The 'xref' table
Introduction
The 'xref' (cross-reference table) table is used by Apple's font tools ftxdumperfuser and ftxenhancer. ftxenhancer lets font designers generate 'just', 'kern', and 'morx' tables using symbolic names. These names are not used by the tables themselves, however, and so are lost when ftxenhancer is run. The 'xref' table is used to store these symbolic names.
The table begins with a header which is structured as follows:
|
Type
|
Name
|
Description
|
|---|---|---|
| uint32_t | version | The version of the table format, currently 1 |
| uint32_t | flags | Flags, currently unused and set to 0 |
| uint32_t | numEntries | The number of entries in the table |
| uint32_t | stringOffset | Offset from the beginning of the table to the string data |
The header is followed by a number of entries which map from data in the font table to strings:
|
Type
|
Name
|
Description
|
|---|---|---|
| FourCharCode | tableTag | The tag for the table that uses this entry (currently 'just', 'kerx', or 'morx') |
| int16_t | chainIndex | The 'morx' chain corresponding to this name |
| int16_t | subtableIndex | The index of the subtable corresponding to this name |
| int16_t | tableType | The type of object corresponding to this name. Values are 0 (class name), 1 (state name), 2 (entry/transition name), 3 (action name), 4 (column class name), and 5 (row class name). |
| int16_t | tableIndex | The index of the object corresponding to this name |
| uint16_t | stringOffset | The offset from the beginning of the string data to this name |
| uint16_t | stringLength | The length of the string data for this name |
For the chainIndex, subtableIndex, tableType, and tableIndex fields can all use -1 as a wildcard, rarely though that may be appropriate.
String data is in UTF-8. Indices are zero-based.
For example, let us suppose that a font's 'morx' table has one chain with four subtables. The fourth subtable is a contextual subtable with two classes defined in addition to the standard four, and the first of the two one is named "myClass." The entry { tableTag:'morx', chainIndex:0, subtableIndex:3, tableType:0, tableIndex:4 }. The offset and length would indicate the bytes 6D 79 43 6C 61 73 73. Note that the tableIndex field counts the four implicit classes, even though they have standard names. Similarly, data for state tables counts the two implicit states, "StartOfText" and "StartOfLine."
Warning
The 'xref' table is generally useful for production purposes only and should be removed from shipping fonts accordingly.