The history of luaTEX 2006–2009 / v 0.50 - Pragma ADE

Therefore, it will be no surprise that LuaTEX does not simply implement the OpenType
specication as such. On the one hand it implements a way to load information stored in
the font, on the other hand it implements mechanisms to fulll the demands of such fonts
and more. The glue between both is done with Lua. In the simple case of ligatures and
kerns this goes as follows. A user (or macropackage) specied a font, and this call can be
intercepted using a callback. This callback can use a built in function that loads an otf or ttf
font. From this table, a font table is constructed that is passed on to TEX. The construction
may involve building ligature and kerning tables using the information present in the font
le, but it may as well mean more. So, given a bare LuaTEX system, OpenType font support
is not giving you automatically handling of features, or more precisely, there is no hard
coded support for features.
This may sound as a disadvantage but as soon as you start looking at how TEX users use
their system (in most cases by using a macro package) you may understand that exibility
is larger this way. Instead of adding more and more control and exceptions, and thereby
making the kernel more instable and complex, we delegate control to the macro package.
The advantage is that there are no (everlasting) discussions on how to deal with
things and in the end the user will use a high level interface anyway. Of course the macro
package needs proper access to the font's internals, but this is provided: the code used
for reading in the data comes from FontForge (an advanced font editor) and is presented
via Lua tables in a well organized way.
Given that users expect OpenType features to be supported, how do we provide an interface.
In ConTEXt the user interface has always be an important aspect and consistency is
a priority. On the other hand, there has been the tradition of specifying the size explicity
and a new custom introduced by X TEX to enhance fontname with directives. Traditional
TEX provides:
\font \name filename [optional size]
X TEX accepts
E
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\font \name "fontname[:optional features]" [optional size]
\font \name fontname[:optional features] [optional size]
Instead of a fontname one can pass a lename between square brackets. LuaTEX handles:
\font \name anything [optional size]
\font \name {anything} [optional size]
where anything as well as the size are passed on to the callback.
This permits us to implement a traditional specication, support X TEX like denitions, and
easily pass information from a macro package down to the callback as well. Interpreting
anything is done in Lua.
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36 A fresh look at fonts