User Guide

FontLab 4
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P o s i t i o ni n g Lo o k u p s
The OpenType specification allows you to define many positioning
lookups. The lookup types may be separated into three groups:
1. Basic lookups, single and pair positioning:
2. The Cursive attachment lookup that allows smooth connection of script
and cursive glyphs:
3. Mark attachment lookups that define relative positions of glyphs and
marks:
FontLab 4 can support only the first group of lookups: single and
pair positioning. We expect to support cursive and mark attachment
lookups in one of the next releases of FontLab.
As in the case with substitution, positioning lookups may be context-free
and context-dependent. Context-dependent lookups are the same as
context-free but add a context that is verified against the source sequence
of glyphs. Only when the context is matched is positioning performed.
Please note that glyph positioning is performed after substitution and that
all positioning lookups must be defined for the glyph string that is a result
of substitution.
Glyph positioning rules begin with the keyword "position"; this keyword
may be abbreviated as "pos". (The "enumerate" or "ignore" keywords may
precede the "position" keyword in some cases.) The GPOS lookup type is
auto-detected from the format of the rest of the rule.