There are two types of contextual positioning modes: Contextual Positioning
and Chained Contextual Positioning. In the simplest form of the first you
may specify a list glyphs and specify repositioning to occur if that sequence
is matched. For example you might specify (in English)
5 t h
and if that sequence were found raise the "t" and "h" to 5th.
You can also specify classes of glyphs so you could say something like:
[0-9] t h
to raise "t" and "h" after any digit.
After applying this lookup, a word processor will skip over the three glyphs matched by the pattern, at least it will in a simple "Contextual Positioning" lookup. The "Chained Contextual Positioning" lookup is a bit more general and allows you to divide the pattern into three parts: A part before the current glyph (called backtracking), a part including the current glyph, and a part after the current glyph (called lookahead). Positioning changes may only be made to the part including the current glyph, and the word processor will advance by the number of glyphs in that subset of the pattern.
Substitutions come in three types. The first two are similar to the two types for positioning, the third is designed to handle a very specific case of arabic typography and is applied backwards.
Suppose you had a script font where most letters join at the base line, but
after some letters (b,o,v,w) the join is near the x-height, so a special
version of each lower case letter needs to be created designed for a left
side join near the x-height. You would want to be able to say:
[bovw] [a-z]
Note that this just specifies the cases in which the substitution may be
applied. It does not specify the substitution itself, that is done in a separate
lookup.
You may pull down an example script font with this substitution from FormalScript.tgz, this example is worked out in detail in the tutorial.
For more information on contextual features see Adobe's Docs:
In some cases a contextual or chaining contextual substitution can be converted into one of Apple's contextual glyph substitution subtables. See the page on Apple Advanced Typography for more information as to when and caveats about how.
Before you can edit one of these features you must create it from the Element->Font Info->Contextual dialog.
Except for the reverse chaining features, any of these may come in one of three formats.
The simplest format is a list of glyphs. You may specify
several glyph sequences in a single feature. So the script example above
could be specified (quite inefficiently) by 4*26 lines like:
b a
b b
...
o a
...
The next format allows you to specify a list of
classes. In the script example we would define 2
classes:
[bovw]
[ac-np-ux-z]
and define two patterns as:
class1 [class2]
class1 [class1]
The third format is the most general and allows you to specify your pattern
by a separate list of glyphs for each glyph
position:
[bovw] [a-z]
This display shows a list whose entries are coverage tables -- that is lists of glyphs. A match occurs if the current glyph matches one of the glyphs named in the current coverage table.
In the example at right there is only a single coverage table here (but there could be more).
In a Chaining Contextual feature it is also possible to specify a list of coverage tables to match glyphs before the current glyph, and another list to match glyphs after.
The buttons under the coverage list allow you to create new coverage tables to go to the end of the list (ie. to be added to the end of the match string), to edit current coverage tables, to remove a coverage table, or to reorder the coverage tables.
This feature does not directly describe how transformations are to happen to the glyphs, instead it provides a list of nested features (actually lookups, but I'm trying to hide that distinction) that are to be applied to specific positions in the match string (positions in the backtrack and lookahead lists my not be transformed).
In the example at right, the feature 'L041' will be applied to any glyph
that matches the coverage table at the top of the list. Again the buttons
under the sequence lookup list allow you to add, change, remove or reorder
these transformations. (And yes, the order the transformations are applied
can matter in complex situations).
This dialog allows you to specify which position in the match string is to have which nested feature applied to it.
A nested feature is very similar to a normal feature (position, pair,
substitution, ligature, anchor, etc.) except that instead of having a
script/language list it has a special value of "Nested".
If you press the [New] or [Edit] buttons from the coverage dialog above you get the page to the right. The text field provides the names of all glyphs in the coverage table. You may edit this list directly by typing in it, or you may press the [Set From Font] button which sets the text field to contain the names of all glyphs selected in the font view. And the [Select In Font] button changes the selection of the font view to match the contents of the text field.
You may also add some standard classes of glyphs (as shown in the pull down menu). When adding letters only those letters will be used which are in the scripts for which this lookup is active.
If you wish to cancel this operation press the [<Prev] button and you
will be returned to the coverage list page (and any changes made here will
be lost), while pressing [Next>] will accept those changes.
This format allows you to specify several glyph lists to match. In the example at right the string "A,B,C" will be matched in the glyphs before the current glyph, then "D E" starting at the current location, and finally "F G H" after them. If everything matched then the lookup "Smallcaps" will be applied to location 0 (in this case "D") , and "superscript" to location 1 (here, "E").
The buttons underneath allow you to add, change, remove and reorder these
entries. The order is significant, word processors will stop at the first
match they find so in the following pattern strings:
b a
b a f
the second entry would never be matched because "b a" would be applied first.
While:
b a f
b a
would apply match "b a f" when an "f" was present, and "b a"
otherwise.
The text field may be manipulated as above. However here we are specifying a string of glyphs each of which must be matched, rather than a class of glyphs any of which could match. So in the example at right, "DE" would match, but "E" would not.
As above a set of lookups can be applied after the match occurs.
When specifying a class match, you must specify at least one set of glyph classes. (you may also specify separate sets of classes for the backtrack and lookahead lists, or you use the same set for everything). You must also specify a list of patterns where each pattern is a list of numbers and each number is the index of the class into the class list.
Every set of classes has a magic class at index 0 which matches whatever
glyphs are not explicitly mentioned in another class. FontForge displays
this by calling it "{Everything Else}". You may not delete, edit nor reorder
this class. Other classes are just unordered lists of glyph names (rather
like coverage tables above), however any specific glyph may belong to at
most one class at a time (whereas a glyph may occur in as many coverage tables
as desired).
Once you have set up your classes, you may then edit the patterns you want to match. To remind you of what your classes are, FontForge displays the class list underneath the pattern. Clicking on a class will insert that class's index into the pattern.
Finally you must set the nested lookups. This is exactly the same as previous
sequence / lookup settings.