Editing PDF417 (section)

=== Codewords ===

PDF417 uses a [[radix|base]] 929 encoding. Each codeword represents a number from 0 to 928.

The codewords are represented by patterns of dark (bar) and light (space) regions. Each of these patterns contains four bars and four spaces (where the 4 in the name comes from). The total width is 17 times the width of the narrowest allowed vertical bar (the X dimension); this is where the 17 in the name comes from. Each pattern starts with a bar and ends with a space.

The row height must be at least 3 times the minimum width: Y &ge; 3 X.<ref name="iso2006" />{{rp|5.8.2}}

There are three distinct bar&ndash;space patterns used to represent each codeword. These patterns are organized into three groups known as ''clusters''. The clusters are labeled 0, 3, and 6. No bar&ndash;space pattern is used in more than one cluster. The rows of the symbol cycle through the three clusters, so row 1 uses patterns from cluster 0, row 2 uses cluster 3, row 3 uses cluster 6, and row 4 again uses cluster 0.

Which cluster can be determined by an equation:<ref name="iso2006" />{{rp|5.3.1}}

: <math>K = b_1 - b_2 + b_3 - b_4 + 9 \,\, \pmod 9</math>

Where ''K'' is the cluster number and the ''b<sub>i</sub>'' refer to the width of the ''i''-th black bar in the symbol character (in ''X'' units).

Alternatively:<ref name="iso2006" />{{rp|76–78}}

: <math>K = E_1 - E_2 + E_5 - E_6 + 9 \,\, \pmod 9</math>

Where ''E<sub>i</sub>'' is the ''i''-th edge-to-next-same-edge distance. Odd indices are the leading edge of a bar to the leading edge of the next bar; even indices are for the trailing edges.

One purpose of the three clusters is to determine which row (mod 3) the codeword is in. The clusters allow portions of the symbol to be read using a single scan line that may be skewed from the horizontal.<ref name="iso2006" />{{rp|5.11.1}} For instance, the scan might start on row 6 at the start of the row but end on row 10. At the beginning of the scan, the scanner sees the constant start pattern, and then it sees symbols in cluster 6. When the skewed scan straddles rows 6 and 7, then the scanner sees noise. When the scan is on row 7, the scanner sees symbols in cluster 0. Consequently, the scanner knows the direction of the skew. By the time the scanner reaches the right, it is on row 10, so it sees cluster 0 patterns. The scanner will also see a constant stop pattern.