Table Organization
Two TrueType Open tables, GSUB and GPOS, use the same data formats to describe the typographic functions of glyphs and the languages and scripts that they support: a ScriptList table, a FeatureList table, and a LookupList table. In GSUB, the tables define glyph substitution data. In GPOS, they define glyph positioning data. This chapter describes these common table formats.
The ScriptList identifies the scripts in a font, each of which is represented by a Script table that contains script and language-system data. Language system tables reference features, which are defined in the FeatureList. Each feature table references the lookup data defined in the LookupList that describes how, when, and where to implement the feature.
Figure 2d. The relationship of scripts, language systems, features, and lookups for substitution and positioning tables
Note: The data in the BASE and JSTF tables also is organized by script and language system. However, the data formats differ from those in GSUB and GPOS, and they do not include a FeatureList or LookupList. The BASE and JSTF data formats are described in the BASE and JSTF chapters.
The information used to substitute and position glyphs is defined in Lookup subtables. Each subtable supplies one type of information, depending upon whether the lookup is part of a GSUB or GPOS table. For instance, a GSUB lookup might specify the glyphs to be substituted and the context in which a substitution occurs, and a GPOS lookup might specify glyph position adjustments for kerning. TrueType Open has five types of GSUB lookups (described in the GSUB chapter) and seven types of GPOS lookups (described in the GPOS chapter).
Each subtable includes a Coverage table that lists the "covered" glyphs that will result in a glyph substitution or positioning operation. The Coverage table formats are described in this chapter.
Some substitution or positioning operations may apply to groups, or classes, of glyphs. GSUB and GPOS Lookup subtables use the Class Definition table to assign glyphs to classes. This chapter includes a description of the Class Definition table formats.
Lookup subtables also may contain device tables, described in this chapter, to adjust scaled contour glyph coordinates for particular output sizes and resolutions. This chapter also describes the data types used in TrueType Open. Sample tables and lists that illustrate the common data formats are supplied at the end of this chapter.