User Guide

FontLab 4
676
Pair Positioning
Rules for this lookup type are usually used for kerning and must follow this
format:
position <glyph|glyphclass> <glyph|glyphclass> <valuerecord format A>;
This format is provided since it closely parallels the way kerning is
expressed in a plain pair kerning table. Here, the <valuerecord> must be of
value record format A only, and corresponds to the first
<glyph|glyphclass>.
Kerning can most easily be expressed with this format. This will result in
adjusting the first glyph's X advance, except when in the 'vrkn' feature, in
which case it will adjust the first glyph's Y advance. Some examples:
pos T a -100; # specific pair (no glyph class present)
pos [T] a -100; # class pair (singleton glyph class present)
pos T @a -100; # class pair (glyph class present, even if singleton)
pos @T [a o u] -80; # class pair
Note that if at least one glyph class is present (even if it is a singleton glyph
class), then the rule is interpreted as a class pair; otherwise, the rule is
interpreted as a specific pair.
In the 'kern' feature, the specific glyph pairs will typically precede the glyph
class pairs in the feature file, mirroring the way that they will be stored in
the font.
feature kern {
# specific pairs for all scripts
# class pairs for all scripts
} kern;