Video writing in Computer Vision - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a video writer with the correct fourcc codec.

Computer Vision

import cv2
fourcc = cv2.VideoWriter_fourcc(*'[1]')
out = cv2.VideoWriter('output.avi', fourcc, 20.0, (640,480))

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

AH264

BMP4V

CMJPG

DXVID

Attempts:

3 left

💡 Hint

Common Mistakes

Using an unsupported codec string causes the video writer to fail.

Forgetting to unpack the string with * in VideoWriter_fourcc.

The 'XVID' codec is commonly used for AVI video writing in OpenCV.

2fill in blank

medium

Complete the code to write a frame to the video file.

Computer Vision

ret, frame = cap.read()
if ret:
    [1].write(frame)

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

Acap

Bout

Cframe

Dcv2

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to write frames using the capture object 'cap'.

Calling write on the frame itself instead of the writer.

The VideoWriter object 'out' is used to write frames to the video file.

3fill in blank

hard

Fix the error in the code to release the video writer properly.

Computer Vision

out = cv2.VideoWriter('output.avi', fourcc, 20.0, (640,480))
# ... writing frames ...
[1].release()

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

Acv2

Bframe

Cout

Dcap

Attempts:

3 left

💡 Hint

Common Mistakes

Calling release on the capture object instead of the writer.

Forgetting to release the writer causing file corruption.

The release() method must be called on the VideoWriter object 'out' to properly close the file.

4fill in blank

hard

Fill both blanks to create a VideoWriter with MJPG codec and 30 FPS.

Computer Vision

fourcc = cv2.VideoWriter_fourcc(*'[1]')
out = cv2.VideoWriter('output.avi', fourcc, [2], (640,480))

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

AMJPG

BXVID

C30.0

D20.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong codec string causing unsupported format.

Setting FPS to a wrong value causing playback issues.

MJPG is a common codec and 30.0 sets the frames per second to 30.

5fill in blank

hard

Fill all three blanks to write frames only if the frame is read successfully and convert it to grayscale before writing.

Computer Vision

ret, frame = cap.read()
if [1]:
    gray = cv2.cvtColor(frame, [2])
    [3].write(gray)

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

Aret

Bcv2.COLOR_BGR2GRAY

Cout

Dframe

Attempts:

3 left

💡 Hint

Common Mistakes

Writing frames without checking if read was successful.

Using wrong color conversion code causing errors.

Writing to the wrong object instead of the VideoWriter.

Check if ret is True, convert frame to grayscale using cv2.COLOR_BGR2GRAY, then write using 'out'.

Practice

1. What is the main purpose of using cv2.VideoWriter in computer vision?

easy

A. To save a series of images as a video file

B. To read video files frame by frame

C. To apply filters on images

D. To display images on screen

Solution

1. Step 1: Understand the function of cv2.VideoWriter

   cv2.VideoWriter is designed to save multiple images as a continuous video file.

2. Step 2: Differentiate from other OpenCV functions

   Functions like cv2.VideoCapture read videos, while VideoWriter writes videos.

3. Final Answer:

   To save a series of images as a video file -> Option A

4. Quick Check:

   Video writing = save images as video [OK]

Hint: VideoWriter saves frames into video files [OK]

Common Mistakes:

• Confusing VideoWriter with VideoCapture
• Thinking VideoWriter displays images
• Assuming VideoWriter applies image filters

2. Which of the following is the correct way to create a cv2.VideoWriter object for a 640x480 video at 30 fps using the 'XVID' codec?

easy

A. cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc(*'XVID'), 30, (640, 480))

B. cv2.VideoWriter('output.avi', 'XVID', 30, (640, 480))

C. cv2.VideoWriter('output.avi', cv2.VideoCapture_fourcc('XVID'), 30, (640, 480))

D. cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc('X', 'V', 'I', 'D'), 30, 640, 480)

Solution

1. Step 1: Use cv2.VideoWriter_fourcc correctly

   The codec must be passed as cv2.VideoWriter_fourcc(*'XVID') to get the fourcc code.

2. Step 2: Check the parameters order and types

   Parameters are filename, fourcc, fps, and frame size as a tuple (width, height).

3. Final Answer:

   cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc(*'XVID'), 30, (640, 480)) -> Option A

4. Quick Check:

   Correct codec and frame size syntax = cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc(*'XVID'), 30, (640, 480)) [OK]

Hint: Use cv2.VideoWriter_fourcc(*'XVID') for codec [OK]

Common Mistakes:

• Passing codec as string directly
• Using cv2.VideoCapture_fourcc instead of VideoWriter_fourcc
• Incorrect frame size format (not a tuple)

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

import cv2
fourcc = cv2.VideoWriter_fourcc(*'MJPG')
out = cv2.VideoWriter('test.avi', fourcc, 20.0, (320,240))
frame = 255 * np.ones((240,320,3), dtype=np.uint8)
out.write(frame)
out.release()
print('Video saved')

medium

A. Nothing happens, no file is created

B. A video file named 'test.avi' with a white frame is saved and 'Video saved' is printed

C. RuntimeError because frame size does not match

D. NameError due to missing import of numpy

Solution

1. Step 1: Check imports

   The code uses np.ones but numpy (np) is not imported, causing a NameError.

2. Step 2: Understand error impact

   Without importing numpy, the code will fail before writing or saving video.

3. Final Answer:

   NameError due to missing import of numpy -> Option D

4. Quick Check:

   Missing numpy import = NameError [OK]

Hint: Always import numpy when using np [OK]

Common Mistakes:

• Assuming code runs without numpy import
• Ignoring frame size correctness
• Thinking print means success

4. Identify the error in this video writing code and select the fix:

import cv2
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi', fourcc, 25, (640, 480))
frame = cv2.imread('image.jpg')
out.write(frame)
out.release()

medium

A. Fix: Change codec to 'MJPG'

B. Fix: Resize frame to (640, 480) before writing

C. Fix: Use grayscale image instead of color

D. Fix: Increase fps to 30

Solution

1. Step 1: Check frame size vs VideoWriter size

   VideoWriter expects frames of size (640, 480). If frame size differs, write() fails or produces errors.

2. Step 2: Correct frame size by resizing

   Resize the loaded image to (640, 480) before writing to match VideoWriter settings.

3. Final Answer:

   Fix: Resize frame to (640, 480) before writing -> Option B

4. Quick Check:

   Frame size must match VideoWriter size [OK]

Hint: Match frame size to VideoWriter size [OK]

Common Mistakes:

• Ignoring frame size mismatch
• Changing codec unnecessarily
• Changing fps unrelated to error

5. You want to create a video from a list of images with different sizes. Which approach correctly handles this situation?

hard

A. Use VideoWriter without resizing; it auto-adjusts frame size

B. Write images directly without VideoWriter; just save as separate files

C. Resize all images to the same size before writing to VideoWriter

D. Change codec to 'X264' to support variable frame sizes

Solution

1. Step 1: Understand VideoWriter frame size requirement

   VideoWriter requires all frames to have the same size as specified at creation.

2. Step 2: Handle varying image sizes

   Resize all images to a consistent size before writing to avoid errors and ensure smooth video.

3. Final Answer:

   Resize all images to the same size before writing to VideoWriter -> Option C

4. Quick Check:

   Consistent frame size needed for video writing [OK]

Hint: Resize images to uniform size before video writing [OK]

Common Mistakes:

• Assuming VideoWriter auto-resizes frames
• Trying to write images without VideoWriter
• Changing codec to fix size issues