Reading images (cv2.imread) in Computer Vision - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the function cv2.imread() do?

It reads an image file from your computer and loads it into memory as an array of pixels that your program can use.

beginner

What are the common flags used with cv2.imread() and what do they mean?

• cv2.IMREAD_COLOR: Loads the image in color (default).
• cv2.IMREAD_GRAYSCALE: Loads the image in black and white (grayscale).
• cv2.IMREAD_UNCHANGED: Loads the image including the alpha channel (transparency).

beginner

What type of data does cv2.imread() return?

It returns a NumPy array representing the image pixels. Each pixel has values for color channels depending on the flag used.

beginner

What happens if cv2.imread() cannot find or open the image file?

It returns None. This means the image was not loaded and you should check the file path or file existence.

beginner

Why is it important to check the result of cv2.imread() before using the image?

Because if the image failed to load, your program might crash or behave unexpectedly. Checking ensures your code handles missing files safely.

What does cv2.imread('image.jpg', cv2.IMREAD_GRAYSCALE) do?

A Saves the image to disk

B Loads the image in black and white

C Loads the image with transparency

D Loads the image in color

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

What type of object does cv2.imread() return?

A A NumPy array of pixel values

B A string with the image path

C A Python list of colors

D An integer representing image size

cv2.imread() returns a NumPy array representing the image pixels.

If cv2.imread() returns None, what does it mean?

A The image was loaded successfully

B The image is empty but loaded

C The image is corrupted but loaded

D The image file was not found or could not be opened

Returning None means the image file was not found or could not be opened.

Which flag loads an image including its transparency channel?

A cv2.IMREAD_UNCHANGED

B cv2.IMREAD_COLOR

C cv2.IMREAD_GRAYSCALE

D cv2.IMREAD_ALPHA

cv2.IMREAD_UNCHANGED loads the image including the alpha (transparency) channel.

What should you do after calling cv2.imread() before processing the image?

A Convert the image to text

B Immediately display the image

C Check if the returned value is not <code>None</code>

D Save the image again

Always check if the image loaded successfully by verifying the return is not None.

Explain how cv2.imread() works and what you need to check after using it.

Think about loading an image and what can go wrong.

Describe the difference between cv2.IMREAD_COLOR and cv2.IMREAD_GRAYSCALE flags.

Consider how images look in color vs black and white.

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