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OpenType feature reference material
This project addresses best practices for applications to implement access to advanced font features, primarily those defined in the OpenType specification, but also those defined elsewhere, like Graphite and Apple Advanced Typography (AAT).
This repository holds reference material outlining the syntax, meaning, and interaction of the various OpenType features adressed by the Open Source OpenType project.
Where necessary, it includes additional reference information about the various OpenType internals (like GPOS and GSUB tables) and language/script information, but in all cases we will try to point to external resources.
What it will not contain, though, is code. For that, you'll want to look in the other repositories.
What are advanced font features?
The features of interest break into a few broad categories. Some define script-shaping behavior that is mandatory for making text look correct in a given language / writing-system combination. Some others are intended to be active all the time, making adjustments to glyph selection or placement without user intervention. Still others are entirely optional---there to be activated by the user on an as-desired basis.
Features in the first category should be handled by the glyph shaping engine (e.g., HarfBuzz), and do not require special attention from the application developer. Features in the second category should, for the most part, also be handled automatically by the text-rendering stack, rather than by application code.
For features in the last category, applications that merely need to display typographically feature-rich text should again require no new work, but it may be important to understand the features and how they can affect layout, bounding boxes, and other such details. For applications that provide text-editing capabilities, this last class of feature will likely require new preferences and user-interface considerations.
Registered features
The bulk of modern fonts use the OpenType format, which has a lengthy list of pre-defined features. This list comes from the OpenType specification.
Other fonts may use the AAT or Graphite formats, which have their own sets of feature definitions.
Many of these features map conceptually to corresponding features also defined in OpenType. HarfBuzz, the free-software OpenType shaping library, comes with a mapping.
Feature classes
Unfortunately, despite the clean-sounding "three categories" described above, it is not always obvious whether any particular feature belongs in a particular category---especially when it comes to deciding between the "invisible and active all the time" and "entirely optional" categories.
Our attempt to break down the set of well-known features into easily understood categories can be found on the feature breakdown page.