Printing barcodes

The most common cause of “my barcode does not scan” is not encoding – the bars are too small. Scanners need the narrowest bar (1D) or one module (2D) – the X-dimension – to be wide enough for their optics to resolve. As features shrink below that threshold, read rates drop like a stone.

Most symbology specs leave X-dimension to the application. The figures below are practical recommendations for the typical pyStrich print chain: a consumer-grade laser printer (300-600 DPI) plus either a low-cost handheld scanner or a modern mobile phone camera. Push the X-dimension up if either end of that chain is weaker – worn ribbons, blurry cameras, scans from a distance – or if the printed surface is reflective or contoured.

Symbology	X-dimension	Notes
Code 39	0.19 - 0.50 mm	0.25 mm is a safe default for industrial labelling.
Code 128	0.19 - 0.50 mm	0.25 mm reads reliably with most fixed and handheld scanners.
EAN-13 (retail)	0.26 - 0.66 mm	GS1 specifies a nominal X of 0.33 mm (100% magnification); retail scanning typically allows 80%-200% of nominal.
Data Matrix	0.25 - 0.50 mm	~0.3 mm is the common practical floor for handheld imagers.
QR Code	0.25 - 0.50 mm	Mobile phone cameras typically need >=0.4 mm at arm’s length.
PDF417	0.25 - 0.50 mm	Row height must be >=3x the X-dimension (pyStrich default).
Aztec Code	0.38 - 0.76 mm	Most reliable range; smaller works with good imagers, but mobile-camera reads degrade quickly below ~0.4 mm.

For print-quality verification, see ISO/IEC 15415 (2D) or ISO/IEC 15416 (1D); for retail and supply-chain applications, follow the GS1 General Specifications.

Tip

Reach for SVG or EPS where your toolchain supports them. Vector output scales losslessly, so you size the symbol at layout time rather than locking in a physical size at encode time.

Vector output (SVG / EPS)

For SVG and EPS, pyStrich doesn’t determine the print size at all. The output is resolution-independent – pick any bar_width (or cellsize), and let the consuming layout tool scale to the X-dimension you need:

• SVG: set width and height in HTML / CSS, or via the viewBox in your design tool.

• EPS: scale at inclusion time – \includegraphics[width=40mm]{...} in LaTeX, or via the placement size in Illustrator / InDesign.

The EPS bar_width is in PostScript points (1 pt = 1/72 inch ≈ 0.353 mm), so the default already produces a physically sensible size if you don’t override it at inclusion time.

Tip

For PDF output, embed the SVG in an HTML template and render with WeasyPrint. This is a clean route to multi-up label sheets, packing slips and similar print-ready documents.

Raster output (PNG)

PNG is the only output format where bar_width / cellsize fixes the print size, because raster images don’t scale losslessly. Working back from a target X-dimension and printer DPI:

bar_width (px) = X-dimension (mm) * DPI / 25.4

At 300 DPI, an X-dimension of 0.25 mm requires bar_width=3 (rounded up from 2.95). At 600 DPI, the same X-dimension requires bar_width=6.

Round up to the nearest integer pixel; rounding down pushes the X-dimension below the readable limit.

Quiet zones

Every symbology requires white space (the quiet zone) around the symbol so scanners can locate its boundaries. pyStrich emits a quiet zone automatically; do not crop into the white margin when compositing the output into another image.

Symbology	Quiet zone applied by pyStrich
Code 39	10x bar_width on each side.
Code 128	10x bar_width on each side.
EAN-13	11x bar_width left, 7x right (mandated by GS1).
Data Matrix	Configurable via quiet_zone (default 2 modules, spec minimum 1 module).
QR Code	4 modules on each side (mandated by spec).
PDF417	Configurable via quiet_zone (default 2 modules, spec minimum 2 modules on every side).
Aztec Code	Configurable via quiet_zone (default 2 modules; spec does not require any quiet zone).

Damage tolerance

2D barcodes offer some tolerance to physical and environmental damage – scuffing, smudging, marking, partial occlusion – thanks to their Reed-Solomon error correction. This tolerance can be improved by increasing the error correction level, increasing print dimensions, and positioning the barcode away from the edges of the label or document (which take the most wear). Using suitable media and printing is essential.

For harsh environments, if the encoded data has a predictable length and character set, raise the error correction level as high as the available label or document space permits at a sensible module size.

Error correction protects only the data area: the symbol’s fixed patterns (finder, locator or bullseye) must remain identifiable for the decoder to lock on at all.

The examples below each survive a dramatic diagonal smudge placed clear of those fixed patterns.

Aztec (ecc=50)	Data Matrix	QR Code (ecl="H")

Verification

If a printed barcode does not scan reliably:

1. Try to scan it on screen first. If it fails on screen, the encoding or sizing is wrong; if it succeeds on screen but fails in print, the print process is the issue.

2. Measure the X-dimension with a ruler or loupe and verify the quiet zone is intact – not cropped, not printed against a coloured background.

3. Check print contrast – low-DPI thermal printers or worn ribbons can produce bars that are too grey to be read.

For production deployments printing barcodes at scale, consider using a print-quality verifier (an instrument that scores symbols against ISO/IEC 15415 / 15416). A passing grade once during integration is worth more than re-printing a thousand failed labels.