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

In practice we prefer a more abstract interface (at the macro level) but the idea stays the
same. Interesting is that having access to the internals this way already makes our TEX Live
more interesting. (We cannot demonstrate this trickery here because when this document
is processed you cannot be sure if the experimental interface is still in place.)
When playing with this we ran into problems with le searching. When performing the
backend role, LuaTEX will look in the TEX tree if there is a corresponding virtual le. It took
a while and a bit of tracing (which is not that hard in the Lua based reader) to gure out that
the omega related path denitions in texmf.cnf les were not correct, something that
went unnoticed because omega never had a backend integrated and the dvi processors
did multiple searches to get around this.
Currently, if you want to enable extensive tracing of le searching and loading, you can
set an environment variable:
MTX.INPUT.TRACE=3
This will produce a lot of information about what le is asked for, what types (tex, font, etc)
determines the search, along what paths is being searched, what readers and locators are
used (le, zip, protocol), etc.
AFM
While Taco implemented the virtual font reader —eventually its data will be merged with
the tfm table— I started playing with constructing tfm tables directly. Because ConTEXt
has a rather systematic naming scheme, we can rather easily see which encoding we are
dealing with. This means that in principle we can throw all encoded tfm les out of our
tree and construct the tables using the afm le and an encoding vector.
It took us a good day to gure out the details, but in the end we were able to trick LuaTEX
into using afm les. With a bit of internal caching it was even reasonable fast. When the
basic conversion mechanism was written we tried to compare the results with existing
tfm metrics as generated by afm2tfm and afm2pl. Doing so was less trivial than we rst
thought. To mention a few aspects:
• heights and depths have a limited number of values in TEX
• we need to convert to TEX's scaled points
• rounding errors of one scaled point occur
• afm2tfm can only add kerns when virtual fonts are used
• afm2tfm adds some extra ligatures and also does some kern magic
• afm2pl adds even more kerns
• the tools remove kern pars between digits
In this perspective we need not be too picky on what exactly a ligature is. An example
of a ligature is fi and such a character can be in the font. In the tfm le, the denition
32 A fresh look at fonts