QR code reading in Raspberry Pi

Introduction

QR code reading helps your Raspberry Pi understand information stored in QR codes quickly and easily.

You want to scan product information using a camera connected to your Raspberry Pi.

You need to read QR codes for a simple check-in system at an event.

You want to automate reading URLs or text from QR codes for a project.

You are building a Raspberry Pi robot that reads QR codes to navigate.

You want to decode QR codes from images stored on your Raspberry Pi.

Syntax

Raspberry Pi

import cv2
from pyzbar import pyzbar

# Load image or capture from camera
image = cv2.imread('qrcode.png')

# Decode QR codes
codes = pyzbar.decode(image)

for code in codes:
    print(code.data.decode('utf-8'))

Use pyzbar library to decode QR codes easily.

cv2.imread() loads an image file; you can also capture from a camera.

Examples

This reads and prints QR code data from a saved image.

Raspberry Pi

import cv2
from pyzbar import pyzbar

# Read QR code from image file
image = cv2.imread('mycode.png')
codes = pyzbar.decode(image)
for code in codes:
    print(code.data.decode('utf-8'))

This captures one frame from the camera and reads QR codes from it.

Raspberry Pi

import cv2
from pyzbar import pyzbar

# Capture from Raspberry Pi camera
cap = cv2.VideoCapture(0)
ret, frame = cap.read()
if ret:
    codes = pyzbar.decode(frame)
    for code in codes:
        print(code.data.decode('utf-8'))
cap.release()

Sample Program

This program loads a QR code image named 'example_qr.png' and prints the text inside the QR code.

Raspberry Pi

import cv2
from pyzbar import pyzbar

# Load an example QR code image
image = cv2.imread('example_qr.png')

# Decode QR codes in the image
codes = pyzbar.decode(image)

# Print all decoded QR code data
for code in codes:
    print(f"QR Code data: {code.data.decode('utf-8')}")

Important Notes

Make sure to install pyzbar and opencv-python packages before running.

Camera access may require permissions and proper setup on Raspberry Pi.

QR code images should be clear and well-lit for best results.

Summary

QR code reading lets your Raspberry Pi understand data from QR codes using a camera or images.

Use pyzbar with opencv to decode QR codes easily.

Works for images and live camera captures.

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