The error correction function of the two-dimensional bar code is achieved by repeatedly representing (redundancy) part of the information. For example, in the PDF417 code, a line contains information that reflects characters (error correction codes) at other positions in addition to the information in the line. In this way, even when a part of the bar code is damaged, the information can be restored by the error correction code existing in other places.
PDF417's error correcting ability is divided into 0 to 8 total 9 levels depending on the number of error correction code words, as shown in Figure 4. The higher the level, the greater the number of correction code words, the stronger the correction ability, and the larger the bar code. When the correction level is 8, even if the bar code is 50% dirty, it can be correctly read out.