JSTF Table Examples

The rest of this chapter describes examples of all the JSTF table formats. All the examples reflect unique parameters described below, but the samples provide a useful reference for building tables specific to other situations.

The examples have three columns showing hex data, source, and comments.

Example 1: JSTF Header Table and JstfScriptRecord

Example 1 demonstrates how a script is defined in the JSTF Header with a JstfScriptRecord that identifies the script and references its JstfScript table.

Example 1

Hex Data	Source	Comments
	JSTFHeader
	TheJSTFHeader	;JSTFHeader table definition
00010000	0x00010000	;version
0001	1	;JstfScriptCount
		;JstfScriptRecord[0]
74686169	"thai"	;JstfScriptTag
000C	ThaiScript	;offset to JstfScript table

Example 2: JstfScript Table, ExtenderGlyph Table, JstfLangSysRecord, and JstfLangSys Table

Example 2 shows a JstfScript table for the Arabic script and the tables it references. The DefJstfLangSys table defines justification data to apply to the script in the absence of language-specific information. In the example, the table lists two justification suggestions in priority order.

JstfScript also supplies language-specific justification data for the Farsi language. The JstfLangSysRecord identifies the language and references its JstfLangSys table. The FarsiJstfLangSys lists one suggestion for justifying Farsi text.

The ExtenderGlyph table in JstfScript lists the indices of all the extender glyphs used in the script.

Example 2

Hex Data	Source	Comments
	JstfScript
	ArabicScript	;JstfScript table definition
000C	ArabicExtenders	;ExtenderGlyph
0012	ArabicDefJstfLangSys	;offset to DefJstfLangSys table
0001	1	;JstfLangSysCount
		;JstfLangSysRecord[0]
50455220	"FAR "	;JstfLangSysTag
0018	FarsiJstfLangSys	;JstfLangSys
	ExtenderGlyph
	ArabicExtenders	;ExtenderGlyph table definition
0002	2	;GlyphCount
01D3	TatweelGlyphID	;ExtenderGlyph[0]
01D4	LongTatweelGlyphID	;ExtenderGlyph[1]

	JstfLangSys
	ArabicDefJstfLangSys	;JstfLangSys table definition
0002	2	;JstfPriorityCnt
000A	ArabicScriptJstfPriority1	;offset to JstfPriority[0] table
001E	ArabicScriptJstfPriority2	;offset to JstfPriority[1] table

	JstfLangSys
	FarsiJstfLangSys	;JstfLangSys table definition
0001	1	;JstfPriorityCnt
002C	FarsiLangJstfPriority1	;offset to JstfPriority[0] table

Example 3: JstfPriority Table, JstfGSUBModList Table,
and JstfGPOSModList Table

Example 3 shows the JstfPriority and JstfModList table definitions for two justification suggestions defined in priority order. The first suggestion uses ligature substitution to shrink the lengths of text lines, and it extends line lengths by replacing ligatures with their individual glyph components. Other lookup actions are not recommended at this priority level and are set to NULL. The associated JstfModList tables enable and disable three substitution lookups.

The second suggestion enables glyph kerning to reduce line lenths and disables glyph kerning to extend line lengths. Each action uses three lookups. This suggestion also includes a JstfMax table to extend line lengths, called WordSpaceExpandMax, which is described in Example 4.

Example 3

Hex Data	Source	Comments
	JstfPriority
	USEnglishFirstJstfPriority	;JstfPriority table definition
0028	EnableGSUBLookupsToShrink	;offset to ShrinkageEnableGSUB
		; JstfGSUBModList table
0000	NULL	;offset to ShrinkageDisableGSUB
		; JstfGSUBModList table
0000	NULL	;offset to ShrinkageEnableGPOS
		; JstfGPOSModList table
0000	NULL	;offset to ShrinkageDisableGPOS
		; JstfGPOSModList table
0000	NULL	;offset to Shrinkage JstfMax
		; table

