DataMatrix Barcode Decoding question

[Jamie Tan]

Any DataMatrix barcode expert here?

I am having a problem decoding a DataMatrix barcode. It is a 18x18 barcode,
encoded with a string "P5803.....". I used a handheld reader and verified
the encoding is correct. However, when I try to read it by myself with my
own image processing code, I am having trouble decoding the "58". I read all
other characters correctly. After passing ReedSolomon correction, the code
words are: 230 (latch to C40), 182,181,254(unlatch),133,... From the code
words 182 and 181, I calculated the three C40 values: 29,2,36. From the C40
character set table, 29 is 'P', 2 is Shift3, and 36 in the Shift3 Set is
undefined. I do not understand how I can get a "58" from the C40 values of 2
and 36.
I am using the spec ANSI/AIM BC11-1997, published by AIM.
Thanks for any help.

Jamie Tan

[J]

It is because your second and third C40 values are calculated wrong:

C40_sum = 182*256 + 181 - 1 = 46772

1'st C40 byte: C40_sum / 1600 = 29 = 'P'
2'nd C40 byte: (C40_sum - (1600*29)) / 40 = 9 = '5'
3'rd C40 byte: C40_sum - (1600*29) - (40*9) = 12 = '8'

hence you get the decoded string: "P58"

--
Jens

"Jamie Tan" <jt...@alum.mit.edu> wrote in message
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[Jamie Tan]

thanks for the help.
"J" <J...@com.com> wrote in message
news:3e020749$0$136$9a6e...@news.newshosting.com...

[J]

You are welcome.

A couple of questions:

- what are you using your DM decoder algorithm for - is it educational?
- do you know if any QR code decoder algorithms?

Thanks
Jens

"Jamie Tan" <jt...@alum.mit.edu> wrote in message

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[J]

That is:

> - do you know of any QR code decoder algorithms?

"J" <J...@com.com> wrote in message

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