| PDF417 s a multi-row,
variable-length symbology offering high data capacity and
error-correction capability. PDF417 can be scanned by linear
PDF417 barcode scanners, rastering laser scanners, or
two-dimensional imaging devices. One PDF417 symbol is
capable of encoding more than 1100 bytes, 1800 ASCII
characters, or 2700 digits, depending on the selected data
Every PDF417 symbol is composed of stack of rows, from a minimum of 3 to a maximum of 90 rows. Each PDF417 row contains start and stop patterns, left and right row indicators, and from one to thirty data symbol characters. Since both the number of rows and their length are selectable when printed, the aspect ratio of a PDF417 symbol can be varied to suit spatial requirements for printing.
A PDF417 symbol character consists of seventeen modules arranged into four bars and four spaces. The entire set of symbol characters is divided into three mutually exclusive encodation sets, or "clusters". Each cluster encodes the 929 available PDF417 symbol character values, or codewords, with distinct bar-spaces patterns so that one cluster cannot be confused with another. Because any two adjacent rows use different clusters, the decoder can utilize data from scans that cross rows while decoding a PDF417 symbol.
By adding error correction codewords to the data message, PDF417 supports correction of lost or missing data. Each PDF417 symbol requires two error correction codewords for error detection. In addition, up to 510 codewords of error correction can be added when printing the symbol. This allows for decoding security that mathematically is many orders of magnitude stronger than that of bar codes with simple check characters.
PDF417 offers three data compaction modes. Each mode defines a conversation or mapping between codeword sequences and byte sequences. The three modes are Text Compaction mode, Byte Compaction mode, and Numeric Compaction mode.
The interpretation of the byte sequences encoded by a compaction mode is determined by the Global Label Identifier (GLI). A GLI is a special codeword sequence which activates a set of interpretations. The implementation of GLIs enables PDF417 to encode international character sets, and industry - and user-defined character sets, as well as ASCII.
Marco PDF417 provides a means
for creating a distributed representation of files too large
to be represented by a single PDF417 symbol. Marco PDF417
symbols differ from ordinary PDF417 symbols in that they
contain additional control information used to support this
distributed representation. This allows a decoder to make
use of this information to correctly reconstruct and verify
the file, independent of the symbol scanning order.
|Table below summarized PDF417's characteristics:|
|Each PDF 417 symbol consists of a stack of vertically aligned rows framed by clear areas called quiet zones.|
|Above Figure shows a typical PDF417 symbol and its structure.|
Every PDF417 symbol contains a minimum or 3
to a maximum of 90 rows. Each row consists of:
a. Leading quiet zone
b. Start pattern
c. Left row indicator symbol character
d. One to thirty data symbol characters
e. Right row indicator symbol character
f. Stop pattern
g. Trailing quiet zone
A symbol character consists of seventeen modules arranged into four bars and four spaces. each symbol character represents a value in the range of 0 to 928; within this documents, these symbol character value are referred to as "codewords"
Because the number of row is variable, and rows are variable in length (i.e., in the number of symbol character "columns"), the height/width proportion, or aspect ration, of a PDF417 symbol can be varied to suit spatial requirements for printing. However, the number of symbol characters in all rows of a given symbol must be the same.
The data region of a PDF417 symbol is the central area of codeword columns between the left row indicator column and the right row indicator column. The first (upper left) codeword of the data region is the symbol length descriptor. Its value indicates the total number of codewords in the data region, including the symbol length descriptor itself, but excluding the error correction codewords. The remaining codewords in the data region (including the data codewords, pad codewords, and error correction code words, in that order) are arranged with the most significant codeword adjacent to the symbol length descriptor and are read from left to right, top row to bottom. The number of codewords in the data region of a single PDF417 symbol cannot exceed 928.
|Symbol Character Encodation|
|A symbol character consists of four bars and four spaces; each bar or space contains one to six modules. In all cases, the four bars and four spaces of any symbol character measure 17 modules in total. The width of one module is the X dimension of that symbol.|
|Clusters and Symbol Character Definitions|
|The entire set of PDF417 symbol characters is divided into three mutually exclusive encodation sets, or "clusters". Each cluster encodes all 929 defined PDF417 codewords with distinct bar and space patterns. Within each cluster, each symbol character is associated with a unique value in the range of 0 to 928; this value is called the symbol character value or codeword.|