0000	NULL	;offset to ExtensionEnableGSUB
		; JstfGSUBModList table
0038	DisableGSUBLookupsToExtend	;offset to ExtensionDisableGSUB
		; JstfGSUBModList table
0000	NULL	;offset to ExtensionEnableGPOS
		; JstfGPOSModList table
0000	NULL	;offset to ExtensionDisableGPOS
		; JstfGPOSModList table
0000	NULL	;offset to Extension JstfMax
		; table

	JstfPriority
	USEnglishSecondJstfPriority	;JstfPriority table definition
0000	NULL	;offset to ShrinkageEnableGSUB
		; JstfGSUBModList table
0000	NULL	;offset to ShrinkageDisableGSUB
		; JstfGSUBModList table
0000	NULL	;offset to ShrinkageEnableGPOS
		; JstfGPOSModList table
001C	DisableGPOSLookupsToShrink	;offset to ShrinkageDisableGPOS
		; JstfGPOSModList table
0000	NULL	;offset to Shrinkage JstfMax
		; table

0000	NULL	;offset to ExtensionEnableGSUB
		; JstfGSUBModList table
0000	NULL	;offset to ExtensionDisableGSUB
		; JstfGSUBModList table
002C	EnableGPOSLookupsToExtend	;offset to ExtensionEnableGPOS
		; JstfGPOSModList table
0000	NULL	;offset to ExtensionDisableGPOS
		; JstfGPOSModList table
0000	NULL	;offset to Extension JstfMax
		; table

	JstfGSUBModList
	EnableGSUBLookupsToShrink	;JstfGSUBModList table
		; definition
		; enable three ligature
		; substitution lookups
0003	3	;LookupCount
002E	46	;LookupIndex[0]
0035	53	;LookupIndex[1]
0063	99	;LookupIndex[2]

	JstfGPOSModList
	DisableGPOSLookupsToShrink	;JstfGPOSModList table
		; definition
		; disable three tight kerning
		; lookups
0003	3	;LookupCount
006C	108	;LookupIndex[0]
006E	110	;LookupIndex[1]
0070	112	;LookupIndex[2]

	JstfGSUBModList
	DisableGSUBLookupsToExtend	;JstfGSUBModList table
		; definition
		; disable three ligature
		; substitution lookups
0003	3	;LookupCount
002E	46	;LookupIndex[0]
0035	53	;LookupIndex[1]
0063	99	;LookupIndex[2]

	JstfGPOSModList
	EnableGPOSLookupsToExtend	;JstfGPOSModList table
		; definition
		; enable three tight kerning
		; lookups
0003	3	;LookupCount
006C	108	;LookupIndex[0]
006E	110	;LookupIndex[1]
0070	112	;LookupIndex[2]

Example 4: JstfMax Table

The JstfMax table in Example 4 defines a lookup to expand the advance width of the word space glyph and extend line lengths. The lookup definition is identical to the SinglePos lookup type in the GPOS table although it is enabled only when justifying text. The ValueRecord in the WordSpaceExpand lookup subtable specifies an XAdvance adjustment of 360 units, which is the maximum value the font developer recommends for acceptable text rendering. The text-processing client may implement the lookup using any value between zero and the maximum.

Example 4

Hex Data	Source	Comments
	JstfMax
	WordSpaceExpandMax	;JstfMax table definition
0001	1	;LookupCount
0004	WordSpaceExpandLookup	;offset to Jstf Lookup[0] table

	Lookup
	WordSpaceExpandLookup	;Jstf Lookup table definition
0001	1	;LookupType
		; SinglePos Lookup
0000	0x0000	;LookupFlag
0001	1	;SubTableCount
0008	WordSpaceExpandSubtable	;offset to Subtable[0]
		; SinglePos subtable

	SinglePosFormat1
	WordSpaceExpandSubtable	;SinglePos subtable definition
0001	1	;PosFormat
0008	WordSpaceCoverage	;offset to Coverage table
0004	0x0004	;ValueFormat
		; XAdvance only
0168	360	;Value
		; XAdvance value
		; in Jstf, this is a max value
		; expand word space from zero
		; to this amount

	CoverageFormat1
	WordSpaceCoverage	;Coverage table definition
0001	1	;CoverageFormat
0001	1	;GlyphCount
0022	WordSpaceGlyphID	;GlyphArray[0]