BaseScript Table

A BaseScript table organizes and specifies the baseline data and min/max extent data for one script. Within a BaseScript table, the BaseValues table contains baseline information, and one or more MinMax tables contain min/max extent data.

The BaseValues table identifies the default baseline for the script and lists coordinate positions for each baseline named in the corresponding BaseTagList. Each script can assign a different position to each baseline, so each script can be aligned independently in relation to any other script. (For more details, see the BaseValues table description later in this chapter.)

The DefaultMinMax table defines the default min/max extent values for the script. (For details, see the MinMax table description below.) If a language system or feature defined in the font has no effect on the script's default min/max extents, TrueType Open will use the default script values.

Sometimes language-specific overrides for min/max extents are needed to properly render the glyphs in a specific language system. For example, a glyph substitution required in a language system may result in a glyph whose extents exceed the script's default min/max extents. Each language system that specifies min/max extent values must define a BaseLangSysRecord. The record should identify the language system (BaseLangSysTag) and contain an offset to a MinMax table of language-specific extent coordinates.

Feature-specific overrides for min/max extents also may be needed to accommodate the effects of glyph actions used to implement a specific feature. For example, superscript or subscript features may require changes to the default script or language system extents. Feature-specific extent values not limited to a specific language system may be specified in the DefaultMinMax table. However, extent values used for a specific language system require a BaseLangSysRecord and a MinMax table. In addition to specifying coordinate data, the MinMax table must contain offsets to FeatMinMaxRecords that define the feature-specific min/max data.

A BaseScript table has four components:

An offset to a BaseValues table (BaseValues). If no baseline data is defined for the script or the corresponding BaseTagList is set to NULL, the offset to the BaseValues table may be set to NULL.

An offset to the DefaultMinMax table. If no default min/max extent data is defined for the script, this offset may be set to NULL.

An array of BaseLangSysRecords (BaseLangSysRecord). The individual records stored in the BaseLangSysRecord array are listed alphabetically by BaseLangSysTag.

A count of the BaseLangSysRecords included (BaseLangSysCount). If no language system or language-specific feature min/max values are defined, the BaseLangSysCount may be set to zero (0).

Example 2 at the end of this chapter shows a sample BaseScript table.

BaseScript table

Type	Name	Description
Offset	ð BaseValues	Offset to BaseValues table
		—from beginning of BaseScript table
		—may be NULL
Offset	ð DefaultMinMax	Offset to MinMax table
		— from beginning of BaseScript table
		—may be NULL
uint16	BaseLangSysCount	Number of BaseLangSysRecords defined
		—may be zero (0)
struct	BaseLangSysRecord	Array of BaseLangSysRecords
	[BaseLangSysCount]	—in alphabetical order by BaseLangSysTag