QR code detecting in python with OpenCV raises UnicodeDecodeError: 'utf-8' codec can't decode byte

[MKraemer85]

System Information

OpenCV python version: 4.7.0
Operating System: Windows 10
Python Version: Python 3.10.5

Detailed description

Related to this Question:
https://stackoverflow.com/questions/76362026/qr-code-detecting-in-python-with-opencv-raises-unicodedecodeerror-utf-8-codec

As I'm trying to detect an qr code and the qr code values, I get this error:

Traceback (most recent call last):
  File "C:\Users\m7073\Repos\Chronos_New\invoice_extraction\qr_code_scan.py", line 128, in <module>
    qrcode.set_qr_values()
  File "C:\Users\m7073\Repos\Chronos_New\invoice_extraction\qr_code_scan.py", line 73, in set_qr_values
    text, points, straight_qrcode = detect.detectAndDecode(img)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xfc in position 157: invalid start byte

Steps to reproduce

import cv2
img = cv2.imread(f"page_1.png")
detect = cv2.QRCodeDetector()
text, points, straight_qrcode = detect.detectAndDecode(img)

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files (videos, images, onnx, etc)

[cpoerschke]

Looking into this I enjoyed learning more about QR codes and python:

• different input modes are possible including byte encoding -- https://en.wikipedia.org/wiki/QR_code
• if qr_code_data.data_type == 4 i.e. byte encoding is detected this is not passed back to the calling code? -- https://github.com/opencv/opencv/blob/4.7.0/modules/objdetect/src/qrcode.cpp#L2734
• it seems python (bindings?) goes with UTF-8 encoding and the byte encoding defaults to ISO/IEC 8859-1 as per wikipedia.
• lowercase U umlaut is the character at position 157 -- https://en.wikipedia.org/wiki/Ü

Code snippet:

try:
    text, points, straight_qrcode = detect.detectAndDecode(img)
except UnicodeDecodeError as ex:
    print(ex)
    print(ex.object)
    print(ex.object.decode('ISO-8859-1').encode('UTF-8'))

These two issues sound related:

• wechat qrcode UnicodeDecodeError in python3.6.9 opencv_contrib#2852
• Swiss QR-bills (ISO 20022) not working in some cases #22131

[MKraemer85]

Hi cpoerschke,
I will try that in like a week or so.
I'm ill for an week now and can't try it out right now, sadly.
But thanks for the suggestion, I will reply back when I can try that.

[MKraemer85]

I've switched to zxingcpp and this way it works:

import cv2
img = cv2.imread(f"./qr_codes/page_1.png")
detect = cv2.QRCodeDetector()
for result in zxingcpp.read_barcodes(img):
print(result.text)

[flauer7]

zxingcpp seems way more reliable in this usecase... thanks @MKraemer85

otherwise if you wish to stick with cv2,
try: data, bbox, rectifiedImage = cv2.QRCodeDetector().detectAndDecode(image) except UnicodeDecodeError as ex: data = ex.object.decode('ISO-8859-1')
In my case it was the letter "à" that generates a UnicodeDecodeError

[MKraemer85]

a cool, good to know @flauer7 . Thanks for the input

[dkurt]

Fixed by this PR: #24350