User Guide

Making OpenType Fonts
679
K n o w n Fe a t u r e s
OpenType feature processing is an application-centric system. The
application knows which features it needs to apply and then searches the
font for these features. If a feature is present in the font, it is applied. To
implement this system features must be standardized and registered so
everybody will know what is done by a feature with a particular name.
The Microsoft Typography group performs feature registration:
http://www.microsoft.com/typography
The full list of registered features may be found in this document:
OpenType Layout tag registry
http://www.microsoft.com/typography/otspec/ttoreg.htm
This document contains descriptions of all name tags that can be used in
an OpenType font for script, language and feature names.
We will provide brief definitions of the most commonly used features.
init, medi, fina and isol Features
These features are most commonly used in Arabic scripts and allow one to
define four forms of Arabic characters: initial, final, medial and isolated:
Isolated Final Medial Initial
Note that Arabic text is written right-to-left.
Please note that it is the application or the operating system that detects
word boundaries and applies one of the features. You are not expected to
pay attention to that in the feature definition.
Usually these features are simple substitutions:
feature init{
sub @isolated_forms by @initial_forms;
}init;