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This is a label printing plugin forInvenTree, which providessupport for Zebra Label printers. It allows two modes of operation:

• Use the ZPL library to convert the png data provided by InvenTree to Zebra's bitmapformat and send this to the printer.
• Write a ZPL template and let the printer do the rendering.

It can output the print data either to a local printer connected to the computer viaUSB or to a network printer with an IP address. The output can be configured in theInvenTree plugin user interface. A preview using the labalary API is also possible.The plugin allows to print the same label multiple times e.g. to put in on the frontand back side of a box. Just increase the Number of labels parameter in the printform. This feature just adds a ^PQ command to the ZPL code. The maximum value is 99.

The latest release of the plugin is on pypi. Install this plugin using pip withrthe following command:

pip install inventree-zebra-plugin

Here you can chose between local printer, network printer or the labelary.com API.The last one is useful for preview of labels, especially when ZPL templates are used.Default value is a local printer.

In case you use an IP printer set the IPv4 address here. This can be overwrittenby the template. See below.

In case you use an IP printer set the port number here. The default port number is 9100.

In case of a local printer set the device here. The plugin actually puts the data directly to thedevice /dev/usb/lp0. No printer spooler is involved so far.

The image from pillow comes in greyscale. The plugin converts it ti pure BW because this gives a muchbetter print result. The threshold between black and white can be adjusted here.

This 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 thedarkness. This can be overwritten by the template.

This sets the resolution of the printer. You can choose between 8, 12 and 24dpmm depending on your printer model.

This string added to the printer output. It can be used to set special commandse.g. label rotation, mirror or white on black. Please refer to the ZPL manualfor more information.

Zebra printers store settings after printing. So if a rotated label has beenprinted all following label will be rotated unless you change it. The defaultsets the printer to settings that have been useful for me. You might want tochange it according to your requirements. Please keep in mind that this stringis passed directly to the printer without any checks. So be careful when editinghere.

Turn this switch on to display a collection of all IP printers below on this page.

The label needs a template described in html and css. The template shouldstart 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 panelin the label section. These values have to fit the label size that is inthe printer. See the example templates for details on template definition.

We have the requirement to print labels in different sizes. As we do notwant to change the reel for each print we set up a second printer loadedwith a different label size. The plugin allows to add multiple printerscontrolled by the label template. Just define a key with an IPaddress in the label meta data:

{"ip_address":"xxx.yyy.zzz.eee"}{"darkness":xx}

If the printer driver finds that key, the IP address from the printer settingsis overwritten with the address from the meta data. So the print will end upin another printer.

Only the IP address and darkness can be overwritten so far. All other settings remain.

The InvenTree printer system uses a graphical representation of the label. The label is describedin HTML, converted to a pixel graphic and printed. The advantage is independency from printermodels 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 ishardly readable using my mobile. The right one reads easily even as it is smaller.

The printer resolution is 8 dots per mm resulting in a dot size of 0.125mm. The QR code pixeland the printer pixel size should be integrally divisible. The code in the picture has 21pixels 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 differentscaling takes place and the result might be worse. If you like a larger printout select moredots per pixel. From a certain size upwards the value does not matter any more because the codegets large enough to be readable in any quality.

Zebra 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 of 30. The black dots tend to blur out a bit resultingin smaller white areas. The right code was printed with a value of 25 resulting in larger whitepixels. 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 mightneed an adjustment from time to time.

You can also bypass the InvenTree print engine and render the label inside the printer.The printer knows how to render the label for best quality. Inspired by theinventree-zpl-plugin a similarfunction was aded to the zebra printer driver. You can write a ZPL template and uploadit to the InvenTree Label templates as usual. Add a command to the template's metadata:

{"zpl_template": "True"}

In that case the printer driver ignores the picture rendered by WeasyPrint. Insteadit calls the render_to_string function of the template and sends theresult to the printer. The result can look like:

The upper label was created using this template:

{% autoescape off %}^FT30,25^A0N,18,22^FDIPN^FS^FT150,30^FB100,1,,C,,^A0N,24,32^FDACME^FS^FT320,25^A0N,18,22^FD{{ item.pk }}^FS^FT100,70^FB200,2,,C,,^A0N,18,22^FD{{ part.name }}^FS^FT100,100^FB200,1,,C,,^A0N,18,22^FD{{ part.manufacturer_parts.first.manufacturer.name }}^FS^FT30,150^FB340,1,,C,,^A0N,30,40^FD{{ part.IPN }}^FS^FT20,210^FB360,3,,L,,^A0N,18,22^FD{{ part.description }}^FS^FT15,110^BQ,2,3^FDQA,{{ part.IPN }}^FS^FT310,130^BQ,2,3^FDQA,{{ qr_data }}^FS{% endautoescape %}

Autoescape must be off. We do not need &quot and similar escapes here.Context variables can be used as usual. Start and stop commands ^XA and^XZ are added by the driver and not allowed in the template. The printerinit string as given in the settings is added as usual.

!!! warning "Limitation"ZPL commands starting with backslash like \& cannot be used so far.

The printer driver allows an output device called "preview". If this is selectedthe ZPL code is sent to the API of labelary.com. The API sends back pdf datawhich is displayed in a new browser window. This is helpful while writing ZPLtemplates but works with HTML templates too. Please be careful and do not sendconfidential information to the API.

In case you need to pass a proxy for the POST requests set the environmentvariables PROXY_CON and PROXY_URL on the server. The plugin does not havesettings for this.

With the multi printer feature it can happen that you have several printers inyour setup. When the Get Printer Info switch is set ON, the driver calls each printeronce a minute and collects some info about it. It calls the printer configuredin the settings as well as all printers it finds in the label templates.In case a printer is unreachable, an error message is shown. If a printer is usedin several templates it is listed only once.

The printer info feature works for local USB printers too.

This info is only visible in the classical user interface of InvenTree. ForPUI the UI api as changed. The plugin does not support that so far.

First 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 socketfor 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 labels"SLUG="zebra"TITLE="Zebra Label Printer"

The name of the class can be freely chosen but should be different from SLUG. Otherwise it does notshow up. 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 andneed to be like that. The values are found in the UI as shown in the picture 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 secondone 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_data
• user
• filename
• label_instance
• item_instance
• width
• height
• png_file

The item_instance is the part to be printed. This allows direct access to all part data. The arguments width and heightcome 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)li.set_darkness(darkness)...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 scalingcan 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.

Happy printing.