Hagen - Pragma ADE

14
Font formats
can be rules that turn 1/2 into one glyph, or transfer the numbers into superior and
inferior alternatives, but leaves us with an unacceptable rendered 1/a, given that the
frac features is enabled. It looks like features like this are to be applied to a manually
selected range of characters.
The fact that an OpenType font can contain many features and rules to apply them
makes it possible to typeset scripts like Arabic. And this is where it gets vague. A generic
OpenType sub-engine can do clever things using these rules, but if you read the specification
for some scripts additional intelligence has to be provided by the typesetting
engine.
While users no longer have to care about encodings, map files and back-end issues,
they do have to carry knowledge about the possibilities and limitations of features. Even
worse, he or she needs to be aware that fonts can have bugs. Also, as font vendors have
no tradition of providing updates this is something that we might need to take care of
ourselves by tweaking the engine.
One of the problems with the transition from Type1 to OpenType is that font designers
can take an existing design and start from that basic repertoire of shapes. If such a
design had oldstyle figures only, there is a good chance that this will be the case in the
OpenType variant too. However, such a default interferes with the fact that the onum
feature is one that we explicitly have to enable. This means that writing a generic style
where a font is later plugged in becomes somewhat messy if it assumes that features
need to be turned on.
T E X users expect more control, which means that in practice just an OpenType engine
is not enough, but for the average font the T E X model using the traditional approach
still is quite acceptable. After all, not all users use complex scripts or need advanced
features. And, in practice most readers don’t notice the difference anyway.
1.7 Lua
A.7 As mentioned support for virtual fonts is built into LuaT E X and loading the so called vf
files happens when needed. However, that concerns traditional fonts that we already
covered. In ConT E Xt we do use the virtual font mechanism for creating missing glyphs
out of existing ones or add fallbacks when this is not possible. But this is not related to
some kind of font format.
In 2010 and 2011 the first public OpenType math fonts showed up that replace their
Type1 originals. In ConT E Xt we already went forward and created virtual Unicode fonts
out of traditional fonts. Of course eventually the defaults will change to the OpenType
alternatives. The specification for such a virtual font is given in Lua tables and therefore
you can consider Lua to be a font format as well. In ConT E Xt such fonts can be defined
in so called goodies files. As we use these files for much more tuning, we come back to
that in a later chapter. In a virtual font you can mix real Type1 fonts and real OpenType
fonts using whatever metrics suit best.
An extreme example is the virtual Unicode Punk font. This font is defined in the
METAPOST language (derived from Don Knuths METAFONT sources) where each glyph is
one graphic. Normally we get PostScript, but in LuaT E X we can also get output in a