`ValueError: glog(0)` when encoding binary data

[bindreams]

I'm encoding a binary image file using a QR code. In the example below I'm creating a QR code version 20 with error correction H, which should allow me to pass 382 bytes in. After calling make I'm getting an error:

from qrcode import QRCode
from qrcode.constants import ERROR_CORRECT_H

data = b'         0\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01\x01\x00\x00H\x00H\x00\x00\xff\xe1\x00@Exif\x00\x00MM\x00*\x00\x00\x00\x08\x00\x01\x87i\x00\x04\x00\x00\x00\x01\x00\x00\x00\x1a\x00\x00\x00\x00\x00\x02\xa0\x02\x00\x04\x00\x00\x00\x01\x00\x00\x00\x8c\xa0\x03\x00\x04\x00\x00\x00\x01\x00\x00\x00\xb4\x00\x00\x00\x00\xff\xed\x008Photoshop 3.0\x008BIM\x04\x04\x00\x00\x00\x00\x00\x008BIM\x04%\x00\x00\x00\x00\x00\x10\xd4\x1d\x8c\xd9\x8f\x00\xb2\x04\xe9\x80\t\x98\xec\xf8B~\xff\xe2\x02(ICC_PROFILE\x00\x01\x01\x00\x00\x02\x18appl\x04\x00\x00\x00mntrRGB XYZ \x07\xe6\x00\x01\x00\x01\x00\x00\x00\x00\x00\x00acspAPPL\x00\x00\x00\x00APPL\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xf6\xd6\x00\x01\x00\x00\x00\x00\xd3-appl\xec\xfd\xa3\x8e8\x85G\xc3m\xb4\xbdOz\xda\x18/\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\ndesc\x00\x00\x00\xfc\x00\x00\x000cprt\x00\x00\x01,\x00\x00\x00Pwtpt\x00\x00\x01|\x00\x00\x00\x14rXYZ\x00\x00\x01\x90\x00\x00\x00\x14gXYZ\x00\x00\x01\xa4\x00\x00\x00\x14bXYZ\x00\x00\x01\xb8\x00\x00\x00\x14rTRC\x00\x00'

qr = QRCode(version=20, error_correction=ERROR_CORRECT_H, box_size=9, border=4)
qr.add_data(data)
qr.make()

Traceback (most recent call last):
  File "D:\OneDrive\Develop\vis-transfer\test.py", line 8, in <module>
    qr.make()
  File "C:\Users\andreasxp\AppData\Local\Programs\Python\Python311\Lib\site-packages\qrcode\main.py", line 162, in make
    self.makeImpl(False, self.best_mask_pattern())
                         ^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\andreasxp\AppData\Local\Programs\Python\Python311\Lib\site-packages\qrcode\main.py", line 252, in best_mask_pattern
    self.makeImpl(True, i)
  File "C:\Users\andreasxp\AppData\Local\Programs\Python\Python311\Lib\site-packages\qrcode\main.py", line 189, in makeImpl
    self.data_cache = util.create_data(
                      ^^^^^^^^^^^^^^^^^
  File "C:\Users\andreasxp\AppData\Local\Programs\Python\Python311\Lib\site-packages\qrcode\util.py", line 586, in create_data
    return create_bytes(buffer, rs_blocks)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\andreasxp\AppData\Local\Programs\Python\Python311\Lib\site-packages\qrcode\util.py", line 528, in create_bytes
    modPoly = rawPoly % rsPoly
              ~~~~~~~~^~~~~~~~
  File "C:\Users\andreasxp\AppData\Local\Programs\Python\Python311\Lib\site-packages\qrcode\base.py", line 279, in __mod__
    ratio = glog(self[0]) - glog(other[0])
            ^^^^^^^^^^^^^
  File "C:\Users\andreasxp\AppData\Local\Programs\Python\Python311\Lib\site-packages\qrcode\base.py", line 236, in glog
    raise ValueError(f"glog({n})")
ValueError: glog(0)

Can you please help me out?

[bindreams]

Soon after posting I also discovered that the code above works on 7.3.1, but not on any 7.4.* version.

[MayankFawkes]

i am having same problem with 7.4.* its working with the strings but not with bytes

[echoechoin]

Maybe you can encode your data to base64 before add_data. It works in my scripts.

[bindreams]

OP here. For anyone encountering the problem, I eventually created my own QR encoder (also supports other barcodes). It's based on a very established C library and has very good support for any kind of data (binary included).

https://github.com/bindreams/zint-bindings

[McOgurcik]

I found a solution for myself: When using qr.make, I explicitly specify the fit=False parameter.
(qr.make(fit=False))

[obilodeau]

This bug is annoying and fit=False didn't work for me. A simple 👍 here.

[wtc492]

It's not a real solution, but I was able to resolve this error for my application by removing long strings of 0x00 (messes with the high error correction, perhaps?) Going to a lower error correction wasn't an option because I'm embedding images in my QR codes, but since I'm encoding C-strings in fixed-size arrays, I could just random-fill the bytes after the null terminator.

[Kaboutersuper]

Here the same problem. As the previous comment suggests, it has to do with long string of zeroes. However, it is pretty sensitive for what precedes the zeroes.

The root problem in the code is that mul and mod are calculated by taking the glog, adding up the numbers and take gexp. This is a neat trick, but introduces problems when one of the numbers is zero. Proposed solution is to check on zeroes.

Sample code that forces zeroes in the data:

from qrcode import *
import io


qr = QRCode(
    version=6,
    error_correction=constants.ERROR_CORRECT_L,
    box_size=10,
    border=4,
    mask_pattern=0
    )

#url consists of
# a part that is byte or alphanumeric
# a part that is numeric


url="http://test.com/abc"+"000"*90
# this url gives the value error on glog(0)

#url="http://test.com/ab"+"000"*90
# this url works well surprisingly

qr.clear()
qr.add_data(url)

qr.make()

[SmileyChris]

Thanks for the discussion and explanation! It does seem the easiest way is skipping the zeros. I added a test and it works without breaking anything else.