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Features and characteristics of two-dimensional barcodes (take PDF417 as an example)
A) Introduction to PDF417
The PDF417 code was invented by Dr. Yinying Wang, a Chinese student in the United States. PDF is the abbreviation of the three words of English Portable Data File, which means "portable data file". Because each symbol character constituting the barcode is composed of 4 bars and 4 spaces, if the narrowest bar or space constituting the barcode is called a module, the total number of modules in the above 4 bars and 4 spaces Set to 17, so called 417 code or PDF417 code.
B) Features of PDF417
1. Large information capacity
PDF417 code can express binary numbers besides letters, numbers and ASCII characters. In order to make the encoding more compact and improve the information density, PDF417 has three formats when encoding:
* The expanded alphanumeric compression format can accommodate 1850 characters;
* Binary / ASCII format can hold 1108 bytes;
* Digital compression format can accommodate 2710 numbers.
2. Error correction capability
One-dimensional bar codes usually have a check function to prevent misreading. Once the bar codes are defaced, they will be rejected. The two-dimensional barcode not only prevents errors, but also corrects errors. Even if the barcode is partially damaged, the correct information can be restored.
3. Printing requirements are not high
Ordinary printing equipment can print, and faxes can also be read.
4. Can be read by multiple reading devices
PDF417 code can be read with laser reader with raster, image reader with linear and surface scanning.
5. The size can be adjusted to suit different printing spaces
6. The open code system has formed an international standard, and China has also formulated a national standard of 417 yards.
Three) PDF417 error correction function
The error correction function of the two-dimensional barcode is realized by repeating (redundant) part of the information. For example, in the PDF417 code, in addition to the information of this line, there is some information reflecting the characters (error correction codes) in other positions. In this way, even when a part of the barcode is damaged, the information can be restored by the error correction code that exists in another location.
The error correction capability of PDF417 is divided into 9 levels from 0 to 8 according to the number of error correction codes. See Figure 4. The higher the level, the more correction code words, the stronger the correction capability, and the larger the bar code. When the correction level is 8, even 50% of the bar code can be read correctly.
Four) Several variants of PDF417
PDF417 also has several variants of code systems:
* PDF417 truncated code
In a relatively "clean" environment, the possibility of barcode damage is very small, you can omit the line indicator on the right and reduce the terminator.
* PDF417 microcode
PDF code further reduced.
* Macro PDF417 code
When the file content is too long to be represented by a PDF417 code, it can be represented by a macro PDF417 code that contains multiple (1 to 99999) barcode blocks.
Advantages of 2D barcodes
As can be seen from the above introduction, compared with the one-dimensional bar code, the two-dimensional bar code has obvious advantages. In summary, there are the following aspects:
A) Larger data capacity
B) Beyond the alphanumeric limit
C) The relative size of the barcode is small
D) Ability to resist damage