Reading images (cv2.imread) in Computer Vision - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read an image from a file using OpenCV.

Computer Vision

import cv2
image = cv2.[1]('photo.jpg')

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

Aimshow

Bimread

Cimwrite

Dresize

Attempts:

3 left

💡 Hint

Common Mistakes

Using cv2.imshow instead of cv2.imread to read images.

Using cv2.imwrite which is for saving images.

The cv2.imread function reads an image from a file and loads it into memory.

2fill in blank

medium

Complete the code to read an image in grayscale mode.

Computer Vision

import cv2
image = cv2.imread('photo.jpg', [1])

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

Acv2.IMREAD_UNCHANGED

Bcv2.IMREAD_COLOR

Ccv2.IMREAD_ANYCOLOR

Dcv2.IMREAD_GRAYSCALE

Attempts:

3 left

💡 Hint

Common Mistakes

Using cv2.IMREAD_COLOR which loads a color image.

Using cv2.IMREAD_UNCHANGED which keeps alpha channel.

Using cv2.IMREAD_GRAYSCALE loads the image as a grayscale (black and white) image.

3fill in blank

hard

Fix the error in the code to correctly read an image file.

Computer Vision

import cv2
img = cv2.imread[1]

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A[0]

B('image.png', 1)

C('image.png', cv2.IMREAD_COLOR)

D('image.png')

Attempts:

3 left

💡 Hint

Common Mistakes

Missing parentheses causing syntax errors.

Passing arguments outside parentheses.

The cv2.imread function requires parentheses around the filename and optional flag. Using cv2.imread('image.png', cv2.IMREAD_COLOR) correctly reads the image in color mode.

4fill in blank

hard

Fill both blanks to read an image and check if it was loaded successfully.

Computer Vision

import cv2
image = cv2.imread('test.jpg', [1])
if image is [2]:
    print('Failed to load image')

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Acv2.IMREAD_COLOR

BNone

Ccv2.IMREAD_GRAYSCALE

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking if the image is None before using it.

Using wrong flag values for reading.

We read the image in color mode with cv2.IMREAD_COLOR. If image is None, it means loading failed.

5fill in blank

hard

Fill all three blanks to read an image, convert it to grayscale, and display it.

Computer Vision

import cv2
image = cv2.imread('pic.jpg', [1])
gray = cv2.[2](image, [3])
cv2.imshow('Gray Image', gray)
cv2.waitKey(0)
cv2.destroyAllWindows()

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

Acv2.IMREAD_COLOR

BcvtColor

Ccv2.COLOR_BGR2GRAY

Dcv2.IMREAD_GRAYSCALE

Attempts:

3 left

💡 Hint

Common Mistakes

Reading image directly in grayscale and then converting again.

Using wrong conversion flags.

First, read the image in color mode. Then convert it to grayscale using cv2.cvtColor with the flag cv2.COLOR_BGR2GRAY. Finally, display the grayscale image.

Practice

1. What does the function cv2.imread() do in computer vision?

easy

A. Loads an image from a file into a format that can be processed

B. Saves an image to a file

C. Displays an image on the screen

D. Converts an image to grayscale

Solution

1. Step 1: Understand the purpose of cv2.imread()

   The function cv2.imread() is designed to read image files and load them into memory as arrays for further processing.

2. Step 2: Differentiate from other functions

   Functions like cv2.imwrite() save images, and cv2.imshow() display images, so they are not the correct answer.

3. Final Answer:

   Loads an image from a file into a format that can be processed -> Option A

4. Quick Check:

   cv2.imread() loads images [OK]

Hint: Remember: imread means 'image read' from file [OK]

Common Mistakes:

• Confusing imread with imwrite (saving images)
• Thinking imread displays images
• Assuming imread converts image color

2. Which of the following is the correct way to read an image in grayscale using OpenCV?

easy

A. cv2.imread('image.jpg', cv2.IMREAD_GRAYSCALE)

B. cv2.imread('image.jpg', cv2.IMREAD_COLOR)

C. cv2.imread('image.jpg', cv2.IMREAD_UNCHANGED)

D. cv2.imread('image.jpg')

Solution

1. Step 1: Identify the flag for grayscale reading

   OpenCV uses cv2.IMREAD_GRAYSCALE to load images in grayscale mode.

