Decodability and 2D Barcode Symbols

Recently we were asked why the important—and sometimes confusing—parameter Decodability appears in the ISO standard for 1D barcodes but not in the standard for 2D barcodes.

Decodability is based on the reference decode algorithm for the particular symbology, and is defined as the amount of tolerance remaining after the printing process.

Decodability is Location and Dimensional Accuracy

Those familiar with the ISO 15416 Standard for 1D barcodes are acquainted with Decodability. This is the only ISO parameter based on measuring the widths of elements (bars and spaces) and their locations in a 1D barcode. All of the other parameters in this standard are based on reflectance and calculations of two reflectance values (Rmin and RMax). Recently we were asked why Decodability is not included in ISO 15415 for 2D barcodes. Aren’t element widths and positions just as important in 2D symbols?

Of course they are. But first, let’s take a closer look at the differences in scanning 1D and 2D barcodes. A scanner decodes a 1D barcode either from left to right or from right to left. Differences in the start/stop patterns or in the formatting of the encoded data inform the scanner of what is left and right, and how to correctly sequence the captured data.

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Sidebar: Digital Scanning

Sidebar: Technically, a digital camera-based scanner doesn’t scan the barcode directionally. It captures the entire image of the barcode. Decoding is done by analyzing the captured data and extracting it directionally.

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The decode algorithm relies on the accurate placement of the elements and their correct widths. Scanners do not measure wide and narrow elements in a linear sense: they detect differences in reflectivity. For example, 59 pulses of energy that start with two equally brief pulses of low reflectivity and end with two equally brief pulses of energy must be a UPC symbol. Pulses of different durations in between are the decoded information for a consumer product. This is an oversimplification, but you get the idea. It’s really quite remarkable.

Scanners See Light, Not Locations and Dimensions

If that barcode gets distorted somehow, the pulses of energy also distort. Let’s say the barcode was printed on a flexible plastic that was stretched in installation or handling. Even if the number of transitions from light to dark comes to the same familiar total—30 bars and 29 spaces in the case of a UPC, the durations and reflectance values are no longer recognizable to the scanner. This is what Decodability measures and grades. Here is a helpful visualization of Decodability from an Axicon linear verification report:

As with the other ISO 15416 parameters, low Decodability can imply that a barcode will fail to scan. it is that important, why is it not also a parameter in the standard for 2D barcode symbols?

Well actually it is, but it goes by a different name (or two), and for good reasons.

 

2D Decodability has (2) Names

Unlike 1D barcodes which are, by definition, encoded and decoded in one axis, 2D barcodes accomplish the same thing in a serpentine path on two axes. There is another important difference. Encoded information, called code words, is comprised of elements of the same size, clustered together. Elements in certain 1D barcodes are not comprised of a standard-sized building block. For example, UPC and Code 128 have many different element widths. Just like elements in a 1D barcode, the clusters of elements of the same size in a 2D barcode must be placed in predictable locations, and, just like 1D barcodes, they can withstand a defined amount of distortion.

This is done by placing 2D barcode elements in a grid pattern. Furthermore, the grid must be uniform vertically and horizontally. These two attributes are the GNU (Grid Nonuniformity) and ANU (Axial Nonuniformity) parameters in the 2D ISO Standard.

There are other similarities—and subtle differences—between the ISO Standards for 1D and 2D barcodes. We will discuss these in later articles.

 

If you have questions or comments about this or other barcode quality and compliance issues, contact us here. There you can also schedule a free 15-minute conversation.