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https://github.com/peterhinch/micropython-font-to-py

A Python 3 utility to convert fonts to Python source capable of being frozen as bytecode
https://github.com/peterhinch/micropython-font-to-py

fonts micropython python-3

Last synced: 29 days ago
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A Python 3 utility to convert fonts to Python source capable of being frozen as bytecode

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README 

        
# MicroPython font handling



This repository defines a method of creating and deploying fonts for use with

MicroPython display drivers. A PC utility renders industry standard font files

as a bitmap in the form of Python sourcecode. A MicroPython module enables such

files to be displayed on devices with suitable device drivers. These include

OLED displays using the SSD1306 chip and the official device driver. Compatible

drivers for a variety of display technologies are available as part of the

[nano-gui repository](https://github.com/peterhinch/micropython-nano-gui).



# 1. Introduction



MicroPython platforms generally have limited RAM, but more abundant storage in

the form of flash memory. Font files tend to be relatively large. The

conventional technique of rendering strings to a device involves loading the

entire font into RAM. This is fast but RAM intensive. The alternative of storing

the font as a random access file and loading individual glyphs into RAM on

demand is too slow for reasonable performance on most display devices.



This alternative implements a font as a Python source file, with the data being

declared as `bytes` objects. Such a file may be frozen as bytecode: this

involves building the firmware from source with the Python file in a specific

directory. On import very little RAM is used, yet the data may be accessed

fast. Note that the use of frozen bytecode is entirely optional: font files may

be imported in the normal way if RAM usage is not an issue.



The resultant file is usable with two varieties of display device drivers:



 1. Drivers where the display class is subclassed from the official

 `framebuffer` class.

 2. Drivers for displays where the frame buffer is implemented in the display

 device hardware.



# 2. Solution



This comprises four components, links to docs below:



 1. [font_to_py.py](./FONT_TO_PY.md) This utility runs on a PC and converts an

 industry standard font file to Python source. See below.

 2. [Writer and CWriter classes](./writer/WRITER.md) These facilitate rendering

 text to a monochrome or colour display having a suitable device driver.

 3. [Creating icon fonts](./icon_fonts/README.md) Ways to incorporate icons in

 a Python font file.

 4. [Device driver notes](./writer/DRIVERS.md). Notes for authors of display

 device drivers. Provides details of the font file format and information on

 ensuring compatibility with the `Writer` classes.



# 3. font_to_py.py



This command line utility is written in Python 3 and runs on a PC. To convert

a scalable font to Python the utility takes input a font file in `ttf` or `otf`

form together with a height in pixels and outputs a Python source file

containing the font as a bitmap. Fixed and variable pitch rendering are

supported. The design has the following aims:



 * Independence of specific display hardware.

 * The path from font file to Python code to be fully open source.



The first is achieved by supplying hardware specific arguments to the utility.

These define horizontal or vertical mapping and the bit order for font data.



The second is achieved by using Freetype and the Freetype Python bindings. Its

use is documented [here](./FONT_TO_PY.md). This also details measurements of

RAM usage when importing fonts stored as frozen bytecode.



## 3.1 Small fonts



Converting scalable `ttf` or `otf` files programmatically works best for larger

fonts. For small fonts it is best to use hand-designed bitmapped font files.

These are now supported: `bdf` or `pcf` font files may be converted to Python

source in the same format as files originating from scalable fonts.



## 3.2 Limitations



Kerning is not supported. Fonts are one bit per pixel. Colour displays are

supported by the `CWriter` class which adds colour information at the rendering

stage. This assumes that all pixels of a character are coloured identically.



By default the `font_to_py.py` utility produces the ASCII character set from

`chr(32)` to `chr(126)` inclusive. Command line options enable the character

set to be modified to include arbitrary Unicode characters. Alternative sets

may be specified such as for non-English languages. Efficient support is now

provided for sparse character sets.



# 4. Font file interface



A font file is imported in the usual way e.g. `import font14`. Python font

files contain the following functions. These return values defined by the

arguments which were provided to `font_to_py.py`:



`height` Returns height in pixels.  

`max_width` Returns maximum width of a glyph in pixels.  

`baseline` Offset from top of glyph to the baseline.  

`hmap` Returns `True` if font is horizontally mapped.  

`reverse` Returns `True` if bit reversal was specified.  

`monospaced` Returns `True` if monospaced rendering was specified.  

`min_ch` Returns the ordinal value of the lowest character in the file.  

`max_ch` Returns the ordinal value of the highest character in the file.



Glyphs are returned with the `get_ch` function. Its argument is a Unicode

character and it returns the following values:



 * A `memoryview` object containing the glyph bytes.

 * The height in pixels.

 * The character width in pixels.



The `font_to_py.py` utility allows a default glyph to be specified (typically

`?`). If called with an undefined character, this glyph will be returned.



The `min_ch` and `max_ch` functions are mainly relevant to contiguous character

sets.



# 5. Licence



All code is released under the MIT licence.