User Guide

FontLab 4
428
Cl a ss - Ba se d K e r ni ng
Some glyphs in a font may have a similar shape and may be kerned equally.
For example, “A” and “Acaron” will have the same kerning value if kerned
with “T” or “Tcaron”. With the standard kerning system this will require 4
kerning pairs. If the number of similar glyphs in the group increases, the
number of necessary pairs will rise dramatically.
A better way to handle the kerning of similar glyphs is to define glyph
groups or classes. In our example the first class will contain the glyphs “A”
and “Acaron” and the second “T” and “Tcaron”. Then we will need to define
only one pair and all glyphs in both classes will be covered.
Kerning classes may save a lot of time if you need to kern a font that
contains a lot of similar glyphs. The typical number of class kerning pairs
(which define kerning between classes) is a few hundred. Compare this to
several thousand pairs that are necessary to define the same kerning data
using traditional methods.
Classes are also actively used to define OpenType features that are
applicable to a set of glyphs, so we will return to this subject in the
“OpenType” chapter.