[notpachet]

As someone who has spent an unfortunate amount of time working on shipping label software for ecommerce companies, my advice is to stay the hell away from ZPL. It's the layout language equivalent of C -- too many low-level footguns. Just send 4x6 PDF's to your thermal printer and be done with it. There are so many more tools, of much better quality, available for composing PDF's than you'll ever find for ZPL or other vendor-specific layout languages.

Sure, if you know you're always going to be using a Zebra printer, and you control the whole pipe from software to the physical label, then you're going to have a lot more explicit control if you use ZPL over a PDF. But the second any of those presuppositions change, it's back to the drawing board. (Actually, even the Zebra assumption isn't sound, because they themselves are not always 100% reliable in following their own specifications. And the specifications... boy howdy.)

The old argument in favor of ZPL is that you can be much more precise with the instructions you're sending to your thermal, resulting in higher print quality, and therefore more reliable barcode scans and OCR. But the world doesn't stand still, and shipping carriers have been steadily improving their scanning capabilities over the last 20 years, even while thermal printer capabilities have remained largely stagnant. After all, USPS has to also optimize for people who are printing labels on shitty desktop inkets. The increase in barcode quality you'd get out of using ZPL is overkill.

TLDR: Save yourself a lot of heartache and just sling PDF's. Postscript/Ghostscript is so much easier to cajole into cooperation.

[tcas]

As someone who's dealt with these protocols as well I agree.

I find the print quality of PDFs perfect on Zebra's as long as you keep the following in mind:

  - Make sure the DPI of the document is correct (e.g. 203dpi)
  - Make sure the document does not need to be scaled (and scaling to fit is disabled) (for 4x6 that means 812 x 1218 pixels max)
  - Make sure the document is black and white (no grayscale)

ZPL is a beast.

Some PDFs embed a single image in them, in which case you can do the following;

  pdfimages -all <input_file>.pdf <out_prefix>
  convert <generated_png>.png -filter point -density 203 -units PixelsPerInch out.png

And then print the generated png.

[pastudan]

Completely agree. I tried scaling down the PDFs but Ghostscript on the Raspberry Pi is far slower than just scaling the 1200px wide PNG to 800px. I still got very readable barcodes, and a 100% delivery rate on my first batch of orders, even with that downsampling.

I could be wrong, but I don't think pixel perfect barcodes are necessary with modern scanners. The only thing going for ZPL is the ability to store the label data as simple ASCII text.

[kayodelycaon]

Did you try using imagemagick to convert to the pdf?

[eyelidlessness]

Heck, if:

- you can target PDF

- you’re married to a JS stack

- your motivation is (quoting the article)

> When you purchase a shipping label through them, they give you the output in either a PDF, PNG, or ZPL format. ZPL would be ideal, since it's just ASCII text which can easily be modified, or even stored in a database along side each order.

It seems like a much more reasonable intermediate format is SVG or JSX, both of which have mature to-PDF solutions, and both of which have much more well-known manipulation stories. Of course, that doesn’t lead to an absolutely bonkers blog title like this. But for anyone considering a similar setup, there are much more conservative options.