Package: inventree-zebra-plugin
inventree-zebra-pluginThis is a label printing plugin forInvenTree, which provides support for Zebra Label printers .
It was only tested with  GK420T but should work for other ZPL printers too. It uses the ZPL library to
convert the png data provided by InvenTree to Zebra's bitmap format.It can output the print data either to a local printer connected to the computer via USB or to a network printer
with an IP address. The output can be configured in the InvenTree plugin user interface.Error handling is very basic.InstallationThe plugin is on pypi. Install this plugin using pip with the following command:pip install inventree-zebra-pluginConfiguration OptionsPrinter InterfaceHere you can chose between Local printer or network printer. Default value is a local printer.IP addressIn case you use an IP printer set the IPv4 address here.PortIn case you use an IP printer set the port number here. The default port number is 9100.Local DeviceIn case of a local printer set the device here. The plugin actually puts the data directly to the
device /dev/usb/lp0. No printer spooler is involved so far.ThresholdThe image from pillow comes in greyscale. The plugin converts it ti pure BW because this gives a much
better print result. The threshold between black and white can be adjusted here.DarknessThis is a value that influences the darkness of the print. Allowed values are 0 (white) to 30 (black).
It is directly converted to a SD command in ZPL. If your black areas tend to blur out reduce the
darkness.Dots per mmThis sets the resolution of the printer. You can choose between 8, 12 and 24
dpmm depending on your printer model.Printer initThis string added to the printer output. It can be used to set special commands
e.g. label rotation, mirror or white on black. Please refer to the ZPL manual
for more information.Zebra printers store settings after printing. So if a rotated label has been
printed all following label will be rotated unless you change it. The default
sets the printer to settings that have been useful for me. You might want to
change it according to your requirements. Please keep in mind that this string
is passed directly to the printer without any checks. So be careful when editing
here.Label TemplateThe label needs a template described in html and css. The template should start with a page definition
that defines the label size as shown below:@page {
        {% localize off %}
        height: {{ height }}mm;
        width: {{ width }}mm;
        {% endlocalize %}
        padding: 0mm;
        margin: 0px 0px 0px 0px;
        background-color: white;
    }The height and width parameters are defined in the InvenTree admin panel in the label section. These values
have to fit the label size that is in the printer. See the example templates for details on template definition.How it worksFirst import all the stuff you need. Here we use the translation mechanism from Django for multi language support.
The import the InvenTree libs and everything you need for plugin. Here we have ZPL for the Zebra bitmaps and socket
for the IP connection to the printer.The next part is this:classZebraLabelPlugin(LabelPrintingMixin,SettingsMixin,IntegrationPluginBase):AUTHOR="Michael Buchmann"DESCRIPTION="Label printing plugin for Zebra printers"VERSION=ZEBRA_PLUGIN_VERSIONNAME="Zebra"SLUG="zebra"TITLE="Zebra Label Printer"The name of the class can be freely chosen. You reference to it in the entry_points section of the setup.py file.
The parameters need to be like in the example. Then there is the description block. The keywords are fixed and
need to be like that. The values are found in the UI as shown in the pictures below.Then we add the configuration parameters.SETTINGS={'CONNECTION':{'name':_('Printer Interface'),'description':_('Select local or network printer'),'choices':[('local','Local printer e.g. USB'),('network','Network printer with IP address')],'default':'local',},'PORT':{'name':_('Port'),'description':_('Network port in case of network printer'),'default':'9100',},}We need to define a dict with the name SETTINGS. Please be aware the keys need to be in all CAPITAL letters like CONNECTION.
Simple parameters are just text strings like the port. We can set a default. The name and description shows up in the UI.
Instead of a simple text we can also use choices. The first string like "local" it the key you use in the code. The second
one is the description in the UI.
After that we need to define a function:defprint_label(self,**kwargs){The kwargs is a dict with the following keys:pdf_datauserfilenamelabel_instanceitem_instancewidthheightpng_fileThe item_instance is the part to be printed. This allows direct access to all part data. The arguments width and height
come from the settings of the label in the admin interface. NOT from the html template.
For the Zebra printer we use the png_file. This is a PIL (python Pillow) object with the graphic of the label in PNG format.
The PIL object is a greyscale image. Because the printer can just print pure BW we convert this to a BW picture.fn=lambdax:255ifx>Thresholdelse0label_image=label_image.convert('L').point(fn,mode='1')The threshold can by modified by a plugin parameter. 200 is a good starting value.  This trick gives much better prints.
We can put the result of this directly into the ZPL library.l=zpl.Label(Height,Width,dpmm)l.origin(0,0)...l.write_graphic(label_image,Width)l.endorigin()Width and Height define is the size of the label in millimeters as described above.
The third parameter is the resolution of the printer in dots per mm.
write_graphic converts the pillow data to ZPL.The plugin was tested with a labels of various sizes defined using css and html. The DPI scaling
can be chosen in the InvenTree settings. 800 is a good value because it gives high quality.The rest of the code is just output to the printer on different interfaces.Quality mattersThe InvenTree printer system uses a graphical representation of the label. The label is described
in HTML, converted to a pixel graphic and printed. The advantage is independency from  printer
models and systems. Disadvantage is larger data and quality problems with darkness and scaling.
Let's have a look at the following printout:Both codes have been printed with the same printer on the same reel. The left one is
hardly readable using my mobile. The right one reads easily even as it is smaller.Secret 1, ScaleThe printer resolution is 8 dots per mm resulting in a dot size of 0.125mm. The QR code pixel
and the printer pixel size should be integrally divisible. The code in the picture has 21
pixels plus one in the frame, so 23 pixel. The frame is set in the HTML description.{% qrcode qr_data border=1 %}I selected two dots per pixel. So 23 * 2 * 0.125 = 6.125mm. If the size is something different
scaling takes place and the result might be worse. If you like a larger printout select more
dots per pixel. From a certain size upwards the value does not matter any more because the code
gets large enough to be readable in any quality.Secret 2: DarknessZebra printers allow to set the darkness of the print in values between 0 (white) and 30 (max)
The left code was printed with a value 0r 30. The black dots tend to blur out a bit resulting
in smaller white areas. The right code was printed with a value of 25 resulting in larger white
pixels.  The darkness values are just examples. Your values will differ based on printer model,
media type and printer age. The printer head tends to wear out and the darkness value might
need an adjustment from time to time.AlternativeYou can also bypass the InvenTree template and printing system and directly create ZPL from
the parts data. The printer knows best how to render the label and the print quality is best.
If you are interested in this way have a look at theinventree-zpl-pluginthat does exactly that.