GSUB: Glyph Substitution

The 'GSUB' table provides information on glyph-substitution features that are required or appropriate when the font is used to render certain languages and scripts. These can be classified according to TrueType Open Script, Language, and Feature tags.

Requirements

Fonts should use only registered script, language, and feature tags. For complete information on the tags available, see the "Tag Registry" in the TrueType Open specification. Registered script tags include:

'hani'	Han Ideographic
'kana'	Kana (Hiragana & Katakana)
'hang'	Korean Hangul
'latn'	Latin

Registered language tags include:

'ZHS '	Chinese (PRC)
'ZHT '	Chinese (Taiwan)
'JAN '	Japanese
'KOR '	Korean

Note that the language tags end with a space character (0x20).

The 'vert' feature lets a font identify a list of horizontal glyph forms, and a corresponding list of vertical glyph forms to be substituted for vertical writing. Far East GDI will use this feature to substitute glyphs when an application requests a vertical writing font.

'vert'	Vertical glyph variants

Although there are many possible ways of implementing the 'vert' feature, Far East editions of Windows 95 and Windows NT 3.51 are guaranteed to read only one specific implementation. This table describes the format requirements. Wherever a value is listed in the "Value" column, fonts must use that value to ensure compatibility.

Section	Value	Type	Meaning
GSUBHeader	00010000	Fixed32	Version 1.0 of GSUB format
	000A	Offset	From start of GSUBHeader to start of ScriptList
	0024	Offset	From start of GSUBHeader to start of FeatureList
	0032	Offset	From start of GSUBHeader to start of LookupList
ScriptList	0001	Uint16	Script count
	xxxxxxxx	Tag	ScriptRecord[0]: Script tag, e.g. 'kana'
	0008	Offset	ScriptRecord[0]: from start of ScriptList to start of this Script
Script	000A	Offset	From start of Script to start of default LangSys
	0001	Uint16	LangSys count
	xxxxxxxx	Tag	LangSysRecord[0]: Language tag, e.g. 'JAN '
	000A	Offset	LangSysRecord[0]: from start of Script to start of this LangSys
LangSys	0000	Offset	LookupOrder (reserved, set to NULL)
	FFFF	Uint16	RequiredFeature index (none for this LangSys)
	0001	Uint16	Feature index count
	0000	Uint16	Feature index [0]: index into FeatureList
FeatureList	0001	Uint16	Feature count
	76657274	Tag	FeatureRecord[0]: Feature tag (only 'vert' is supported)
	0008	Offset	FeatureRecord[0]: from start of FeatureList to start of this Feature
Feature	0000	Offset	FeatureParams (reserved, set to NULL)
	0001	Uint16	Lookup index count
	0000	Uint16	Lookup index [0]: index into LookupList
LookupList	0001	Uint16	Lookup count
	0004	Offset	From start of LookupList to start of this Lookup
Lookup	0001	Uint16	LookupType (1 = Single substitution)
	0000	Uint16	LookupFlags
	0001	Uint16	Subtable count
	0008	Offset	From start of Lookup to start of Substitution subtable
Substitution subtable	0002	Uint16	Format identifier (2 = Specified output glyph indices)
	xxxx	Offset	From start of Substitution subtable to start of Coverage table
	xxxx	Uint16	Count of glyph Ids
	...	GlyphID[n]	(glyph IDs of vertical glyphs, in order corresponding to Coverage table)
Coverage	0001	Uint16	Format identifier
	xxxx	Uint16	Count of glyph Ids
	...	GlyphID[n]	(glyph IDs of horizontal glyphs, in sorted order)

Recommendations

None additional to TTO spec.