Reading video with OpenCV in Computer Vision - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What does this OpenCV video reading code output?

Consider this Python code that reads a video file frame by frame using OpenCV. What will be the printed output after running this code snippet?

Computer Vision

import cv2
cap = cv2.VideoCapture('video.mp4')
frame_count = 0
while True:
    ret, frame = cap.read()
    if not ret:
        break
    frame_count += 1
print(frame_count)
cap.release()

A The first frame's pixel values printed as an array

B 0

C The total number of frames in the video

D Raises an error because 'cap.release()' is missing

💡 Hint

Think about what 'cap.read()' returns and when the loop stops.

The code reads each frame until no more frames are available. It counts how many frames were read and prints that number. 'cap.release()' is correctly called to free resources.

❓ Model Choice

intermediate

Which OpenCV method correctly opens a video file for reading?

You want to read a video file named 'clip.avi' frame by frame. Which OpenCV method call correctly opens the video for reading?

A cv2.VideoCapture('clip.avi')

B cv2.VideoOpen('clip.avi')

C cv2.readVideo('clip.avi')

D cv2.VideoReader('clip.avi')

💡 Hint

Check the OpenCV documentation for video capture classes.

cv2.VideoCapture is the correct class to open video files for frame-by-frame reading.

❓ Hyperparameter

advanced

What does setting CAP_PROP_POS_FRAMES do in OpenCV?

In OpenCV, you can set a property of a video capture object using cap.set(cv2.CAP_PROP_POS_FRAMES, frame_number). What effect does this have?

A It moves the video reading position to the specified frame number

B It changes the frame rate of the video playback

C It sets the brightness of the video frames

D It resets the video capture object to the first frame

💡 Hint

Think about how you would jump to a specific frame in a video.

CAP_PROP_POS_FRAMES sets the index of the next frame to be read, allowing random access to frames.

❓ Metrics

advanced

How to calculate video duration using OpenCV properties?

You have opened a video with OpenCV. Which formula correctly calculates the video duration in seconds using OpenCV properties?

A duration = frames_per_second / total_frames

B duration = total_frames / frames_per_second

C duration = total_frames * frames_per_second

D duration = frames_per_second - total_frames

💡 Hint

Duration is how many seconds the video lasts, so think about frames and frame rate.

Duration is total frames divided by frames per second (fps).

🔧 Debug

expert

Why does this OpenCV video reading loop freeze?

This code snippet freezes and never ends when reading a video. What is the most likely cause? import cv2 cap = cv2.VideoCapture('video.mp4') while True: ret, frame = cap.read() if ret: cv2.imshow('Frame', frame) if cv2.waitKey(25) & 0xFF == ord('q'): break cap.release() cv2.destroyAllWindows()

A cap.release() is called before the loop

B cv2.imshow is called without a window name

C cv2.waitKey is missing a delay argument

D The loop never breaks because it does not check for ret == False

💡 Hint

What happens if the video ends and ret is False?

The loop only breaks if 'q' is pressed but does not break when ret is False (no more frames), causing an infinite loop.

Practice

1. What does the cv2.VideoCapture function do in OpenCV when reading a video?

easy

A. It opens the video file or camera stream for reading frames.

B. It displays the video frames on the screen.

C. It saves the video to a new file.

D. It converts video frames to grayscale automatically.

Solution

1. Step 1: Understand the purpose of cv2.VideoCapture

   This function is used to open a video file or a camera stream so that frames can be read one by one.

2. Step 2: Differentiate from other functions

   Displaying frames or saving videos are done by other functions like cv2.imshow or cv2.VideoWriter. Conversion to grayscale is a separate processing step.

3. Final Answer:

   It opens the video file or camera stream for reading frames. -> Option A

4. Quick Check:

   cv2.VideoCapture opens video source [OK]

Hint: Remember: VideoCapture opens video, does not display or save [OK]

Common Mistakes:

• Confusing opening video with displaying frames
• Thinking it saves or processes frames automatically
• Assuming it converts frames to grayscale

2. Which of the following is the correct way to read a frame from a video capture object cap in OpenCV?

easy

A. cap.read() -> frame, ret

B. frame, ret = cap.read()

C. cap.read(frame, ret)

D. ret, frame = cap.read()

Solution

1. Step 1: Recall the return order of cap.read()

   The method returns two values: a boolean ret indicating success, and the frame image.

2. Step 2: Match the correct variable order

   The correct order is ret, frame = cap.read(). Assigning in reverse order causes errors or wrong values.

3. Final Answer:

   ret, frame = cap.read() -> Option D

4. Quick Check:

   ret, frame = cap.read() [OK]

Hint: Remember: ret (bool) comes first, then frame image [OK]

Common Mistakes:

• Swapping ret and frame variables
• Trying to pass variables as arguments to read()
• Using incorrect assignment syntax

3. What will be the output of the following code snippet if the video file path is incorrect or missing?

import cv2
cap = cv2.VideoCapture('wrong_path.mp4')
ret, frame = cap.read()
print(ret)

medium

A. True

B. None

C. False

D. Raises an exception

Solution

1. Step 1: Understand behavior when video path is invalid

   If the video file cannot be opened, cap.read() returns ret = False and frame = None.

