The 'ltag' table

Introduction

The 'ltag' table provides a mapping between numeric codes and IETF language tags. The numeric codes are used in language field of the 'name' table for strings with a Unicode platform. The codes are also used in conjunction with the 'feat' and 'morx' tables to provide language-specific shaping.

IETF language tags are used for a number of reasons. First, they are standard and in common use. In particular, they're used by locales on OS X and iOS. Secondly, they are extensible and don't require a pre-determined ennumeration of supported languages.

The table uses two structures.

The 'ltag' FTStringRange
Type Name Description
UInt16 offset Offset from the start of the table to the beginning of the string
UInt16 length String length (in bytes)

 

The 'ltag' header
Type Name Description
UInt32 version Table version; currently 1
UInt32 flags Table flags; currently none defined
UInt32 numTags Number of language tags which follow
FTStringRange[] tagRange Range for each tag's string

There is one range per tag. Offsets are from the beginning of the table.

The flags field is currently unused and should be 0. The current version is 1.

IETF language tags are in ASCII and can therefore be interpreted as UTF-8 or any other ASCII-compatible text-encoding.

If we had a font with both English and Spanish names, we could build the following 'ltag' table:

Sample 'ltag' table
Type Value Description
UInt32 1 Version
UInt32 0 Flags
UInt32 2 numTags
FTStringRange 0,2 First tag
FTStringRange 2,2 Second tag
char[] "ensp" Tag strings

This maps English to language code 0 and Spanish to language code 1. We could then use this in the name table with Unicode-platform names:

How the 'ltag' is used
Language Code Value
-1 (don't care) Geneva
0 Geneva
1 Ginebra

Language tags are also used by 'feat' and 'morx' tables using the new always-exclusive feature 39 (language code).

A standard example of this is a case where Cyrillic italic letters are written one way in Serbia and another way everywhere else. We can add to our 'ltag' table:

Sample 'ltag' table
Type Value Description
UInt32 1 Version
UInt32 0 Flags
UInt32 3 numTags
FTStringRange 0,2 First tag
FTStringRange 2,2 Second tag
FTStringRange 4,2 Third tag
char[] "enspsr" Tag strings

We can then define a font feature with feature type 39 and selector type 3. (We add one to the 'ltag' index here because a selector type of 0 means "no change" for an exclusive feature.) We can then define a 'morx' subtable that maps the standard Cyrillic glyphs to the Serbian-specific ones.

Note that the use of the 'ltag' means that language tag IDs are internal to a specific font. This is similar to the way that glyphIDs are specific to a font. Anything which needs to know the identity of a glyph outside the context of the specific font needs to convert its glyph ID to a PostScript name via the 'post' table. Similarly, anything which needs to know the identity of a language tag outside the context of a specific font would need to convert the tag ID to an IETF tag via the 'ltag' table.

UI concerns:

IETF language tie naturally in to the OS X and iOS user interface and tools such as ftxdumperfuser. One could use a statement like

[[NSLocale autoupdatingCurrentLocale] displayNameForKey:NSLanguageIdentifier value:tag]

to get the proper string to use in the UI. This also ties in well with locale information as used elsewhere in the system.

Platform-specific Information

Support for the 'ltag' table is available on OS X 10.9 (Mavericks) and later, and on iOS 7.0 and later. The 'ltag' table is currently supported only in conjunction with 'morx' (extended metamorphosis) and 'feat' (font feature) tables. Support may be added for use with 'cmap' (character mapping) tables in future releases.

Tools

'ltag' tables are fully supported by ftxdumperfuser. ftxenhancer will generate or extended 'ltag' tables as needed when processing an ATIF.