- Scan an existing QR code at https://zxing.org/w/decode.jspx to get the text content
- Remove the last 4 characters, which is the checksum (CRC), from the text content
- Spot the substring that contains the amount from the text content
- Replace the amount substring with new amount using the *same number* of characters, for example, replacing
- 1755.00
- with
- 0050.00
- will update the amount from $1,755 to $50
- Calculate the new checksum
- http://www.sunshine2k.de/coding/javascript/crc/crc_js.html
- width: CRC-16
- parametrization: Custom
- Polynomial: 0x1021
- Initial Value: 0xFFFF
- Input Data: String
- Paste the updated text content from above
- Click Calculate CRC
- Generate new QR code
- https://www.qrcode-monkey.com/#text
- Paste the updated text content appended with the new 4-character CRC
- Click Create QR Code
Ref: FPS QR code specification at https://www.hkma.gov.hk/media/eng/doc/key-functions/financial-infrastructure/infrastructure/retail-payment-initiatives/Common_QR_Code_Specification.pdf
