Start and Stop Pattern Structure
A codeword is the unit
for encoding a value representing, or associated with, numbers,
letters, or symbols. There are 929 codeword values defined in PDF417.
These values are 0 - 928.
There are four bars and four spaces in each codeword.
Individual bars or spaces can vary in width from one to six
modules, but the combined total per codeword is always 17 modules.
The width of one module, or narrowest element width, is the x-dimension of that symbol. Its height is the y-dimension.
Each codeword is defined by
an 8 digit sequence, which represents the 4 sets of alternating
bar and space widths within that codeword. This is called the x-sequence. For
instance, in an x-sequence of 31111334, the first element is a
bar three modules wide, followed by five elements one module wide
each, one element three modules wide, and the last element six
modules wide.
Focusing on element widths along the x-sequence (x0 ... x7), we might diagram another PDF417 codeword like this:
Start and stop patterns announce where the symbol
begins and ends. By doing this, they make reading the bar code
possible whether the scan is left-to-right or right-to-left.
For these purposes, PDF417 uses unique start and
stop patterns. The start pattern, or left end of the symbol, has
the unique pattern, or x-sequence, of 81111113. The stop pattern,
or right end of the symbol, has the unique x-sequence of
711311121. No character pattern other than start/stop uses more
than six modules per element. For space termination, the stop
pattern contains one extra bar (or ninth element), of width one,
at the end.
Each row consists of a start pattern, a left row
indicator codeword, data codewords, a right row indicator codeword,
and a stop pattern. Note that codewords are numbered from right
to left.
Row indicators, discussed later, help synchronize
the symbol's structure. They help define which row a particular
one is, which is the left and right side of that row, how many
rows are in the symbol, what security level is
encoded in the symbol, and how many data columns are in the rows.
Any PDF417 symbol is made up of at least 3 rows, and
at most 90 rows. The minimum number of codewords in a row is 3;
this includes the left row indicator codeword, 1 data
codeword, and the right row indicator.
Every symbol contains 1 codeword (the first data
codeword in row 0) indicating the total number of data codewords
within the symbol, and at least 2 error-detection codewords.