On 10/3/24 00:20, Max Nikulin wrote:
Looking at a QR code likely having ~75 pixels per inch I find it
unreasonably small for delivery labels. I am in doubts if its
redundancy is high enough to reliably recognize it if it would be
scratched during delivery. Another limitation may be stability of
optics in scanners in respect to labels. This one is printed using a
laser printer with resolution at least 600 dpi. Each QR code pixel has
still 4x4 printer dots in the case of 300dpi, so when image is
properly aligned, printer quality is not an issue.
Standard thermal label printers are 203DPI (8 dots per mm).
The problem with printing QR codes and bar codes is not the resolution
of the printer but getting the the software drivers to produce a bitmap
that is the same resolution as the printer resolution and aligned at the
pixel level when it goes to the printer
I have written a CUPS driver for a thermal printer and routinely print
labels with barcodes and QR style codes. I have immense problems with
PDF labels generated by my local postal service which are generated for
a 300 DPI printer. The barcodes especially end up with too thick or too
thin bars where the pixel mapping doesn't align. These will not scan.
I have asked the postal service to generate labels at 203dpi which will
print just fine at 600 dpi and so work with laser and thermal printers,
but they will not cooperate.
I am in the process of developing software to take the issued labels,
extract the barcode and QR fields, decode them, and then generate new
bitmaps at 203 dpi to replace the misaligned bitmaps. This is not
straight forward as the postal service does not fully comply with the
coding rules.
In the meantime I have to extract the QR and bar code fields, enlarge
them, and print on separate labels.
As far as printer drivers work, CUPS typically generates a bitmap from a
source document such as PDF or postscript, and passes that to a raster
printer. This in itself can be a problem as any stage of the process of
generation of pdf, 'printing', and rasterisation can misalign and
distort pixels. To be absolutely sure of clean output you need a custom
program directly driving the printer.
