The BaseTagList table identifies the baselines for all scripts in the font that are rendered in the same text direction. Each baseline is identified with a 4-byte baseline tag. The BaseTagList can define any number of baselines, and it may include baseline tags for scripts supported in other fonts.
Each script in the BaseScriptList table must designate one of these BaseTagList baselines as its default, which TrueType Open uses to align all glyphs in the script. Even though the BaseScriptList and the BaseTagList are defined independently of one another, the BaseTagList typically includes a tag for each different default baseline needed to render the scripts in the layout direction. If some scripts use the same default baseline, the BaseTagList needs to list the common baseline tag only once.
The BaseTagList table consists of an array of baseline identification tags (BaselineTag), listed alphabetically, and a count of the total number of baseline Tags in the array (BaseTagCount).
Example 1 at the end of this chapter shows a sample BaseTagList table.
BaseTagList table
Type
Name
Description
uint16
BaseTagCount
Number of baseline identification tags in this text direction