Hagen - Pragma ADE

5 Features
5.1 Introduction
If you look into fonts, it is hard not to bump into kerns (spacing between characters) and
ligatures (combining multiple shapes into one) and apart from monospaced fonts most
Type1 fonts have them. In the OpenType universe we call these properties features and
in such a font there can be many such features.
For those who grew up with T E X or still remember the times of eight bit fonts, it is
no secret that T E X macro packages did some magic to get most out of a font: replacing
missing glyphs, fixing metrics, using commands to access shapes that had a weird code
point, to mention a few. As there is absolutely no guarantee that an OpenType font does
better, there is a good reason to continue messing around with fonts. After all, it’s what
T E X users seem to like: control.
So, when we started writing support for OpenType quite soon a mechanism has been
created that permits adding our own features to the repertoire that comes with a font.
Because OpenType features demand a configuration and control mechanism, it made
sense to generalize that and provide a single interface.
This means that when we talk about font features, we don’t limit ourselves to those
provided by the font, but also those provided by ConT E Xt. As with font features, they
are enabled per font.
Some of the extra features are sort of generic, others are very font specific and their
properties are somewhat bound to a font. Such features are defined in a font goodie
files. Consider these goodies a font extension mechanism.
Some features need information that only the engine can provide. This is why we have
analyzers. Some are generic, others are bound to scripts. They come in action before
features are applied. Rather special is applying features in combination with paragraph
building. This is something very specific to ConT E Xt but it depends on properties of the
font. It falls into the category ‘optimizing’.
It is clear that when we talk of features many aspects of a font play a role. In this
chapter we will discuss all the mentioned aspects. There is quite a bit of Lua code shown
in this chapter, but don’t worry, users will seldom need to tweak fonts this way. On the
other hand it’s good to see what is possible.
5.2 Regulars
5.2.1 Introduction
The OpenType specification, which can be found on the Microsoft website is no easy
reading. Some of the concepts are easy to understand, like relative positioning (that we
call kerning in T E X) or ligature substitution (as we have ligatures in T E X too). It makes
no sense to discuss the bitwise composition of an OpenType or TrueType file here. First
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Features