QR code reading in Raspberry Pi - Mini Project: Build & Apply

QR Code Reading on Raspberry Pi

📖 Scenario: You have a Raspberry Pi with a camera attached. You want to read QR codes from images to get useful information like URLs or text.

🎯 Goal: Build a simple Python program that reads a QR code from an image file and prints the decoded text.

📋 What You'll Learn

Use the opencv-python library to load the image.

Use the pyzbar library to detect and decode QR codes.

Create a variable for the image file path.

Print the decoded QR code text.

💡 Why This Matters

🌍 Real World

Reading QR codes is common in many applications like scanning tickets, URLs, or product information using a Raspberry Pi camera.

💼 Career

Understanding how to process images and decode QR codes is useful for jobs in IoT, embedded systems, and computer vision.

1

Set up the image file path

Create a variable called image_path and set it to the string 'qr_code_sample.png' which is the name of the image file containing the QR code.

Raspberry Pi

# Create a variable called image_path with the value 'qr_code_sample.png'
# Your code here

Hint

Use a string to store the file name exactly as 'qr_code_sample.png'.

2

Import libraries and load the image

Import cv2 from opencv-python and decode from pyzbar.pyzbar. Then load the image from image_path using cv2.imread() and store it in a variable called image.

Raspberry Pi

image_path = 'qr_code_sample.png'
# Import cv2 and decode, then load the image into variable image
# Your code here

Hint

Use cv2.imread(image_path) to read the image file.

3

Decode the QR code from the image

Use decode(image) to find QR codes in the image and store the result in a variable called decoded_objects. Then get the first decoded object's data as a string and store it in a variable called qr_text. Use decoded_objects[0].data.decode('utf-8') to decode bytes to string.

Raspberry Pi

import cv2
from pyzbar.pyzbar import decode

image_path = 'qr_code_sample.png'
image = cv2.imread(image_path)
# Decode the QR code and get the text
# Your code here

Hint

Use decode(image) to get a list of decoded objects, then get the first one's data.

4

Print the decoded QR code text

Write a print statement to display the value of the variable qr_text.

Raspberry Pi

import cv2
from pyzbar.pyzbar import decode

image_path = 'qr_code_sample.png'
image = cv2.imread(image_path)
decoded_objects = decode(image)
qr_text = decoded_objects[0].data.decode('utf-8')
# Print the decoded QR code text
# Your code here

Hint

Use print(qr_text) to show the decoded text on the screen.

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