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Document Type: Tutorial
NI Supported: Yes
Publish Date: Sep 6, 2006

Reading Data Matrix bar codes with missing information

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Overview

For manufacturing engineers who use Data Matrix codes for part identification, this example code shows how to use IMAQ Vision to read partially occluded Data Matrix codes.

Table of Contents

  1. Background
  2. Example
  3. Demo Code

Background

2-D barcodes like DataMatrix are being used extensively in several industries such as automotive, aerospace, consumer goods, electronics, semiconductor, and life sciences. Major benefits of 2D codes over 1D barcodes include:
  1. Small size - more information can be packed into a smaller area
  2. Do not require precise sizing of individual elements (cells), because all elements are the same size. This is a major improvement over 1D barcodes, where bar thickness is critical to decoding.
  3. Built in error correction allows all information to be recovered even when a portion of the code is missing.

DataMatrix codes have 3 basic areas of interest. The sample 2D code image in Figure 1 illustrates these regions.

Figure 1. A DataMatrix code has three areas of interest: the Locator, the alternating black and white boundary cells and the core Data area.

Example

Figure 2 shows sample images of the same data matrix code with missing information. By definition, the data matrix code can recover up to 20% of missing information by using error correction built into the Solomon-Reed algorithm used to construct the code.

(a)

(b) (c)

(d) (e)

(f) (g)
Figure 2. Original barcode (a) and the same bardcode with missing information (b - f) .
These images will be used in the attached demonstration code.

Demo Code


For most applications, the size of the 2D code to be read is known. Sometimes, for a variety of reasons the code gets damaged. As the structure of the Locator template and the matrix size information is pre-known, the 2D code can be reconstructed. The example presented here illustrates how this can be accomplished using LV and IMAQ Vision.

[+] Enlarge Image
(a)


[+] Enlarge Image
(b)
Figure 3 User interface (a) and Block Diagram (b) for a demo code illustrating use of the example VIs for Pre-processing of the Data Matrix codes

The inspection systems can be trained on the size of the barcode and the number of elements in it. When a damaged barcode comes into the system it is then read correctly. Figure 3(a) shows the damaged DataMatrix code is reconstructed with the missing information for the Locator and the alternating black and white cells. The resulting image is then passed through the DataMatrix reader VI and is read correctly.

The VIs used to regenerate the missing information and example code demonstrating their use are available through the link at the end of this document.
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Link to the sample code
Here is the link to the sample code: http://zone.ni.com/devzone/cda/epd/p/id/3251
- Jul 14, 2008

 

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