Frame extraction in Computer Vision - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a video file using OpenCV.

Computer Vision

import cv2

cap = cv2.VideoCapture([1])

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

A'video.mp4'

B0

CTrue

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 0 opens the webcam, not a video file.

Passing True or None causes errors.

To open a video file, you pass the filename as a string to cv2.VideoCapture.

2fill in blank

medium

Complete the code to read a frame from the video capture object.

Computer Vision

ret, frame = cap.[1]()

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

Aread

Bcapture

Cgrab

Dfetch

Attempts:

3 left

💡 Hint

Common Mistakes

Using grab() only grabs the frame but does not decode it.

capture() and fetch() are not OpenCV methods.

The read() method reads the next frame from the video.

3fill in blank

hard

Fix the error in the loop that extracts frames until the video ends.

Computer Vision

while True:
    ret, frame = cap.read()
    if not [1]:
        break
    cv2.imshow('Frame', frame)
    if cv2.waitKey(25) & 0xFF == ord('q'):
        break

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

Aframe

Bret

Ccap

Dcv2

Attempts:

3 left

💡 Hint

Common Mistakes

Checking frame instead of ret can cause errors if frame is None.

Checking cap or cv2 is incorrect.

ret is a boolean indicating if the frame was read successfully; we break if it's False.

4fill in blank

hard

Fill both blanks to release the video and close all windows.

Computer Vision

cap.[1]()
cv2.[2]()

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

Arelease

BdestroyAllWindows

Cclose

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using close() or stop() are not valid OpenCV methods.

Not releasing the video can cause resource leaks.

release() frees the video resource; destroyAllWindows() closes all OpenCV windows.

5fill in blank

hard

Fill all three blanks to save the extracted frame as an image file.

Computer Vision

if ret:
    cv2.[1]('[2]', [3])

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

Aimwrite

Bframe

Coutput.jpg

Dimshow

Attempts:

3 left

💡 Hint

Common Mistakes

Using imshow instead of imwrite displays the image but does not save it.

Swapping filename and image arguments causes errors.

imwrite saves an image to a file; first argument is filename, second is image data.

Practice

1. What does frame extraction mean in computer vision?

easy

A. Compressing video files to smaller size

B. Combining multiple videos into one

C. Changing video colors to grayscale

D. Taking individual pictures from a video one by one

Solution

1. Step 1: Understand the term 'frame'

   A frame is a single image in a sequence that makes up a video.

2. Step 2: Define frame extraction

   Frame extraction means taking these single images out of the video one by one.

3. Final Answer:

   Taking individual pictures from a video one by one -> Option D

4. Quick Check:

   Frame extraction = Taking pictures from video [OK]

Hint: Frames are single images from video, extracted one by one [OK]

Common Mistakes:

• Confusing frame extraction with video compression
• Thinking frame extraction means editing video colors
• Mixing frame extraction with video merging

2. Which Python code snippet correctly opens a video file for frame extraction using OpenCV?

easy

A. cap = cv2.ReadVideo('video.mp4')

B. cap = cv2.VideoCapture('video.mp4')

C. cap = cv2.OpenVideo('video.mp4')

D. cap = cv2.VideoOpen('video.mp4')

Solution

1. Step 1: Recall OpenCV function for video capture

   The correct function to open a video file is cv2.VideoCapture().

2. Step 2: Check the code options

   Only cap = cv2.VideoCapture('video.mp4') uses cv2.VideoCapture with the video filename correctly.

3. Final Answer:

   cap = cv2.VideoCapture('video.mp4') -> Option B

4. Quick Check:

   Open video with VideoCapture() [OK]

Hint: Use cv2.VideoCapture('filename') to open video [OK]

Common Mistakes:

• Using non-existent functions like VideoOpen or OpenVideo
• Forgetting to pass the filename as a string
• Confusing VideoCapture with image reading functions

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

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

medium

A. <class 'numpy.ndarray'>

B. <class 'list'>

C. <class 'NoneType'>

D. <class 'str'>

Solution

1. Step 1: Understand cap.read() output

   cap.read() returns a boolean and a frame image as a numpy array if successful.

2. Step 2: Check the type of frame

   The frame is an image stored as a numpy.ndarray representing pixels.

3. Final Answer:

   <class 'numpy.ndarray'> -> Option A

4. Quick Check:

   Frame type = numpy.ndarray [OK]

Hint: Frames from OpenCV are numpy arrays [OK]

Common Mistakes:

• Assuming frame is NoneType if video loads correctly
• Thinking frame is a list or string
• Ignoring the boolean return value ret

4. Identify the error in this frame extraction code:

import cv2
cap = cv2.VideoCapture('video.mp4')
while True:
    frame = cap.read()
    if frame is None:
        break
    cv2.imshow('Frame', frame)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cap.release()
cv2.destroyAllWindows()

medium

A. cv2.imshow() cannot display frames

B. cv2.waitKey() should be called with argument 0

C. cap.read() returns two values, but only one is assigned

D. cap.release() is missing

Solution

1. Step 1: Check cap.read() usage

   cap.read() returns two values: ret (bool) and frame (image). The code assigns only one variable.

2. Step 2: Identify the error consequence

   Assigning one variable causes frame to be a tuple, so frame is never None and loop logic fails.

3. Final Answer:

   cap.read() returns two values, but only one is assigned -> Option C

4. Quick Check:

   cap.read() returns two values [OK]

Hint: Always unpack cap.read() into ret, frame [OK]

Common Mistakes:

• Ignoring the boolean ret value
• Checking frame is None instead of ret
• Forgetting to release video capture

5. You want to extract every 10th frame from a video and save it as an image. Which code snippet correctly does this?

hard

A. import cv2
cap = cv2.VideoCapture('video.mp4')
count = 0
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()

B. import cv2
cap = cv2.VideoCapture('video.mp4')
count = 0
while cap.isOpened():
frame = cap.read()
if frame is None:
break
if count % 10 == 0:
cv2.imwrite('frame.jpg', frame)
count += 1
cap.release()

C. import cv2
cap = cv2.VideoCapture('video.mp4')
for i in range(0, 100, 10):
ret, frame = cap.read()
if ret:
cv2.imwrite(f'frame_{i}.jpg', frame)
cap.release()

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

Solution

1. Step 1: Understand frame extraction every 10 frames

   We want to save frames where count is divisible by 10 (0, 10, 20...).

2. Step 2: Check code correctness

   import cv2 cap = cv2.VideoCapture('video.mp4') count = 0 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() correctly reads frames, checks ret, saves every 10th frame with unique filename, and releases capture.

3. Final Answer:

   Code that saves every 10th frame with unique filenames and proper checks -> Option A

4. Quick Check:

   Save frames where count % 10 == 0 [OK]

Hint: Use count % 10 == 0 to save every 10th frame [OK]

Common Mistakes:

• Not unpacking ret and frame from cap.read()
• Saving all frames with same filename overwriting images
• Using wrong condition like count == 10 only