2. Step 2: Compare with other flags

   cv2.IMREAD_COLOR loads color images, cv2.IMREAD_UNCHANGED loads with alpha channel, and no flag defaults to color.

3. Final Answer:

   cv2.imread('image.jpg', cv2.IMREAD_GRAYSCALE) -> Option A

4. Quick Check:

   Grayscale flag = cv2.IMREAD_GRAYSCALE [OK]

Hint: Use cv2.IMREAD_GRAYSCALE flag to read grayscale images [OK]

Common Mistakes:

• Using default flag which loads color image
• Confusing IMREAD_COLOR with grayscale
• Using IMREAD_UNCHANGED when grayscale is needed

3. What will be the output type of the variable img after running this code?

import cv2
img = cv2.imread('sample.png')

medium

A. None, because cv2.imread does not return anything

B. A string containing the image file path

C. A list of image file names

D. A NumPy array representing the image pixels

Solution

1. Step 1: Understand cv2.imread return type

   cv2.imread loads the image and returns it as a NumPy array containing pixel values.

2. Step 2: Eliminate other options

   It does not return a string, list, or None. It always returns an array or None if loading fails.

3. Final Answer:

   A NumPy array representing the image pixels -> Option D

4. Quick Check:

   cv2.imread returns NumPy array [OK]

Hint: cv2.imread returns image as NumPy array [OK]

Common Mistakes:

• Thinking it returns file path string
• Assuming it returns None always
• Confusing with list of files

4. Consider this code snippet:

import cv2
img = cv2.imread('nonexistent.jpg')
print(img.shape)

What error will occur and why?

medium

A. FileNotFoundError because the file does not exist

B. AttributeError because img is None and None has no attribute 'shape'

C. SyntaxError due to wrong function usage

D. No error, prints image shape

Solution

1. Step 1: Understand cv2.imread behavior on missing files

   If the file does not exist, cv2.imread returns None instead of an image array.

2. Step 2: Analyze the print statement

   Trying to access img.shape when img is None causes an AttributeError because NoneType has no attribute 'shape'.

3. Final Answer:

   AttributeError because img is None and None has no attribute 'shape' -> Option B

4. Quick Check:

   Missing file -> img=None -> AttributeError on .shape [OK]

Hint: Check if img is None before accessing attributes [OK]

Common Mistakes:

• Expecting FileNotFoundError instead of None return
• Ignoring None check before using img
• Assuming no error occurs

5. You want to load an image with transparency (alpha channel) preserved using OpenCV. Which code snippet correctly does this?

hard

A. img = cv2.imread('image.png', cv2.IMREAD_COLOR)

B. img = cv2.imread('image.png', cv2.IMREAD_GRAYSCALE)

C. img = cv2.imread('image.png', cv2.IMREAD_UNCHANGED)

D. img = cv2.imread('image.png')

Solution

1. Step 1: Identify flag for loading alpha channel

   cv2.IMREAD_UNCHANGED loads the image as-is, including the alpha transparency channel if present.

2. Step 2: Compare with other flags

   IMREAD_COLOR ignores alpha, IMREAD_GRAYSCALE loads single channel, and default is color without alpha.

3. Final Answer:

   img = cv2.imread('image.png', cv2.IMREAD_UNCHANGED) -> Option C

4. Quick Check:

   Use IMREAD_UNCHANGED to keep transparency [OK]

Hint: Use cv2.IMREAD_UNCHANGED to keep alpha channel [OK]

Common Mistakes:

• Using IMREAD_COLOR which drops alpha channel
• Assuming default read keeps transparency
• Using grayscale flag by mistake