_HSV2RGB

Dcv2.COLOR_GRAY2HSV

Attempts:

3 left

💡 Hint

Common Mistakes

Using cv2.COLOR_BGR2HSV when the image is RGB

Using a conversion code that converts HSV back to RGB

To convert from RGB to HSV, use cv2.COLOR_RGB2HSV. This changes the color space to HSV.

3fill in blank

hard

Fix the error in the code to convert a grayscale image back to BGR color space.

Computer Vision

bgr_image = cv2.cvtColor(gray_image, [1])

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

Acv2.COLOR_RGB2GRAY

Bcv2.COLOR_BGR2GRAY

Ccv2.COLOR_GRAY2BGR

Dcv2.COLOR_BGR2RGB

Attempts:

3 left

💡 Hint

Common Mistakes

Using cv2.COLOR_BGR2GRAY which converts color to grayscale

Using color order conversions instead of grayscale to BGR

To convert a grayscale image to BGR, use cv2.COLOR_GRAY2BGR. This adds color channels.

4fill in blank

hard

Fill both blanks to create a dictionary that maps color space names to their OpenCV conversion codes.

Computer Vision

color_conversions = {
    'RGB_to_HSV': [1],
    'BGR_to_Gray': [2]
}

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

Acv2.COLOR_RGB2HSV

Bcv2.COLOR_BGR2GRAY

Ccv2.COLOR_HSV2RGB

Dcv2.COLOR_GRAY2BGR

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the conversion codes for the keys

Using HSV to RGB instead of RGB to HSV

The dictionary maps 'RGB_to_HSV' to cv2.COLOR_RGB2HSV and 'BGR_to_Gray' to cv2.COLOR_BGR2GRAY.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each color space name to its conversion code if the code converts to grayscale.

Computer Vision

gray_conversions = {name: code for name, code in color_conversions.items() if code [1] cv2.COLOR_BGR2GRAY or code [2] cv2.COLOR_RGB2GRAY or code [3] cv2.COLOR_HSV2GRAY}

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

A==

B!=

Cis

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using != or > instead of ==

Using 'is' which is not reliable for value comparison

The dictionary comprehension filters conversion codes that are exactly equal to grayscale conversion codes using ==.

Practice

1. Which color space is commonly used by OpenCV as the default when reading images?

easy

A. HSV

B. BGR

C. Grayscale

D. RGB

Solution

1. Step 1: Understand OpenCV image reading default

   OpenCV reads images using the BGR color space by default, not RGB.

2. Step 2: Compare common color spaces

   RGB is common in many libraries, but OpenCV specifically uses BGR order for color images.

3. Final Answer:

   BGR -> Option B

4. Quick Check:

   OpenCV default color space = BGR [OK]

Hint: Remember OpenCV uses BGR, not RGB by default [OK]

Common Mistakes:

• Confusing RGB with BGR as default
• Thinking grayscale is default for color images
• Assuming HSV is default color space

2. Which of the following is the correct OpenCV Python code to convert an image from BGR to grayscale?

easy

A. gray = cv2.cvtColor(image, cv2.COLOR_RGB2GRAY)

B. gray = cv2.cvtColor(image, cv2.COLOR_GRAY2BGR)

C. gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)

D. gray = cv2.cvtColor(image, cv2.COLOR_HSV2GRAY)

Solution

1. Step 1: Identify correct color conversion code

   To convert from BGR to grayscale, use cv2.COLOR_BGR2GRAY in cv2.cvtColor.

2. Step 2: Check other options for correctness

   Options A, C, and D use wrong conversions or directions.

3. Final Answer:

   gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY) -> Option C

4. Quick Check:

   BGR to grayscale uses cv2.COLOR_BGR2GRAY [OK]

Hint: Use cv2.COLOR_BGR2GRAY to convert BGR to grayscale [OK]

Common Mistakes:

• Using RGB instead of BGR conversion code
• Trying to convert grayscale to BGR instead
• Using HSV conversion code incorrectly

3. What will be the shape of the output image after converting a color image of shape (480, 640, 3) from BGR to grayscale using OpenCV?

medium

A. (480, 640)

B. (640, 480)

C. (480, 640, 3)

D. (480, 640, 1)

Solution

1. Step 1: Understand input image shape

   The input image has shape (480, 640, 3), meaning height=480, width=640, and 3 color channels.

2. Step 2: Effect of BGR to grayscale conversion

   Converting to grayscale removes color channels, resulting in a 2D array with shape (480, 640).

3. Final Answer:

   (480, 640) -> Option A

4. Quick Check:

   Grayscale image shape = (height, width) [OK]

Hint: Grayscale images have 2D shape, no color channels [OK]

Common Mistakes:

• Assuming grayscale keeps 3 channels
• Swapping height and width in shape
• Expecting a single channel dimension like (480,640,1)

4. You wrote this code to convert an image from BGR to HSV but got incorrect results:

hsv = cv2.cvtColor(image, cv2.COLOR_RGB2HSV)

What is the likely cause of the incorrect results?

medium

A. cv2.cvtColor cannot convert to HSV color space

B. cv2.COLOR_RGB2HSV is not a valid conversion code

C. The image must be grayscale before converting to HSV

D. The image is in BGR, but conversion expects RGB input

Solution

1. Step 1: Check image color space and conversion code

   The image is in BGR format by default, but the code uses COLOR_RGB2HSV which expects RGB input.

2. Step 2: Identify mismatch causing incorrect results

   Using COLOR_RGB2HSV on a BGR image causes incorrect conversion; correct code is COLOR_BGR2HSV.

3. Final Answer:

   The image is in BGR, but conversion expects RGB input -> Option D

4. Quick Check:

   Use matching color space codes for input image [OK]

Hint: Match input image color space with conversion code [OK]

Common Mistakes:

• Using RGB conversion code on BGR images
• Thinking grayscale is needed before HSV
• Believing cv2.cvtColor can't convert to HSV

5. You want to detect red objects in an image using HSV color space. Which step is essential before applying a color range mask for red detection?

hard

A. Convert the image from BGR to HSV using cv2.COLOR_BGR2HSV

B. Convert the image from grayscale to HSV

C. Convert the image from RGB to grayscale

D. Apply a Gaussian blur before converting to BGR

Solution

1. Step 1: Understand color detection in HSV

   HSV color space separates color information, making it easier to detect specific colors like red.

2. Step 2: Convert image to HSV from correct input space

   Since OpenCV images are BGR by default, convert from BGR to HSV using cv2.COLOR_BGR2HSV before masking.

3. Final Answer:

   Convert the image from BGR to HSV using cv2.COLOR_BGR2HSV -> Option A

4. Quick Check:

   Convert BGR to HSV before color masking [OK]

Hint: Always convert BGR to HSV before color range masking [OK]

Common Mistakes:

• Skipping conversion or using wrong color space
• Trying to convert grayscale to HSV
• Applying blur before correct color conversion