2. Step 2: Check the printed output

   The code prints the value of ret, which will be False indicating failure to read a frame.

3. Final Answer:

   False -> Option C

4. Quick Check:

   Invalid path makes ret False [OK]

Hint: ret is False if frame read fails (e.g., bad path) [OK]

Common Mistakes:

• Expecting True even if file missing
• Assuming exception is raised automatically
• Confusing None with False for ret

4. Identify the error in this code snippet for reading and displaying video frames:

import cv2
cap = cv2.VideoCapture('video.mp4')
while True:
    ret, frame = cap.read()
    if ret:
        cv2.imshow('Video', frame)
    else:
        break
cv2.destroyAllWindows()

medium

A. Missing cap.release() to free video resource

B. Incorrect use of cv2.imshow function

C. Loop should use while ret: instead of while True:

D. No error, code runs fine

Solution

1. Step 1: Check resource management in video reading

   After finishing reading frames, the video capture object should be released with cap.release() to free resources.

2. Step 2: Verify other parts of the code

   The cv2.imshow usage and loop logic are correct. The missing cap.release() is the main issue.

3. Final Answer:

   Missing cap.release() to free video resource -> Option A

4. Quick Check:

   Always call cap.release() after reading video [OK]

Hint: Always release video capture after use [OK]

Common Mistakes:

• Forgetting cap.release() causes resource leaks
• Thinking cv2.destroyAllWindows() frees video capture
• Changing loop condition unnecessarily

5. You want to read a video file and save only every 10th frame as an image using OpenCV. Which code snippet correctly implements this?

hard

A. import cv2
cap = cv2.VideoCapture('video.mp4')
count = 0
while True:
ret, frame = cap.read()
if not ret:
break
count += 1
if count % 10 == 0:
cv2.imwrite(f'frame_{count}.jpg', frame)
cap.release()

B. import cv2
cap = cv2.VideoCapture('video.mp4')
count = 0
while True:
ret, frame = cap.read()
if ret:
if count % 10 == 0:
cv2.imwrite(f'frame_{count}.jpg', frame)
count += 1
else:
break
cap.release()

C. import cv2
cap = cv2.VideoCapture('video.mp4')
count = 0
while cap.isOpened():
ret, frame = cap.read()
if ret:
if count % 10 == 0:
cv2.imwrite('frame_{count}.jpg', frame)
count += 1
else:
break
cap.release()

D. import cv2
cap = cv2.VideoCapture('video.mp4')
count = 1
while True:
ret, frame = cap.read()
if not ret:
break
if count % 10 == 0:
cv2.imwrite(f'frame_{count}.jpg', frame)
count += 1
cap.release()

Solution

1. Step 1: Understand frame reading loop and counting

   We need a loop that reads frames until no more frames are available, counting frames starting from zero.

2. Step 2: Check saving every 10th frame with correct filename formatting

   Using f-strings like f'frame_{count}.jpg' correctly formats the filename. The count increments only after reading a frame.

3. Step 3: Compare options for correctness

   import cv2 cap = cv2.VideoCapture('video.mp4') count = 0 while True: ret, frame = cap.read() if not ret: break count += 1 if count % 10 == 0: cv2.imwrite(f'frame_{count}.jpg', frame) cap.release() increments count before modulo check, causing off-by-one (saves frame 9 as frame_10.jpg). import cv2 cap = cv2.VideoCapture('video.mp4') count = 0 while cap.isOpened(): ret, frame = cap.read() if ret: if count % 10 == 0: cv2.imwrite('frame_{count}.jpg', frame) count += 1 else: break cap.release() uses wrong string formatting in cv2.imwrite. import cv2 cap = cv2.VideoCapture('video.mp4') count = 1 while True: ret, frame = cap.read() if not ret: break if count % 10 == 0: cv2.imwrite(f'frame_{count}.jpg', frame) count += 1 cap.release() starts count at 1, causing off-by-one error (saves frame 9 as frame_10.jpg). import cv2 cap = cv2.VideoCapture('video.mp4') count = 0 while True: ret, frame = cap.read() if ret: if count % 10 == 0: cv2.imwrite(f'frame_{count}.jpg', frame) count += 1 else: break cap.release() correctly handles all requirements.

4. Final Answer:

   while True with if ret: (modulo check, save, increment) else: break, using f-string -> Option B

5. Quick Check:

   Use f-strings and check ret before saving frames [OK]

Hint: Use f-strings and check ret before saving frames [OK]

Common Mistakes:

• Incrementing count before modulo check (off-by-one error)
• Using wrong string formatting in filename
• Starting count at 1 without adjusting logic (off-by-one)