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Reading Barcodes from an Image - II

, 21 Aug 2010
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An example of how to process an image for barcode strings
Sample Image

Introduction

I recently ran across Benjamin Liedblad's article on reading barcode strings from images. The code was good, but not very useful in the real world. I thought my additions had a place here.

Berend Engelbrecht has vastly improved this project. His new version can be found at BarcodeImaging3.aspx.

Background

Liedblad's barcode reader has two significant flaws:

  • It only reads right-side up barcodes (as he mentioned in his "todo").
  • It assumes that there is nothing other than the barcode in the image.

Frequently, we need to read a barcode from a page full of other text. This is what I set about to do.

Using the Code

First of all, you should check out the original article to see how the barcode reading works. Much of the code is left unchanged.

Changes:

  • Since the barcode we are looking for may not be at the beginning of the text line that we scan, we can't just scan by blocks of 9 'characters'. Instead, we walk along the scanned pattern one 'bar' at a time, testing it and the 8 bars after it to see if they form a character. If we find a real character, then we skip over the character pattern and begin again.
  • Since there are text and other distractions on many pages, we need to cut the page up into sections and look for barcodes in each section. I've found that 50 is a good number of sections. While this seems like a lot, it's enough to ensure that we get the barcode we're looking for. Your mileage may vary. To this end, I added a startheight and endheight to ReadCode39(). Then we just need to calculate where each section begins and ends. This is accomplished easily in a for loop:
    for (int i=0; i < numscans; i++)
    {
        read = ReadCode39(bmp,i * (bmp.Height / numscans), 
               (i * (bmp.Height / numscans))+ (bmp.Height / numscans));
        [...]
    }
  • We need to check all four page orientations, which is handled easily with .NET's Bitmap control:
    for (int i=0; i < 4; i++)
    {
        bmp.RotateFlip(System.Drawing.RotateFlipType.Rotate90FlipNone);
        VScanPageCode39(ref CodesRead, bmp,numscans);
    }

    You can, of course not use this if you know that all of your pages are the right way up.

  • Finally, since we scan so many times and receive so many barcodes (including the backwards version of what we are looking for), we need to store all of the barcodes that we found. For this, I decided to use an ArrayList. The code currently returns all of the barcodes, but since most people supply some sort of pattern to their code (usually beginning and ending with astericks "*"), it's easy enough to pick out the right one.

Running the Demo

When you run the demo and load "SamplePage.jpg", first perform "Scan Barcode". This is essentially identical to Liedblad's original code. You will get "*JR7". Not very helpful. Then perform "Scan Page". Now you have a list of barcodes, one of which is the one you want (*surprise*)... and one is its mirror image (P4 9V*VK P). As you can see, it's pretty easy to pick out what you need.

History

  • 5-15-05: Original
  • 8-20-10: Article updated

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

qlipoth
Software Developer (Senior) InGage Networks
United States United States
James is currently a Software developer after a detour from that profession into IT because he 'didn't want to sit in a cubicle and wanted to deal with people.' He has since learned that people are stupid, and enjoys his cubicle greatly.

Comments and Discussions

 
GeneralThanks Pinmembermr_squall26-Aug-09 21:54 
QuestionAny ideas on how to get this code to work in Windows Mobile? Pinmembers1dd29-May-09 8:01 
AnswerRe: Any ideas on how to get this code to work in Windows Mobile? Pinmembers1dd29-May-09 8:14 
GeneralRe: Any ideas on how to get this code to work in Windows Mobile? PinmemberBerend Engelbrecht3-Oct-09 5:24 
QuestionAnyone to give me how to read code 128 barcode from an image ? PinmemberMember 45876798-Dec-08 8:12 
GeneralCan´t read codebars generated using Word... and other tools Pinmemberneptas13-Feb-08 6:50 
GeneralRe: Can´t read codebars generated using Word... and other tools PinmemberFeurich26-Mar-08 1:12 
GeneralRe: Can´t read codebars generated using Word... and other tools PinmemberBerend Engelbrecht3-Oct-09 4:58 
I had the same problem and looked into the cause. Please consider this enlargement from the leading edge of a barcode, that I generated using a Code39 TrueType font in MS Word. I used the MS Document Image Writer printer to create a tiff image directly from Word, then enlarged and measured it in Photoshop:
 
Click to view image in new window
 
The first dark bar encountered has a width of 4 pixels, being a "narrow" bar. The software assumes that the first bar encountered will always be a dark "narrow" bar and uses that to determine if other bars are "narrow" or "wide". A barcode is seen as a pattern of bars of alternating width in light and dark color.
 
Here is one example of the code that does that determination:
  if ((i-nBarStart) > (nNarrowBarWidth))
  {
    // The light bar was wider than the narrow bar width, it's a wide bar
As you can see, the comparison will go wrong because the "narrow" bars following the first one are not 4 pixels, but 5, 6 or even 7 pixels wide. All bars > 5 pixels will be regarded as "wide" bars by the software, and the barcode is thus not correctly recognized. Word consistently makes the white gaps wider than the black bars in a barcode font, hence the problem is so prominent when Word is used to create barcode bitmaps.
 
To solve the problem, I have applied a small change:
private const float WIDEFACTOR = 2.0f; // minimum width of wide bar compared to narrow bar

...
 
  int nBarWidth = i - nBarStart;
  if (nBarWidth > nNarrowBarWidth * WIDEFACTOR)
  {
    // The light bar was wider than the narrow bar width, it's a wide bar
Now with this version of the code, a bar will not be considered to be "wide" unless it is at least twice as wide as the measured narrow bar width. In the example, only bars >= 9 pixels will be considered to be wide. With this change the barcode created by Word is correctly recognized. I have re-run the software against all my test images and I believe that the change does not negatively affect the detection rate.
GeneralUpdating to read code 128 Pinmembercameron molyneux22-Apr-07 17:53 
GeneralSome question. PinmemberPham Tuan21-Sep-06 21:23 
AnswerRe: Some question. Pinmemberqlipoth22-Sep-06 13:37 
GeneralRe: Some question. PinmemberPham Tuan22-Sep-06 15:43 
GeneralRe: Some question. Pinmembervbdotnetcoder200525-Sep-06 8:35 
GeneralReg:Generating BarCode.. PinmemberKumar Murugesan26-Jul-06 4:48 
GeneralRe: Reg:Generating BarCode.. Pinmemberqlipoth1-Aug-06 4:57 
GeneralRe: Reg:Generating BarCode.. Pinmemberkirankumarannigeri14-Nov-07 22:36 
GeneralImplementation Tips Pinmembervbdotnetcoder20052-Feb-06 5:47 
GeneralRe: Implementation Tips PinmemberSuntai5-May-06 2:32 
GeneralRe: Implementation Tips Pinmembervbdotnetcoder200510-May-06 10:24 
GeneralGreat topic PinmemberMichaelWells6-Jul-05 1:28 
GeneralRe: Great topic Pinmembervbdotnetcoder200516-Jan-06 8:41 
GeneralRe: Great topic PinmemberOllipolli20-Jan-06 2:31 
GeneralRe: Great topic Pinmembervbdotnetcoder200520-Jan-06 11:51 
GeneralRe: Great topic Pinmemberqlipoth5-Feb-06 16:03 
GeneralRe: Great topic Pinmemberqlipoth5-Feb-06 16:08 

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