QR code reading in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the library needed for QR code reading.

Raspberry Pi

import [1]

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

Atime

Bnumpy

Ccv2

Dos

Attempts:

3 left

💡 Hint

Common Mistakes

Importing unrelated libraries like numpy or time.

Forgetting to import the QR code reading library.

The cv2 library (OpenCV) is used for image processing and QR code reading on Raspberry Pi.

2fill in blank

medium

Complete the code to initialize the QR code detector.

Raspberry Pi

detector = cv2.[1]()

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Aimread

BQRCodeDetector

CCascadeClassifier

DVideoCapture

Attempts:

3 left

💡 Hint

Common Mistakes

Using VideoCapture which is for camera input.

Using CascadeClassifier which is for face/object detection.

The QRCodeDetector() class in OpenCV is used to detect and decode QR codes.

3fill in blank

hard

Fix the error in the code to read a frame from the camera.

Raspberry Pi

ret, frame = cap.[1]()

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Aread

Bcapture

Cget

Dgrab

Attempts:

3 left

💡 Hint

Common Mistakes

Using capture() which does not exist.

Using get() which retrieves properties, not frames.

The read() method captures a frame from the video source and returns a success flag and the frame.

4fill in blank

hard

Fill both blanks to decode the QR code from the frame.

Raspberry Pi

data, bbox, _ = detector.[1](frame)
if data [2] "":
    print(f"QR Code data: {data}")

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AdetectAndDecode

B!=

C==

DdetectMultiScale

Attempts:

3 left

💡 Hint

Common Mistakes

Using detectMultiScale which is for object detection.

Checking if data equals empty string instead of not equals.

The detectAndDecode() method detects and decodes the QR code from the image. Checking if data != "" confirms a QR code was found.

5fill in blank

hard

Fill all three blanks to capture video, detect QR codes, and exit on pressing 'q'.

Raspberry Pi

cap = cv2.[1](0)
while True:
    ret, frame = cap.[2]()
    data, bbox, _ = detector.[3](frame)
    if data != "":
        print(f"QR Code: {data}")
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
cap.release()
cv2.destroyAllWindows()

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

AVideoCapture

Bread

CdetectAndDecode

Dimread

Attempts:

3 left

💡 Hint

Common Mistakes

Using imread which reads images from files, not camera.

Not releasing the camera or destroying windows.

VideoCapture(0) opens the default camera. read() captures frames. detectAndDecode() reads QR codes from frames. The loop exits when 'q' is pressed.

Practice

1. What is the main purpose of using the pyzbar library on a Raspberry Pi when working with QR codes?

easy

A. To capture images from the camera

B. To decode and read data from QR codes

C. To display images on the screen

D. To create QR codes from text

Solution

1. Step 1: Understand the role of pyzbar

   The pyzbar library is designed to decode barcodes and QR codes from images.

2. Step 2: Identify its use in QR code reading

   On Raspberry Pi, pyzbar reads the data encoded in QR codes, not capturing or displaying images.

3. Final Answer:

   To decode and read data from QR codes -> Option B

4. Quick Check:

   pyzbar = decode QR codes [OK]

Hint: pyzbar reads QR code data, not captures or displays images [OK]

Common Mistakes:

• Confusing pyzbar with camera capture libraries
• Thinking pyzbar creates QR codes
• Assuming pyzbar displays images

2. Which of the following is the correct way to import the decode function from the pyzbar.pyzbar module in Python on Raspberry Pi?

easy

A. from pyzbar.pyzbar import decode

B. import pyzbar.decode

C. from pyzbar import decode

D. import decode from pyzbar.pyzbar

Solution

1. Step 1: Recall correct Python import syntax

   To import a function from a submodule, use from module.submodule import function.

2. Step 2: Apply to pyzbar

   The decode function is inside pyzbar.pyzbar, so the correct import is from pyzbar.pyzbar import decode.

3. Final Answer:

   from pyzbar.pyzbar import decode -> Option A

4. Quick Check:

   Correct import syntax = from pyzbar.pyzbar import decode [OK]

Hint: Use 'from module.submodule import function' for nested imports [OK]

Common Mistakes:

• Using 'import pyzbar.decode' which is invalid
• Writing 'import decode from ...' which is wrong syntax
• Omitting the submodule pyzbar in import

3. Given the following Python code snippet on Raspberry Pi, what will be printed if the image contains a QR code with data 'Hello123'?

from pyzbar.pyzbar import decode
import cv2

image = cv2.imread('qrcode.png')
decoded_objects = decode(image)
for obj in decoded_objects:
    print(obj.data.decode('utf-8'))

medium

A. b'Hello123'

B. obj.data

C. Error: decode() expects a file path

D. Hello123

Solution

1. Step 1: Understand decode output

   The decode function returns a list of objects with a data attribute as bytes.

2. Step 2: Decode bytes to string

   The code calls obj.data.decode('utf-8') to convert bytes to a readable string, so it prints 'Hello123'.

3. Final Answer:

   Hello123 -> Option D

4. Quick Check:

   Decoded bytes to string = Hello123 [OK]

Hint: Decode bytes with .decode('utf-8') to get readable text [OK]

Common Mistakes:

• Printing raw bytes without decoding
• Assuming decode() takes file path instead of image
• Confusing obj.data with string directly

4. What is the error in the following Raspberry Pi Python code for reading a QR code?

from pyzbar.pyzbar import decode
import cv2

image = cv2.imread('qrcode.png')
result = decode('qrcode.png')
print(result)

medium

A. print() cannot print decode results

B. cv2.imread() is missing

C. decode() should receive an image, not a file path string

D. Missing import for numpy

Solution

1. Step 1: Check decode() argument type

   The decode function expects an image object, not a file path string.

2. Step 2: Identify the mistake in code

   The code passes the string 'qrcode.png' directly to decode, which causes an error.

3. Final Answer:

   decode() should receive an image, not a file path string -> Option C

4. Quick Check:

   decode() needs image object, not filename [OK]

Hint: Pass image object to decode(), not filename string [OK]

Common Mistakes:

• Passing filename string directly to decode()
• Forgetting to read image with cv2.imread()
• Assuming decode() reads files itself

5. You want to build a Raspberry Pi program that continuously reads QR codes from a live camera feed and prints the decoded data only once per unique QR code detected. Which approach best solves this problem?

hard

A. Use a set to store decoded QR data and check before printing each frame

B. Print all decoded QR data every frame without filtering duplicates

C. Clear the camera feed buffer after each frame to avoid repeats

D. Restart the camera after each QR code is detected

Solution

1. Step 1: Understand the problem of repeated QR codes

   Live camera frames may detect the same QR code multiple times, causing repeated prints.

2. Step 2: Use a set to track unique QR codes

   Storing decoded data in a set allows checking if a QR code was already printed, avoiding duplicates.

3. Step 3: Avoid inefficient or incorrect methods

   Clearing buffers or restarting camera won't prevent repeated prints logically.

4. Final Answer:

   Use a set to store decoded QR data and check before printing each frame -> Option A

5. Quick Check:

   Use set to track unique QR codes [OK]

Hint: Track seen QR codes in a set to print each once [OK]

Common Mistakes:

• Printing every detection without filtering duplicates
• Trying to clear camera buffer to fix repeats
• Restarting camera unnecessarily