inventree-zebra-plugin 0.9.0
Zebra label printer plugin for InvenTree
License: MIT
Requires Python: >=3.6
Keywords: inventree label printer printing inventory

[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)


# inventree-zebra-plugin

This is a label printing plugin for [InvenTree](https://inventree.org), which provides
support 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 bitmap
  format 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 via
USB or to a network printer with an IP address. The output can be configured in the
InvenTree 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 front
and back side of a box. Just increase the Number of labels parameter in the print
form. This feature just adds a ^PQ command to the ZPL code. The maximum value is 99.


## Installation

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

```
pip install inventree-zebra-plugin
```

## Configuration Options
### Printer Interface
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.

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

### 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 /dev/usb/lp0. No printer spooler is involved so far.

### Threshold
The 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.

### Darkness
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 the
darkness. This can be overwritten by the template.

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

### Printer init
This 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.

### Get Printer info
Turn this switch on to display a collection of all IP printers below on this page.

## Label Template
The 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.

## Multi printer usage
We have the requirement to print labels in different sizes. As we do not
want to change the reel for each print we set up a second printer loaded
with a different label size. The plugin allows to add multiple printers
controlled by the label template. Just define a key with an IP
address in the label meta data:

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

If the printer driver finds that key, the IP address from the printer settings
is overwritten with the address from the meta data. So the print will end up
in another printer.

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

## Quality matters
The 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:

![QRcodes](https://github.com/SergeoLacruz/inventree-zebra-plugin/blob/master/pictures/qr.png)

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, Scale
The 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: Darkness
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 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.

## In printer rendering
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 the
[inventree-zpl-plugin](https://github.com/yellowcrescent/inventree-zpl-plugin) a similar
function was aded to the zebra printer driver. You can write a ZPL template and upload
it 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. Instead
it calls the render_to_string function of the template and sends the
result to the printer. The result can look like:

![Label Example](https://github.com/SergeoLacruz/inventree-zebra-plugin/blob/master/pictures/example_label.png)

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 printer
init string as given in the settings is added as usual.

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

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

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

## Getting printer info
With the multi printer feature it can happen that you have several printers in
your setup. When the Get Printer Info switch is set ON, the driver calls each printer
once a minute and collects some info about it. It calls the printer configured
in 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 used
in several templates it is listed only once.

The printer info feature works for local USB printers too.

![Printer Info](https://github.com/SergeoLacruz/inventree-zebra-plugin/blob/master/pictures/printer_info.png)

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

## Inventree 1.0
The UI features of the driver are minimal. There is only the printer status list
in the settings. The function get_settings_content is replaced by get_admin_context.
Some lines of javascript code in ./static and some table tweaks are needed.
That's basically it. The printer logic remains the same and works fine with Inventree 1.0.

The machine interface is not used yet. We have no need for it. Our method to support
several printers binds the template fix to a dedicated printer. No way to print a
50x30 label on a printer loaded with 20x12 labels. the only drawback is that the
templates metadata field needs to be edited manually in the admin interface. But
this is done seldom.

## 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 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 not
show 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 and
need to be like that. The values are found in the UI as shown in the picture below.

![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 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:

```python
def print_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 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.

```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 of this directly into the ZPL library.

```python
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 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.


## ZPL deep dive
in Normal Mode (no ZPL templates) the plugin uses the ZPL library to convert the png data
provided by InvenTree to Zebra's bitmap format. ZPL code needs to have a certain sequence
of commands to properly work. Some ZPL commands are generated by the ZPL library, others
can be included using the "Printer Init" parameter of tthe plugin. Here is a closer look
at the ZPL code that the plugin generates. These commands are generated by the ZPL
library and calculated based on the label size and darkness parameter:

| Command | Description             |
| ------- | ----------------------- |
| ^XA     | Start of ZPL file       |
| ^PW400  | Width of label in dots  |
| ^LL240  | Length of label in dots |
| ~SD22   | Darkness                |
| ^LH0,0  | Label home              |

The following commands are from the Printer Init parameter. These are just the defaults.
The can be edited based on our personal setup.

| Command  | Description                                                   |
| -------- | ------------------------------------------------------------- |
| ~TA000   | tear off                                                      |
| ~JSN     | back feed                                                     |
| ^LT0     | top offset                                                    |
| ^MNW     | media tracking. This is for non continuous labels             |
| ^MTT     | media type. This is for thermo transfer. Adjust to your needs |
| ^PMN     | no mirror print                                               |
| ^PON     | normal orientation                                            |
| ^PR2,2 2 | Speed 2 inch per second                                       |
| ^LRN     | No reverse print (white on black)                             |

Finally there are commands that are again inserted by the ZPL library:

| Command| Description        |
| ------ | ------------------ |
| ^FO0,0 | Start field        |
| ^PQ1   | Label count (one)  |
| ^GFA   | Graphics bit field |
| ...    | Graphics data      |
| ^FS    | End of field       |
| ^XZ    | End of file        |

For more details on the commands refer to the ZPL programming guide.
Happy printing.