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Added docu for multi printer hack
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@SergeoLacruz
SergeoLacruz committed Apr 7, 2024
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# inventree-zebra-plugin

This is a label printing plugin for [InvenTree](https://inventree.org), 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.
This is a label printing plugin for [InvenTree](https://inventree.org), 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.
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.
Error handling is very basic.

## Installation

Expand All @@ -19,35 +21,35 @@ The plugin is on pypi. Install this plugin using pip with the following command:
```
pip install 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
### 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.
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
### 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
### 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
It is directly converted to a SD command in ZPL. If your black areas tend to blur out reduce the
darkness.

### Dots per mm
### 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
### 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.
Expand All @@ -60,8 +62,8 @@ is passed directly to the printer without any checks. So be careful when editing
here.

## 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:
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 {
Expand All @@ -75,13 +77,31 @@ that defines the label size as shown below:
}
```

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.
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 hack
We had 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. InvenTree is not yet able to handle different
printers. So I added a multi printer hack. You can define a key with an IP
address in the label meta data:

```
{"ip_address":"xxx.yyy.zzz.eee"}
```

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 can be overwritten so far. All other settings remain.

## 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.
for the IP connection to the printer.

The next part is this:

Expand All @@ -97,7 +117,7 @@ class ZebraLabelPlugin(LabelPrintingMixin, SettingsMixin, IntegrationPluginBase)
```

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
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)
Expand All @@ -123,9 +143,9 @@ SETTINGS = {
```

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.
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.
one is the description in the UI.
After that we need to define a function:

```python
Expand All @@ -143,18 +163,18 @@ The kwargs is a dict with the following keys:
- 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.
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.
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)
Expand All @@ -165,49 +185,49 @@ 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 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 matters
## 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.
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
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.
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
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.
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 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.
need an adjustment from time to time.

### Alternative
You can also bypass the InvenTree template and printing system and directly create ZPL from
You 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 the [inventree-zpl-plugin](https://github.com/yellowcrescent/inventree-zpl-plugin)
that does exactly that.
If you are interested in this way have a look at the [inventree-zpl-plugin](https://github.com/yellowcrescent/inventree-zpl-plugin)
that does exactly that.

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