Afm2tfm's encoding files have the same format as an encoding vector in a PostScript font. Here is a skeletal example:
% Comments are ignored, unless the first word after the percent sign % is `LIGKERN'; see below. /MyEncoding [ % exactly 256 entries follow, each with a leading `/' /Alpha /Beta /Gamma /Delta ... /A /B ... /Z ... /.notdef /xfooaccent /yfooaccent /zfooaccent ] def
These encoding files are downloaded as part of changing the encoding at the PostScript level (see the previous section).
Comments, which start with a percent sign and continue until the end of the line, are ignored unless they start with `LIGKERN' (see below).
The first non-comment word of the file must start with a forward slash
`/' (i.e., a PostScript literal name) and defines the name of the
encoding. The next word must be an left bracket `['. Following
that must be precisely 256 character names; use `/.notdef' for any
that you want to leave undefined. Then there must be a matching right
bracket ]. A final `def' token is optional. All names are
case-sensitive.
Any ligature or kern information is given as a comment. If the first word after the `%' is `LIGKERN', then the entire rest of the line is parsed for ligature and kern information. This ligature and kern information is given in groups of words: each group is terminated by a space and a semicolon and (unless the semicolon is at the end of a line) another space.
In these LIGKERN statements, three types of information may be
specified. These three types are ligature pairs, kerns to ignore, and
the character value of this font's boundary character.
Throughout a LIGKERN statement, the boundary character is
specified as `||'. To set the font's boundary character value for
TeX:
% LIGKERN || = 39 ;
To indicate a kern to remove, give the names of the two characters (without the leading slash) separated by `{}', as in `one {} one ;'. This is intended to be reminiscent of the way you might use `{}' in a TeX file to turn off ligatures or kerns at a particular location. Either or both of the character names can be given as `*', which is a wild card matching any character; thus, all kerns can be removed with `* {} * ;'.
To specify a ligature, specify the names of the pair of characters, followed by the ligature operation (as in Metafont), followed by the replacing character name. Either (but not both) of the first two characters can be `||' to indicate a word boundary.
The most common operation is `=:' meaning that both characters are
removed and replaced by the third character, but by adding the `|'
character on either side of the `=:', you can retain either or both
of the two leading characters. In addition, by suffixing the ligature
operation with one or two `>' signs, you can make the ligature
scanning operation skip that many resulting characters before
proceeding. This works just like in Metafont. For example, the `fi'
ligature is specified with `f i =: fi ;'. A more convoluted
ligature is `one one |=:|>> exclam ;' which separates a pair of
adjacent 1's with an exclamation point, and then skips over two
of the resulting characters before continuing searching for ligatures
and kerns. You cannot give more >'s than |'s in an ligature
operation, so there are a total of eight possibilities:
=: |=: |=:> =:| =:|> |=:| |=:|> |=:|>>
The default set of ligatures and kerns built in to Afm2tfm is:
% LIGKERN question quoteleft =: questiondown ;
% LIGKERN exclam quoteleft =: exclamdown ;
% LIGKERN hyphen hyphen =: endash ; endash hyphen =: emdash ;
% LIGKERN quoteleft quoteleft =: quotedblleft ;
% LIGKERN quoteright quoteright =: quotedblright ;
% LIGKERN space {} * ; * {} space ; 0 {} * ; * {} 0 ;
% LIGKERN 1 {} * ; * {} 1 ; 2 {} * ; * {} 2 ; 3 {} * ; * {} 3 ;
% LIGKERN 4 {} * ; * {} 4 ; 5 {} * ; * {} 5 ; 6 {} * ; * {} 6 ;
% LIGKERN 7 {} * ; * {} 7 ; 8 {} * ; * {} 8 ; 9 {} * ; * {} 9 ;