[REPO] Add inventree-zebra-plugin (#117)

* [REPO] Add inventree-zebra-plugin

* Add SergeoLacruz to publishers

* update readme

_publishers/SergeoLacruz.md

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+---
+short_name: SergeoLacruz
+name: Michael
+github: SergeoLacruz
+website:
+---

_repo/inventree-zebra-plugin.md

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+---
+name: inventree-zebra-plugin
+author: SergeoLacruz
+license: MIT
+open_source: true
+stable: true
+maintained: true
+pypi: false
+package_name:
+github: https://github.com/SergeoLacruz/inventree-zebra-plugin
+gitlab:
+source:
+issue_tracker: https://github.com/SergeoLacruz/inventree-zebra-plugin/issues
+website:
+categories: Printer
+tags: Label Printer Zebra ZPL
+---
+Zebra Label Printer Plugin for Inventree
+
+This is a label printing plugin for [InvenTree](https://inventree.org), which provides support for Zebra Label printers .
+It supports printing to just GK420T with one label size (50x30) so far. So it is very simple. It uses the zpl library to
+convert the png data provided by InvenTree to Zebras 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. So the source code is 
+a good example for this. 
+
+Error handling is very basic. 
+
+## Installation
+
+Install this plugin using pip with the following command::
+
+```
+pip install git+https://github.com/SergeoLacruz/inventree-zebra-plugin
+```
+ 
+## Configuration Options
+### Printer Interface
+Here you can chose between Local printer or network printer. Default value is a local printer.
+
+### IP address
+In case you use an IP printer set the IPv4 address here.
+
+### Port 
+In case you use an IP printer set the port number here. The default port number is 9100.
+
+### Local Device
+In case of a local printer set the device here. The plugin actually puts the data directly to the
+device libe /dev/usb/lp0. No printer spooler is involved so far. 
+
+## How it works
+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 socket
+for the IP connection to the printer. 
+
+The next part is this:
+
+```python
+class ZebraLabelPlugin(LabelPrintingMixin, SettingsMixin, IntegrationPluginBase):
+
+    AUTHOR = "Michael Buchmann"
+    DESCRIPTION = "Label printing plugin for Zebra printers"
+    VERSION = ZEBRA_PLUGIN_VERSION
+    NAME = "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.
+
+![Admin](https://github.com/SergeoLacruz/inventree-zebra-plugin/blob/master/pictures/plugin_admin.png)
+![Config](https://github.com/SergeoLacruz/inventree-zebra-plugin/blob/master/pictures/plugin.png)
+
+
+Then we add the configuration parameters.
+```python
+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 ta simple test 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:
+
+```python
+def print_label(self, **kwargs){
+```
+
+The kwargs is a dict with the following keys:
+
+- pdf_data
+- user
+- filename
+- label_instance
+- width
+- height
+- png_file
+
+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. 
+
+```python
+fn = lambda x : 255 if x > Threshold else 0
+label_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 this directly into the zpl library. 
+
+```python
+l = zpl.Label(50,30,8)
+l.origin(0, 0)
+l.write_graphic(label_image, 50)
+l.endorigin()
+```
+
+50,30 is the size of the label in millimeters. The third parameter is the resolution of the printer in
+dots per mm. As the Zebra printer has 200dpi we put an eight here. write_graphic converts the pillow data
+to zpl. 50 is the with of the image in mm. 
+
+The plugin was tested with a label of 50x30 mm defined using css and html in InvenTree as shown below. The DPI scaling
+can be chosen in the InvenTree settings. 400 is a good value because it is just double of the printers
+resolution. If you save the pillow data to a png file you get a size of 788x473 which fits well to that data.
+
+```
+<style>
+    @page {
+        width: 50mm;
+        height: 30mm;
+        padding: 0mm;
+        margin: 0px 0px 0px 0px;
+        background-color: white;
+    }
+```
+
+The rest of the code is just output to the printer on different interfaces.