如何将 AAT 上下文连字添加到 TTF 字体？

Question

我正在为 Sgaw Karen 开发字体，这是一种需要根据上下文来塑造字符（和连字）的语言。我的字体运行良好，但是，我希望添加一个 AAT MORX 表格以便在 Mac 上获得更好的支持。

但是，我很难理解 AAT 字体功能的语法。特别是上下文类型。我已阅读 Apple 参考和教程文档，并查看了列出的示例 here。

基本上，我有两个字符，比如 X 和 Y，如果后面跟着一组特定的字符，比如 N 和 M，则需要替换为连字。对于 FontLab Studio 中的 OpenType 功能，我简单地说：

sub X' Y' [N M] by X_Y.alt;

而且效果很好。我如何在 AAT 中做同样的事情？

这是我的完整 OT 代码：

feature clig {

script mymr;
@needSpaceOnTop = [uni102B uni1032 uni102D uni102E];

sub uni1000' uni103C' @needSpaceOnTop by uni1000_uni103C.alt;
sub uni1003' uni103C' @needSpaceOnTop by uni1003_uni103C.alt;
sub uni1006' uni103C' @needSpaceOnTop by uni1006_uni103C.alt;
sub uni1010' uni103C' @needSpaceOnTop by uni1010_uni103C.alt;
sub uni1011' uni103C' @needSpaceOnTop by uni1011_uni103C.alt;
sub uni1018' uni103C' @needSpaceOnTop by uni1018_uni103C.alt;
sub uni101C' uni103C' @needSpaceOnTop by uni101C_uni103C.alt;
sub uni101E' uni103C' @needSpaceOnTop by uni101E_uni103C.alt;
sub uni101F' uni103C' @needSpaceOnTop by uni101F_uni103C.alt;
sub uni1001' uni103C' @needSpaceOnTop by uni1001_uni103C.alt;
sub uni1002' uni103C' @needSpaceOnTop by uni1002_uni103C.alt;
sub uni100E' uni103C' @needSpaceOnTop by uni100E_uni103C.alt;
sub uni1004' uni103C' @needSpaceOnTop by uni1004_uni103C.alt;
sub uni1005' uni103C' @needSpaceOnTop by uni1005_uni103C.alt;
sub uni1007' uni103C' @needSpaceOnTop by uni1007_uni103C.alt;
sub uni1012' uni103C' @needSpaceOnTop by uni1012_uni103C.alt;
sub uni1015' uni103C' @needSpaceOnTop by uni1015_uni103C.alt;
sub uni1016' uni103C' @needSpaceOnTop by uni1016_uni103C.alt;
sub uni1019' uni103C' @needSpaceOnTop by uni1019_uni103C.alt;
sub uni101D' uni103C' @needSpaceOnTop by uni101D_uni103C.alt;
sub uni1065' uni103C' @needSpaceOnTop by uni1065_uni103C.alt;

} clig;

这是来自 OS X 字体工具文档的上下文示例：

------------------------------------------------------------------
//  Turn medial s into long s
------------------------------------------------------------------
Type        Contextual
Name        Smart Swashes
Namecode    8
Setting     Medial Long-s
Settingcode 8
Default     no
Orientation H
Forward     yes
Exclusive   no

Ess     s
Lower   a b c d e f g h i j k l m n o p q r t u v w x y z

            EOT OOB DEL EOL Ess Lower
StartText   1   1   1   1   2   1
StartLine   1   1   1   1   2   1
SawS        1   1   1   1   3   4
SawSS       1   1   1   1   3   4

    GoTo        Mark?   Advance?    SubstMark   SubstCurrent
1   StartText   no      yes         none        none
2   SawS        yes     yes         none        none
3   SawSS       yes     yes         ToLongS     none
4   StartText   no      yes         ToLongS     none

ToLongS
    s   slong

Answer 1

好的，我自己想通了。 AAT 不直接支持上下文连字。但是您可以通过使用两次传递来使其工作，如下所示：

如果后跟 N 或 M，将 X Y 更改为 X_Y：

//====================================================
//  Step 1: change Y to an arbitrary high glyph number
//  if it is preceded by X and followed by N or M  
//====================================================

Type Contextual
Name NULL
Namecode 7
Setting NULL
Settingcode 0
Default yes
Orientation HV
Forward yes
Exclusive no

// Define some classes
X X
Y Y
NM N M

// Define what action to take for state/action
            EOT     OOB     DEL     EOL   X     Y    NM
StartText   1       1       1       1     2     1    1
StartLine   1       1       1       1     2     1    1
SawX        1       1       1       1     1     3    1
SawY        1       1       1       1     1     1    4

// The state machine starts off in the StartText or StartLine state.
// If it sees an X in one of those states, it changes to the SawX
// state. If it's in the SawX state and sees a Y, then it changes to
// the SawY state and marks that character (the Y) for possible
// future processing. Then if it sees an N or M in the SawY state,
// it runs the DoSub on the marked Y, and returns to the StartText state.

// Actions - the Goto column tells what state to take for
// the next round.

  GoTo      Mark? Advance?    SubstMark   SubstCurrent
1 StartText no    yes         none        none
2 SawX      no    yes         none        none
3 SawY      yes   yes         none        none
4 StartText no    yes         DoSub       none

// Subs Y by 5999
DoSub
      Y 5999

//====================================================
// Step 2: Change the X 5999 to X_Y
//====================================================

Type          LigatureList
Name          NULL
Namecode      7
Setting       NULL
Settingcode   0
Default       yes
Orientation   HV
Forward       yes
Exclusive     no

// Replace X 5999 by X_Y
List
    X_Y X 5999

当然，要使其正常工作，5999 不能是以前存在的字形 ID。