Writing/saving images in Computer Vision - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to save an image using OpenCV.

Computer Vision

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

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

Aresize

Bimshow

Cimwrite

Dimread

Attempts:

3 left

💡 Hint

Common Mistakes

Using cv2.imshow instead of cv2.imwrite.

Trying to read the image again instead of saving.

The cv2.imwrite function saves an image to a file.

2fill in blank

medium

Complete the code to save a grayscale image using OpenCV.

Computer Vision

import cv2
image = cv2.imread('input.jpg', [1])
cv2.imwrite('gray_output.jpg', image)

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

Acv2.IMREAD_UNCHANGED

Bcv2.IMREAD_GRAYSCALE

Ccv2.IMREAD_COLOR

Dcv2.COLOR_BGR2GRAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using color conversion codes instead of read flags.

Using IMREAD_COLOR which reads a color image.

To read an image in grayscale, use cv2.IMREAD_GRAYSCALE as the flag.

3fill in blank

hard

Fix the error in saving an image with compression parameters.

Computer Vision

import cv2
image = cv2.imread('input.jpg')
cv2.imwrite('compressed.jpg', image, [1])

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

A[cv2.IMWRITE_JPEG_QUALITY, 90]

Bcv2.IMWRITE_JPEG_QUALITY=90

C{cv2.IMWRITE_JPEG_QUALITY: 90}

D(cv2.IMWRITE_JPEG_QUALITY, 90)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing parameters as a dictionary or tuple.

Using assignment syntax inside the function call.

The compression parameters must be passed as a list of pairs: [flag, value].

4fill in blank

hard

Fill both blanks to save a PNG image with compression level 3.

Computer Vision

import cv2
image = cv2.imread('input.png')
cv2.imwrite('output.png', image, [[1], [2]])

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

Acv2.IMWRITE_PNG_COMPRESSION

Bcv2.IMWRITE_JPEG_QUALITY

C3

D90

Attempts:

3 left

💡 Hint

Common Mistakes

Using JPEG flags for PNG images.

Passing quality value instead of compression level.

Use cv2.IMWRITE_PNG_COMPRESSION flag with compression level 3 for PNG images.

5fill in blank

hard

Fill all three blanks to save a JPEG image with quality 85 and grayscale read.

Computer Vision

import cv2
image = cv2.imread('input.jpg', [1])
cv2.imwrite('output.jpg', image, [[2], [3]])

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

Acv2.IMREAD_GRAYSCALE

Bcv2.IMWRITE_JPEG_QUALITY

C85

Dcv2.IMREAD_COLOR

Attempts:

3 left

💡 Hint

Common Mistakes

Using color read flag instead of grayscale.

Mixing PNG compression flags with JPEG quality.

Read the image in grayscale with cv2.IMREAD_GRAYSCALE, then save with JPEG quality 85 using cv2.IMWRITE_JPEG_QUALITY flag and value 85.

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