incorrect encoding for repeating 0

[mutoo]

Hi Patrick, thanks for your great work on this project. I am using luaqrcode in my aseprite plugin to generate qrcode from pixel art.

But I found an issue when I try to generate a qrcode with a long repeating 0x00 (pixels with 0th palette) with ec_level = M, the decoder returns a lot of 1s among the 0s, which causing dirty dots when we decode the image from qrcode.

Here is the code to replica the bug:

local data = {}
for i = 1, 255 do
    table.insert(data, 0)
end

qrencode.qrcode(data, 2)

this code generates a qrcode which is decoded by https://zxing.org/w/decode.jspx as following data:

40 0f f0 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 01 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   01 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 01 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
01 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 01 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   01 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 01 00 00
00 00 ec 11 ec 11 ec 11   ec 11 ec 11 ec 11 ec 11
ec 11 ec 11 ec 11 ec 11   ec 11 ec 11 ec 11 ec 11
ec 11 

As you can see there are 01s inside the data, which should be all 00.

I had tried to figure it out for a while, but it's far out of my scope, I'd appreciate it if you could help.

Thanks,
mutoo

[pgundlach]

Hello mutoo, thanks for the report. I'll try to find out what goes wrong, but I cannot promise anything. The last time I looked into the code I felt like somebody else has written it...

[pgundlach]

You have used a function encode() which is (as far as I can see) not part of the qrenocde library. So you need to provide more feedback.

[mutoo]

You have used a function encode() which is (as far as I can see) not part of the qrenocde library. So you need to provide more feedback.

oh sorry, it's my typo. it should be qrencode.qrcode(data, 2)

[pgundlach]

OK, then I get the following error:

lua: qrencode.lua:179: bad argument #1 to 'match' (string expected, got table)
stack traceback:
	[C]: in function 'string.match'
	qrencode.lua:179: in upvalue 'get_mode'
	qrencode.lua:310: in upvalue 'get_version_eclevel_mode_bistringlength'
	qrencode.lua:1311: in field 'qrcode'
	foo.lua:8: in main chunk
	[C]: in ?

You don't get the same error?

[mutoo]

@pgundlach the pseudo-code above may not express my idea. could you try this one:

local data = {}
for i = 1, 255 do
    table.insert(data, 0)
end

local bitstr = ""
for i, v in ipairs(data) do
    bitstr = bitstr .. string.char(v)
end

qrencode.qrcode(bitstr, 2)

[pgundlach]

OK, I can produce a qr code now.

[mutoo]

Hi, Patrick, I spent some time to dig a little bit into the code with the help of this article:
https://www.thonky.com/qr-code-tutorial/error-correction-coding

I found there are some issues within the calculate_error_correction() function. Once there is a block of the message with all 0s, it's like an edge case, the polynomial long division may yield a wrong result.

The article doesn't mention how to deal with such a case, I may keep digging when I've got time.

[pgundlach]

I have no idea what could be wrong.

[mutoo]

Hi Patrick, with the fix I had committed, the QRcode now works well:

40 0f f0 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00   00 00 00 00 00 00 00 00
00 00 ec 11 ec 11 ec 11   ec 11 ec 11 ec 11 ec 11
ec 11 ec 11 ec 11 ec 11   ec 11 ec 11 ec 11 ec 11
ec 11 

Could you please review the pull request and let me know if I need to do anything.

Thanks,
mutoo

[pgundlach]

Thank you very much!