Writing/saving images in Computer Vision - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to write or save an image in computer vision?

Writing or saving an image means storing the image data from memory to a file on disk, so it can be viewed or used later.

beginner

Name a common file format used to save images.

JPEG, PNG, BMP, and TIFF are common image file formats used to save images.

beginner

In Python, which library is often used to write images to disk?

OpenCV (cv2) is often used in Python to write images to disk using the function cv2.imwrite().

beginner

What is the basic syntax of cv2.imwrite() to save an image?

cv2.imwrite(filename, image) saves the image to the file named filename.

intermediate

Why is it important to check the return value of cv2.imwrite()?

Because cv2.imwrite() returns True if the image was saved successfully and False if it failed, checking helps confirm the save worked.

Which function in OpenCV saves an image to disk?

A cv2.imshow()

B cv2.imread()

C cv2.imwrite()

D cv2.resize()

cv2.imwrite() is used to write or save images to disk.

What does cv2.imwrite() return if saving the image fails?

A False

B True

C None

D -1

cv2.imwrite() returns False if it fails to save the image.

Which image format supports transparency when saving images?

A PNG

B TIFF

C BMP

D JPEG

PNG supports transparency (alpha channel), unlike JPEG.

What is the minimum information needed to save an image using cv2.imwrite()?

A Filename only

B Filename and image

C Image only

D Image and format

cv2.imwrite() requires both the filename and the image data.

If you want to save an image as 'output.png', which is the correct call?

A cv2.imwrite('output', image.png)

B cv2.imwrite(image, 'output.png')

C cv2.imwrite('image', output.png)

D cv2.imwrite('output.png', image)

The first argument is the filename string, the second is the image data.

Explain how to save an image using OpenCV in Python.

Think about the function name and what inputs it needs.

List common image file formats and mention one key feature of each.

Consider which formats are good for photos, transparency, or quality.

Practice

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

easy

A. Converts an image to grayscale

B. Reads an image from a file

C. Displays an image in a window

D. Saves an image to a file on disk

Solution

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

   This function is used to save image data to a file on your computer.

2. Step 2: Differentiate from other OpenCV functions

   Functions like cv2.imread() read images, and cv2.imshow() display images, but cv2.imwrite() specifically saves images.

3. Final Answer:

   Saves an image to a file on disk -> Option D

4. Quick Check:

   cv2.imwrite() = Save image [OK]

Hint: Remember 'imwrite' means 'image write' to file [OK]

Common Mistakes:

• Confusing imwrite with imread
• Thinking it displays images
• Assuming it converts image formats automatically

2. Which of the following is the correct syntax to save an image stored in variable img to a file named output.jpg using OpenCV?

easy

A. cv2.save('output.jpg', img)

B. cv2.imwrite('output.jpg', img)

C. cv2.write('output.jpg', img)

D. cv2.saveImage('output.jpg', img)

Solution

1. Step 1: Recall the correct OpenCV function name

   The function to save images is cv2.imwrite(), not save or write.

2. Step 2: Check the argument order

   The first argument is the filename as a string, the second is the image variable.

3. Final Answer:

   cv2.imwrite('output.jpg', img) -> Option B

4. Quick Check:

   Correct function and argument order = cv2.imwrite('output.jpg', img) [OK]

Hint: Use 'imwrite' with filename first, then image [OK]

Common Mistakes:

• Using non-existent functions like cv2.save
• Swapping argument order
• Missing quotes around filename

3. What will be the output of the following code snippet?

import cv2
img = cv2.imread('input.png')
success = cv2.imwrite('saved.png', img)
print(success)

medium

A. True if image saved successfully, False otherwise

B. The saved image data

C. The filename 'saved.png'

D. An error message

Solution

1. Step 1: Understand cv2.imwrite() return value

   This function returns a boolean: True if saving worked, False if it failed.

2. Step 2: Analyze the print statement

   The code prints the boolean stored in success, so output is True or False.

3. Final Answer:

   True if image saved successfully, False otherwise -> Option A

4. Quick Check:

   imwrite() returns success boolean [OK]

Hint: imwrite returns True/False for success [OK]

Common Mistakes:

• Expecting image data as output
• Thinking it prints filename
• Assuming it throws error on failure

4. Identify the error in this code snippet that tries to save an image:

import cv2
img = cv2.imread('photo.jpg')
cv2.imwrite(img, 'output.jpg')

medium

A. cv2.imread cannot read jpg files

B. cv2.imwrite requires file extension in filename

C. Arguments to cv2.imwrite are in wrong order

D. cv2.imwrite cannot save images

Solution

1. Step 1: Check cv2.imwrite() argument order

   The first argument must be the filename string, second the image data.

2. Step 2: Identify the mistake in the code

   The code passes img first and filename second, which is incorrect.

3. Final Answer:

   Arguments to cv2.imwrite are in wrong order -> Option C

4. Quick Check:

   Filename first, image second in imwrite() [OK]

Hint: Filename must be first argument in imwrite() [OK]

Common Mistakes:

• Swapping filename and image arguments
• Assuming imread can't read jpg
• Thinking imwrite can't save images

5. You want to save a grayscale image stored in gray_img as a PNG file and ensure the save was successful. Which code snippet correctly does this?

hard

A. if cv2.imwrite('gray.png', gray_img): print('Saved successfully') else: print('Save failed')

B. cv2.imwrite(gray_img, 'gray.png') and print('Saved successfully')

C. cv2.imwrite('gray.png', gray_img)
print('Saved successfully')

D. if cv2.imwrite('gray.png'): print('Saved successfully')

Solution

1. Step 1: Use cv2.imwrite() with correct arguments

   The first argument is the filename string, second is the image data variable.

2. Step 2: Check the return value to confirm success

   Use an if statement to check if cv2.imwrite() returns True, then print success message; else print failure.

3. Final Answer:

   if cv2.imwrite('gray.png', gray_img): print('Saved successfully') else: print('Save failed') -> Option A

4. Quick Check:

   Check imwrite() return before confirming save [OK]

Hint: Check imwrite() return value to confirm save success [OK]

Common Mistakes:

• Swapping arguments in imwrite
• Not checking if save succeeded
• Passing wrong argument types