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Basic Barcode Concepts
Bars and Spaces
All barcodes are comprised of bars and spaces (dark and light
regions). A bar is a continuous dark area, a space is a continuous light
area. From here on, I will refer to bar and/or space as simply bar. Most
barcodes have a fixed number of possible bar widths. That is, each and
every bar must be one of a fixed number of sizes. Each bar's width must
also be a multiple of the smallest bar width. For instance, if the
narrowest bar is 10 mil, then possible bar widths can be 10 mil, 20 mil,
30 mil, 40 mil, etc... But not, 15mil, or 25mil since these sizes are not
multiples of the narrowest bar size of 10 mil.
The width of the smallest bar is defined as a barcode's 'X' dimension. Each X' dimension is sometimes referred to as a module. In the following picture, you can easily pick out the smallest, or narrowest bar. By measuring this bar, we can determine the barcodes 'X' dimension. You can also see that all of the other bar widths are multiples of the smallest bar or 'X' dimension. Each bar's width is often expressed relative to the 'X' dimension, for instance, 3X refers to a bar that is 3times as wide as the narrowest bar.
The following picture is a PDF417 barcode that looks like it is printed on graph paper. This image makes it easier to count the number of 'X' dimensions, or modules in each bar. If we look closely, we will see that each and every bar is an exact multiple of the minimum bar width. The first bar is 8X wide, the following space 1X wide, etc... In a validly printed code, without inkspread, you should never see a bar that is 1.5X, 4.2X, or not a whole multiple of X. In a PDF417 symbol , you will always see 1X, 2X, 3X, 4X, 5X, 6X, 7X, or 8X bars. Keep this in mind, you will see later how to detect, and avoid printing problems related to this topic. (See Scaling Errors)
PDF417 Structure Like other barcodes, PDF417 has a well defined physical structure. This section describes each of these structural components.
Start / Stop Patterns Every barcode has a start pattern on the left, and a stop pattern on the right. These patterns are unique for each type of barcode. PDF417's unique start and stop patterns are:
Codewords In a PDF417 barcode, each bar and space does not store data. Data is actually stored in codewords. A codeword is a consecutive sequence of 4 bars and 4 spaces totaling 17X wide. The 417 in PDF417 refers to this codeword structure. Codewords reside between the start pattern on the left, and the stop pattern on the right. This region contains several types of codewords including data codewords, control codewords, and row indicator codewords. Each of these codewords abides by the rules described above. The picture below points out a single PDF417 codeword:
Row Indicators Codewords Adjacent to the start and stop patterns, are PDF's right and left row indicators. These indicators are codewords that are used to store information required to decode the PDF417 symbol. This includes the row number, number or rows, number of columns, and the error correction level used.
Data Codeword Region User data is first encoded into codeword values. These codeword values are then converted into physical codewords represented by bars and spaces as described above. Data codewords are physically located between the left and right row indicator codewords. Below, you can see the Data Codeword Region:
Rows If you look closely at a PDF417 symbol, you will notice that it appears to be made of many "1D-like" barcodes. In reality, it is made up of multiple rows. A PDF417 barcode can have anywhere from 3 to 90 rows. This allows a PDF417 symbol to be reshaped by adjusting the number of rows. The following PDF417 symbol has 5 rows:
Columns A PDF417 symbol is made up of multiple data columns, which are sometimes referred to as the data column area. The number of data columns can vary from 3 to 30, to accomadate user's real estate requirements. These columns contain encoded data, as well as error correction information. Within the data column area, a single PDF417 can contain no more than 928 codewords. An example of a 3 column PDF417 symbol appears below: Error Correction The PDF417 symbology has error correction capability. This capability enables scanners to read the barcode even if it has been torn, written on, or damaged in other ways. How much damage a symbol can withstand depends on the amount of error correction in each PDF417 symbol. The user has the ability to select 1 of 9 error correction levels for each symbol printed. Error correction is specified by selecting a level from 0 to 8. At level 0, a damaged PDF417 cannot be read, but the damage can be detected. At levels 1 through 8, a PDF417 symbol can still be read, even when damaged. As the error correction level increases, more damage can occur to the symbol and still be read. Consequently, the the higher the error correction level, the larger the symbol becomes, while the data capacity goes down. The following table illustrates these facts: