Video writing in Computer Vision - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this OpenCV video writing code?

Consider the following Python code snippet that writes a video file using OpenCV. What will be the output after running this code?

Computer Vision

import cv2
import numpy as np

fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi', fourcc, 10.0, (640,480))

frame = np.zeros((480,640,3), dtype=np.uint8)
frame[:] = (0,255,0)  # green frame

for _ in range(5):
    out.write(frame)
out.release()

print('Video written with 5 green frames')

A RuntimeError because frame size does not match

B Video written with 10 green frames

C SyntaxError due to incorrect fourcc code

D Video written with 5 green frames

💡 Hint

Check how many times the frame is written in the loop.

The loop runs 5 times, writing 5 identical green frames to the video file. The fourcc code and frame size are correct, so no errors occur.

❓ Model Choice

intermediate

Which codec is best for writing a high-quality MP4 video using OpenCV?

You want to save a video file in MP4 format with good compression and quality using OpenCV's VideoWriter. Which fourcc codec should you choose?

A cv2.VideoWriter_fourcc(*'DIVX')

B cv2.VideoWriter_fourcc(*'XVID')

C cv2.VideoWriter_fourcc(*'MP4V')

D cv2.VideoWriter_fourcc(*'MJPG')

💡 Hint

MP4 format usually uses MPEG-4 codecs.

The 'MP4V' codec is commonly used for MP4 files and provides good compression and quality. 'XVID' and 'DIVX' are for AVI containers, and 'MJPG' is motion JPEG.

❓ Hyperparameter

advanced

What happens if the frame size parameter in VideoWriter does not match the actual frame size?

In OpenCV, when creating a VideoWriter object, you specify the frame size. What is the effect if this size does not match the size of frames you write?

A The video file is created but frames are resized automatically

B A runtime error occurs when writing frames with mismatched size

C The video file is created but frames are cropped to the specified size

D The VideoWriter silently ignores the size mismatch and writes frames anyway

💡 Hint

OpenCV expects exact frame size match for VideoWriter.

OpenCV raises a runtime error if the frame size does not match the size specified when creating the VideoWriter. It does not resize or crop frames automatically.

❓ Metrics

advanced

How to calculate the total duration of a video written with OpenCV?

You wrote a video with OpenCV's VideoWriter with frame rate 20 FPS and 300 frames. What is the total duration of the video in seconds?

A 15 seconds

B 20 seconds

C 300 seconds

D 6000 seconds

💡 Hint

Duration = number_of_frames / frames_per_second

Duration = 300 frames / 20 FPS = 15 seconds.

🔧 Debug

expert

Why does this OpenCV video writing code fail to save the video file?

Examine the code below. It runs without errors but the output video file is empty (0 bytes). What is the most likely cause?

Computer Vision

import cv2
import numpy as np

fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi', fourcc, 10.0, (640,480))

frame = np.zeros((480,640,3), dtype=np.uint8)
frame[:] = (255,0,0)  # blue frame

# Missing frame writing loop

out.release()

A No frames were written to the video before releasing

B The VideoWriter object was not released properly

C The fourcc codec 'XVID' is not supported on this system

D The frame size is incorrect and causes no frames to be written

💡 Hint

Check if any frames are actually written before release.

The code creates a VideoWriter and a frame but never calls out.write(frame), so no frames are saved. The file is created but empty.