GS1-128 barcode in a PDF

Where the symbol comes from

This API renders HTML to PDF. It does not generate barcodes, so the GS1-128 symbol itself is produced by an encoder in your own stack and placed in the markup, either as inline SVG or as a data URI. That split matters more than it sounds: the encoders are mature and correct, and almost every barcode that will not scan failed for a layout reason rather than an encoding one. Everything below is about the layout half.

What it can encode

The same character set as Code 128, but the data is structured. An FNC1 in the first position tells the scanner this is GS1 data, and the content is then a run of Application Identifiers with their values: (00) is a serial shipping container code, (01) a trade item number, (10) a batch or lot, (17) an expiry date, (21) a serial number. The GS1 specification caps a single symbol at 48 data characters, which is the constraint that most often forces a second symbol.

The check digit, and who computes it

Two separate things, and confusing them is the classic error. The symbol carries the same modulo 103 check character as Code 128, computed by the encoder. Separately, the data inside some Application Identifiers has its own check digit: a GTIN in AI (01) ends with a modulo 10 check that is part of your data and that you must supply correctly.

Size and quiet zone

The two measurements that decide whether a scanner accepts the symbol, and the two most often lost when a design is adjusted to fit.

• X-dimension: 0.25mm to 1.016mm depending on where the label goes. A carton read by a fixed scanner on a conveyor needs the larger end of that range; a label read by hand can use the smaller.
• Quiet zone: 10 times the X-dimension at each end. On a logistics label the standard layout gives the symbol its own band across the label, which is partly to guarantee that space.

Where it is used

• logistics unit labels, where AI (00) carries the serial shipping container code
• pharmaceutical and food traceability, carrying batch and expiry alongside the item number
• variable measure retail products, where the weight is part of the encoded data
• any supply chain where a trading partner has specified GS1 rather than free-form

Placing it in the markup

The quiet zone is padding on the container rather than part of the encoder's output, so it survives a change of encoder. Dimensions are in millimetres, because a bar width is a physical measurement and a percentage makes it depend on whatever the container happens to be.

<!-- Human-readable interpretation shows the AIs in brackets. The
     brackets are NOT encoded in the symbol: they are punctuation for a
     person, and an encoder that puts them in the data is wrong. -->
<figure class="gs1-symbol">
  <img src="data:image/svg+xml;base64,PHN2ZyB4bWxucz0i..." alt="GS1-128">
  <figcaption>(01)05012345678900(10)A4471(17)271130</figcaption>
</figure>

<style>
.gs1-symbol {
  margin: 0;
  padding: 0 8mm;
}
.gs1-symbol img {
  width: 90mm;
  /* Bar height matters here: a symbol read on a moving conveyor needs
     enough height for the scan line to cross it at an angle. */
  height: 32mm;
  display: block;
}
.gs1-symbol figcaption {
  font: 9pt/1.3 "IBM Plex Mono", monospace;
  margin-top: 1.5mm;
  /* The interpretation must be under the symbol and must be readable.
     It is what a person falls back to, and on a pharmaceutical label it
     is what a regulator expects to see. */
  word-break: break-all;
}
</style>

What goes wrong on paper

Encoding the brackets. The human-readable line shows AIs in parentheses so a person can parse it, and those parentheses are not part of the encoded data. A symbol containing literal brackets scans and then fails validation at the receiving end, which is a far more expensive failure than one that does not scan at all.

First thing to check when it will not scan

Check whether the symbol actually starts with FNC1. Without it the scanner reads plain Code 128 and hands the receiving system a string of digits with no structure, which usually presents as data being rejected rather than as a failed scan.

Vector, not a bitmap

Encode to SVG wherever the encoder offers it. A rasterised barcode is resampled when the PDF is printed, and at 203 or 300 dots per inch the bar edges land between printer dots, so the ratio of bar to space drifts by a fraction of a millimetre. That fraction is the whole tolerance. An SVG stays vector inside the PDF and is rendered at the printer's own resolution, which removes the entire failure mode. Where a bitmap is unavoidable, generate it at the final physical size and set image-rendering so nothing smooths the edges.

Frequently asked

Other symbologies

Codes of the same kind, and one from each of the other kinds.