This is used as the language index in pango_ot_info_find_feature() when the default language system of the script is desired.
It is also returned by pango_ot_info_find_language() if the requested language is not found, or the requested language tag was PANGO_OT_TAG_DEFAULT_LANGUAGE. The end result is that one can always call pango_ot_tag_from_language() followed by pango_ot_info_find_language() and pass the result to pango_ot_info_find_feature() without having to worry about falling back to default language system explicitly.
This is used as a feature index that represent no feature, that is, should be skipped. It may be returned as feature index by pango_ot_info_find_feature() if the feature is not found, and pango_ot_ruleset_add_feature() function automatically skips this value, so no special handling is required by the user.
This is used as a script index that represent no script, that is, when the requested script was not found, and a default ('DFLT') script was not found either. It may be returned as script index by pango_ot_info_find_script() if the script or a default script are not found, all other functions taking a script index essentially return if the input script index is this value, so no special handling is required by the user.
Finds the OpenType script tag corresponding to script.
The %PANGO_SCRIPT_COMMON, %PANGO_SCRIPT_INHERITED, and %PANGO_SCRIPT_UNKNOWN scripts are mapped to the OpenType 'DFLT' script tag that is also defined as %PANGO_OT_TAG_DEFAULT_SCRIPT.
Note that multiple PangoScript values may map to the same
OpenType script tag. In particular, %PANGO_SCRIPT_HIRAGANA
and %PANGO_SCRIPT_KATAKANA both map to the OT tag 'kana'.
Finds the PangoScript corresponding to script_tag.
The 'DFLT' script tag is mapped to %PANGO_SCRIPT_COMMON.
Note that an OpenType script tag may correspond to multiple
PangoScript values. In such cases, the PangoScript value
with the smallest value is returned.
In particular, %PANGO_SCRIPT_HIRAGANA
and %PANGO_SCRIPT_KATAKANA both map to the OT tag 'kana'.
This function will return %PANGO_SCRIPT_HIRAGANA for
'kana'.
A PangoOTTag OpenType script tag
This is used as the property bit in pango_ot_ruleset_add_feature() when a feature should be applied to all glyphs.