Why OpenCV is not able to process quite similar sets of json data?

Question

I want to transfer a data structure through a QR code. So after building this structure MyDataTest, I convert it using JSON to MyDataTest_json. Then, a QR code is created using Python QR Code.

import qrcode
import json
import cv2


A = ['Lorem ipsum dolor sit amet, consectetur ']
B = ['adipiscing elit. Cras at justo ', 'sum dolor sit am']
C = ['sum dolor sit', 'm ipsum dolor', 'ipiscing el']

A2 = ['Lorem ipsum dolor']
B2 = ['adipiscing eli', 'strwater']
C2 = ['dolor', 'btexDelta', 'strcat']

MyDataTest = {
    "A" : A,        # Replace by A2
    "B" : B,        # Replace by B2
    "C" : C,        # Replace by C2
}

MyDataTest_json = json.dumps(MyDataTest, separators=(',', ':'))

qrcode.make(MyDataTest_json).save("test.png")

d = cv2.QRCodeDetector()
retval, points, straight_qrcode = d.detectAndDecode(cv2.imread('test.png'))
print(json.loads(retval))

The error occurs when I use OpenCV to read this QR code: when MyDataTest contains A,B,C JSON is able to decode the retval value, but when I replace them with A2,B2,C2 OpenCV seem unable to open the QR code as retval is empty.

This is really puzzling me, I can't get why the behaviors are different. Here are the two QR codes generated. The first one can be opened and read, and the second one return no retval value.

Answer 1

This seems to be a common issue with OpenCV, as many people reported that they couldn't load their QR code:

• Doesn't correctly recognize some QR
• QR_code not detected
• QRCodeDetector fails randomly (+samples)

Eventually, I managed to load my QR by using pyzbar instead of OpenCV:

import qrcode
import json
import cv2
import pyzbar.pyzbar as pyzbar # Not in original code


A = ['Lorem ipsum dolor sit amet, consectetur ']
B = ['adipiscing elit. Cras at justo ', 'sum dolor sit am']
C = ['sum dolor sit', 'm ipsum dolor', 'ipiscing el']

A2 = ['Lorem ipsum dolor']
B2 = ['adipiscing eli', 'strwater']
C2 = ['dolor', 'btexDelta', 'strcat']

MyDataTest = {
    "A" : A,        # Replace by A2
    "B" : B,        # Replace by B2
    "C" : C,        # Replace by C2
}

MyDataTest_json = json.dumps(MyDataTest, separators=(',', ':'))

qrcode.make(MyDataTest_json).save("test.png")

im = cv2.imread("test.png")         # Not in original code
decodedObjects = pyzbar.decode(im)  # Not in original code
for obj in decodedObjects:          # Not in original code
    print('Type : ', obj.type)      # Not in original code
    print('Data : ', obj.data,'\n') # Not in original code
    print(json.loads(obj.data))     # Not in original code