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Shaperglot - Test font files for OpenType language support

PyPI Version PyPI License

Shaperglot is a library and a utility for testing a font's language support. You give it a font, and it tells you what languages are supported and to what degree.

Most other libraries to check for language support (for example, Rosetta's wonderful hyperglot library) do this by looking at the Unicode codepoints that the font supports. Shaperglot takes a different approach.

What's wrong with the Unicode codepoint coverage approach?

For many common languages, it's possible to check that the language is supported just by looking at the Unicode coverage. For example, to support English, you need the 26 lowercase and uppercase letters of the Latin alphabet.

However, for the majority of scripts around the world, covering the codepoints needed is not enough to say that a font really supports a particular language. For correct language support, the font must also behave in a particular way.

Take the case of Arabic as an example. A font might contain glyphs which cover all the codepoints in the Arabic block (0x600-0x6FF). But the font only supports Arabic if it implements joining rules for the init, medi and fina features. To say that a font supports Devanagari, it needs to implement conjuncts (which set of conjuncts need to be included before we can say the font "supports" Devanagari is debated...) and half forms, as well as contain a languagesystem statement which triggers Indic reordering.

Even within the Latin script, a font only supports a language such as Turkish if its casing behaving respects the dotless / dotted I distinction; a font only supports Navajo if its ogonek anchoring is different to the anchoring used in Polish; and so on.

But there's a further problem with testing language support by codepoint coverage: it encourages designers to "fill in the blanks" to get to support, rather than necessarily engage with the textual requirements of particular languages.

Testing for behaviour, not coverage

Shaperglot therefore determines language support not just on codepoint coverage, but also by examining how the font behaves when confronted with certain character sequences.

The trick is to do this in a way which is not prescriptive. We know that there are many different ways of implementing language support within a font, and that design and other considerations will factor into precisely how a font is constructed. Shaperglot presents the font with different strings, and makes sure that "something interesting happened" - without necessarily specifying what.

In the case of Arabic, we need to know that the init feature is present, and that when we shape some Arabic glyphs, the output with init turned on is different to the output with init turned off. We don't care what's different; we only care that something has happened. (Yes, this still makes it possible to trick shaperglot into reporting support for a language which is not correctly implemented, but at that point, it's probably less effort to actually implement it...)

Shaperglot includes (or will include) the following kinds of test:

  • Certain codepoints were mapped to base or mark glyphs.
  • A named feature was present.
  • A named feature changed the output glyphs.
  • A mark glyph was attached to a base glyph or composed into a precomposed glyph (but not left unattached).
  • Certain glyphs in the output were different to one another.
  • Languagesystems were defined in the font.
  • ...

Using this library of tests, we then create language support definitions which exercise the font's capabilities to obtain a fuller picture of its support for a particular language. These language support definitions, expressed as YAML files, are the core of Shaperglot; to extend and improve Shaperglot, we need as many language support definition files as possible - so if you know a language well and can express what it means to "support" that language properly, please contribute a definition!

Using Shaperglot

To report whether or not a given language is supported, pass a font and one or more ISO639-3 language codes.

$ shaperglot -v -v MyFont.ttf urd
Font does not fully support language 'urd'
 * PASS: All base glyphs were present in the font
 * FAIL: Some mark glyphs were missing: ْ
 * PASS: Required feature 'mark' was present
 * PASS: Mark glyph ◌َ  (FATHA) took part in a mark positioning rule
 * PASS: Mark glyph ◌ُ  (DAMMA) took part in a mark positioning rule
 * PASS: Mark glyph ◌ِ  (KASRA) took part in a mark positioning rule
 * PASS: Mark glyph ◌ٰ  (LONG_A) took part in a mark positioning rule
 * PASS: Required feature 'init' was present
 * PASS: Glyph ع (AINu1) took part in a init rule
 * PASS: Required feature 'medi' was present
 * PASS: Glyph ع (AINu1) took part in a medi rule
 * PASS: Required feature 'fina' was present
 * PASS: Glyph ع (AINu1) took part in a fina rule
 * PASS: Repeated beh forms should produce different shapes
 * PASS: Initial and final forms should differ

Setup

Requirements

  • Python 3.9+

Installation

Install it directly into an activated virtual environment:

$ pip install shaperglot

or add it to your Poetry project:

$ poetry add shaperglot

Usage

After installation, the package can imported:

$ python
>>> import shaperglot
>>> shaperglot.__version__

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Test font files for language support

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