Camera stream access with OpenCV in Drone Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open the default camera stream using OpenCV.

Drone Programming

import cv2

cap = cv2.VideoCapture([1])

if not cap.isOpened():
    print("Cannot open camera")

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

A"camera"

B1

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 for the default camera index.

Passing a string instead of an integer.

The default camera is accessed with index 0 in OpenCV.

2fill in blank

medium

Complete the code to read a frame from the camera stream.

Drone Programming

ret, frame = cap.[1]()

if not ret:
    print("Failed to grab frame")

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

Acapture

Bread

Cget

Dfetch

Attempts:

3 left

💡 Hint

Common Mistakes

Using get() which only retrieves properties.

Using capture() or fetch() which do not exist.

The read() method captures a frame from the video stream.

3fill in blank

hard

Fix the error in the code to release the camera resource properly.

Drone Programming

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

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

Aclose

Bend

Cstop

Drelease

Attempts:

3 left

💡 Hint

Common Mistakes

Using close() which does not exist for VideoCapture.

Using stop() or end() which are invalid.

The release() method frees the camera resource.

4fill in blank

hard

Fill both blanks to convert the captured frame to grayscale and display it.

Drone Programming

gray = cv2.[1](frame, cv2.[2])
cv2.imshow('Gray Frame', gray)

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

AcvtColor

BCOLOR_BGR2GRAY

CCOLOR_RGB2GRAY

DconvertColor

Attempts:

3 left

💡 Hint

Common Mistakes

Using convertColor which is not a valid function.

Using COLOR_RGB2GRAY when the frame is in BGR format.

Use cvtColor with COLOR_BGR2GRAY to convert a BGR image to grayscale.

5fill in blank

hard

Fill all three blanks to create a loop that reads frames, shows them, and breaks on pressing 'q'.

Drone Programming

while True:
    ret, frame = cap.[1]()
    if not ret:
        break
    cv2.imshow('Frame', frame)
    if cv2.waitKey([2]) & 0xFF == ord([3]):
        break
cap.release()
cv2.destroyAllWindows()

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

Aread

B1

C'q'

Dwait

Attempts:

3 left

💡 Hint

Common Mistakes

Using wait() instead of waitKey().

Using a wrong key character or missing quotes around 'q'.

Not breaking the loop properly on key press.

The loop reads frames with read(), waits 1 ms for a key press, and breaks if 'q' is pressed.

Practice

1. What does the cv2.VideoCapture(0) function do in OpenCV when working with a drone's camera?

easy

A. It closes any open camera streams.

B. It saves the video stream to a file named '0'.

C. It opens the default camera stream for capturing video frames.

D. It applies a filter to the video stream.

Solution

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

   The function cv2.VideoCapture() is used to open a connection to a camera or video file.

2. Step 2: Interpret the argument 0

   Passing 0 as an argument tells OpenCV to open the default camera device, usually the first connected camera.

3. Final Answer:

   It opens the default camera stream for capturing video frames. -> Option C

4. Quick Check:

   cv2.VideoCapture(0) opens default camera [OK]

Hint: 0 means default camera device in cv2.VideoCapture [OK]

Common Mistakes:

• Thinking it saves video instead of opening camera
• Confusing argument 0 with file name
• Assuming it closes camera streams

2. Which of the following is the correct syntax to read a frame from a camera stream stored in variable cap?

easy

A. ret, frame = cap.read()

B. frame, ret = cap.read()

C. cap.read(frame, ret)

D. cap.read = (ret, frame)

Solution

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

   The cap.read() method returns two values: a boolean ret and the image frame frame.

2. Step 2: Match the correct variable order

   The correct order is ret, frame = cap.read(), where ret indicates success and frame holds the image.

3. Final Answer:

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

4. Quick Check:

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

Hint: Remember cap.read() returns ret first, then frame [OK]

Common Mistakes:

• Swapping ret and frame variables
• Using assignment inside parentheses incorrectly
• Trying to assign with equal sign inside method call

3. What will the following code print if the camera stream opens successfully and a frame is captured?

import cv2
cap = cv2.VideoCapture(0)
ret, frame = cap.read()
print(ret)
cap.release()

medium

A. None

B. False

C. An error message

D. True

Solution

1. Step 1: Understand cap.read() return value

   If the camera opens and a frame is successfully read, ret will be True.

2. Step 2: Analyze the print statement

   The code prints the value of ret, so it will print True if successful.

3. Final Answer:

   True -> Option D

4. Quick Check:

   Successful frame read returns True [OK]

Hint: ret is True if frame read succeeds, else False [OK]

Common Mistakes:

• Assuming ret is the frame itself
• Expecting printed frame data instead of boolean
• Not releasing the camera after use

4. Identify the error in this code snippet that tries to display a live camera feed:

import cv2
cap = cv2.VideoCapture(0)
while True:
    ret, frame = cap.read
    if not ret:
        break
    cv2.imshow('Camera', frame)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cap.release()
cv2.destroyAllWindows()

medium

A. Missing parentheses after cap.read

B. Incorrect window name in imshow

C. waitKey argument should be 0 instead of 1

D. cap.release() should be inside the loop

Solution

1. Step 1: Check method call syntax

   The line ret, frame = cap.read is missing parentheses. It should be cap.read() to call the method.

2. Step 2: Understand impact of missing parentheses

   Without parentheses, cap.read is a method reference, not a call, so unpacking fails or returns wrong values.

3. Final Answer:

   Missing parentheses after cap.read -> Option A

4. Quick Check:

   Method calls need () after name [OK]

Hint: Always add () to call methods like cap.read() [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Misunderstanding waitKey timing
• Releasing camera inside loop causing errors

5. You want to capture video from a drone's camera and save only frames where the video stream is successfully read. Which code snippet correctly implements this using OpenCV?

hard

A. cap = cv2.VideoCapture(0)
while True:
frame, ret = cap.read()
if ret:
cv2.imwrite('frame.jpg', frame)
cap.release()

B. cap = cv2.VideoCapture(0)
while True:
ret, frame = cap.read()
if ret:
cv2.imwrite('frame.jpg', frame)
else:
break
cap.release()

C. cap = cv2.VideoCapture(0)
while True:
ret, frame = cap.read
if ret:
cv2.imwrite('frame.jpg', frame)
else:
break
cap.release()

D. cap = cv2.VideoCapture(0)
while True:
ret, frame = cap.read()
cv2.imwrite('frame.jpg', frame)
if not ret:
break
cap.release()

Solution

1. Step 1: Verify correct method call and variable order

   cap = cv2.VideoCapture(0) while True: ret, frame = cap.read() if ret: cv2.imwrite('frame.jpg', frame) else: break cap.release() correctly calls cap.read() and assigns ret, frame in the right order.

2. Step 2: Check logic for saving frames

   It saves the frame only if ret is True, otherwise breaks the loop, which is correct behavior.

3. Final Answer:

   Correctly captures and saves frames only when read succeeds -> Option B

4. Quick Check:

   Call cap.read() and check ret before saving [OK]

Hint: Save frames only if ret is True after cap.read() [OK]

Common Mistakes:

• Swapping ret and frame variables
• Missing parentheses on cap.read
• Saving frames even when ret is False