TrueTypeOpen
Support for international typography
The TrueType font format now supports international typography with five new tables, collectively known as TrueType Open: GSUB, GPOS, BASE, JSTF, and GDEF. TrueType Open provides typographic and linguistic information for properly positioning and substituting glyphs, operations that are required for accurate typographic composition in many language environments. With TrueType Open, multiple scripts may be supported by a single font. To assist application and OS text processing clients, the TrueType Open data is organized by script, language system, and typographic feature.
The GSUB and GPOS tables define glyph substitution and positioning features. GSUB supplies five types of substitutions to support different kinds of character-to-glyph mappings, such as many-to-one ligature glyph substitution and one-to-many ligature decomposition. GPOS defines seven types of positioning features that provide two-dimensional positioning data for adjusting glyph placement and for glyph attachment.
The BASE table contains baseline and minimum/maximum extent data for each script. Script baselines can be defined in relation to one another to properly align glyphs from different scripts. Min/max extent values can be modified for particular language systems or features. With JSTF, text processing clients can turn glyph substitution and positioning features on and off to adjust line lengths and glyph spacing when justifying text. GDEF contains assorted tables that define useful information for text processing clients, such as ligature caret positions and lists of glyph attachment points.
The architecture of TrueType Open provides backwards font compatibility, client control, ease of use, and portability across machine platforms. Because TrueType Open fonts are simply TrueType 1.0 fonts that contain additional tables, they are compatible with any software that currently supports TrueType 1.0. And TrueType 1.0 fonts that do not contain TrueType Open data are still valid fonts. Font developers can define their own TrueType Open features in a font, and text processing clients can decide which of these features, if any, to apply to the text. TrueType Open data is decodable; an operating system or application can easily parse and understand it. If the same TrueType Open code is used in cross-platform applications, it will function properly.
When Microsoft releases the Arabic and Japanese versions of Windows 95, the core system fonts will be TrueType Open fonts. These fonts demonstrate some of TrueType Open's versatility.
Arabic Windows 95 will ship with several TrueType Open fonts that define many glyph substitution features, namely single substitution (a one-to-one contextual operation), ligature substitution (a many-to-one operation), and mark set substitutions. In Arabic Windows 95, the operating system itself manages glyph substitution, using data in the GSUB table of each font.
Japanese Windows 95 also will ship with several TrueType Open fonts that include vertical glyph substitution features and baseline positioning. Again, the operating system in Japanese Windows 95 manages glyph substitution, using the GSUB table data in each font. The text-processing client manages baseline positioning, using the BASE table data.