micropython-font-to-py/FONT_TO_PY.md

# font_to_py.py

Convert a font file to Python source code.

# Usage

``font_to_py.py`` is a command line utility written in Python 3. It is run on a
PC. It takes as input a font file with a ``ttf`` or ``otf`` extension and a
required height in pixels and outputs a Python 3 source file. The pixel layout
is determined by command arguments. By default fonts are stored in variable
pitch form. This may be overidden by a command line argument.

Further arguments ensure that the byte contents and layout are correct for the
target display hardware. Their usage should be specified in the documentation
for the device driver.

Example usage to produce a file ``myfont.py`` with height of 23 pixels:  
``font_to_py.py FreeSans.ttf 23 myfont.py``

## Arguments

### Mandatory positional arguments:

 1. Font file path. Must be a ttf or otf file.
 2. Height in pixels.
 3. Output file path. Must have a .py extension.

### Optional arguments:

 * -f or --fixed If specified, all characters will have the same width. By
 default fonts are assumed to be variable pitch.
 * -x Specifies horizontal mapping (default is vertical).
 * -b Specifies bit reversal in each font byte.

Optional arguments other than the fixed pitch argument will be specified in the
device driver documentation. Bit reversal is required by some display hardware.

## The font file

Assume that the you have employed the utility to create a file ``myfont.py``. In
your code you will issue

```python
import myfont
```

The ``myfont`` module name will then be used to instantiate a ``Writer`` object
to render strings on demand. A practical example may be studied
[here](https://github.com/peterhinch/micropython-samples/blob/master/SSD1306/ssd1306_test.py).
The detailed layout of the Python file may be seen [here](./DRIVERS.md).

# Dependencies, links and licence

The code is released under the MIT licence. It requires Python 3.2 or later.

The module relies on [Freetype](https://www.freetype.org/) which is included in most Linux distributions.  
It uses the [Freetype Python bindings](http://freetype-py.readthedocs.io/en/latest/index.html)
which will need to be installed.  
My solution draws on the excellent example code written by Daniel Bader. This
may be viewed [here](https://dbader.org/blog/monochrome-font-rendering-with-freetype-and-python) and [here](https://gist.github.com/dbader/5488053).

# Appendix: RAM utilisation Test Results

A font file was created, frozen as bytecode and deployed to a version 1.0
Pyboard. The font was saved as variable pitch with a height of 19 pixels. The
following code was then pasted at the REPL:

```python
import gc, micropython
gc.collect()
micropython.mem_info()

import freeserif

gc.collect()
micropython.mem_info()

def foo():
    addr, height, width = freeserif.get_ch('a')

foo()

gc.collect()
micropython.mem_info()
print(len(freeserif._font) + len(freeserif._index))
```

The memory used was 5408, 5648, and 5696 bytes. As increments over the initial
state this corresponds to 240 and 288 bytes. The ``print`` statement shows the
RAM which would be consumed by the data arrays: this was 3271 bytes.

The ``foo()`` function emulates the behaviour of a device driver in rendering a
character to a display. The local variables constitute memory which will be
reclaimed on exit from the function. Its additional RAM use was 48 bytes.

## Conclusion

With a font of height 19 pixels RAM saving was an order of magnitude. The
saving will be greater if larger fonts are used