List, one to start a new section (go from back tracking to active, or fromĪctive to lookahead), and one to add a new lookup reference.Ī warning about contextual behavior: Not all software supports them. There are two buttons down at the bottom of the pattern As you enter glyph names they will beĪuto completed for you. You may edit a pattern in the obvious way. YouĬan select glyphs in the font view if that is easier than remembering their This produces another dialog showing a font view. To edit a glyph class simply click on it and type. That line must be in a separate sub-table in the same lookup. The OpenType specĪllows for it, but implementations of OpenType don’t seem to support it. "high-after | high-after |" to handle the second case Now there should be a second line which would look like Includes those glyphs which must be followed by a high variant, so this seems If you look at the glyph class definitions you will see that class “high-afters” Should have simple substitution “To-TopJoin” applied to it. “letter”, then that letter in the match string (that is the current glyph) Rules” field) should be read thus: If a backtracking glyph (the glyph before theĬurrent one) in class “high-after” is followed by the current glyph in class To convert the substitution to Apple’s format). In this example we use the same classes for all categories (this makes it easier Itself (themselves), which may have simple substitutions applied to them, andįinally glyphs after the current glyph (these are called lookahead glyphs).Įach category of glyphs may divide glyphs into a different set of classes, but A contextual chainingĭialog divides the glyph stream into three sections: those glyphs before theĬurrent glyph (these are called backtracking glyphs), the current glyph(s) That are the glyph classes that this substitution uses. Match and substitutions that will be applied if the string matches. The next dialog finally shows something interesting. We want a class based system – we’ve already mentioned what the glyph classes The first dialog allows you to specify the overall format of the substitution. Series of dialogs to edit a contextual subtable And of course we want an associated subtable). Lookup, a contextual chaining lookup which should be associated with a ‘calt’įeature. Go with our lookup, and we can use the button to fill it Not be attached to any feature, instead it will be invoked by a contextual This is a “Simple Substitution” lookup, but it will The first thing we must do is create a simple substitution mapping each low Have one glyph appearing in two classes). Instead of two? Because in this case all my classes must be disjoint, I mayn’t (You might wonder why I don’t just have a class of all letters and use one rule => Apply a substitution to the second letter => Apply a substitution to second letter If either of these matches the second glyph should be transformed Will match a glyph in the “bovw” class followed by another glyph in the “bovw”Ĭlass, while the second will match a glyph in the “bovw” class followed by any We need to create two patterns, the first We divide the set of possible glyphs into three classes: the letters “bovw”, all Them for some other scripts they will work.Īs time has gone on ‘calt’ has been supported by more applications for latnĪnd this warning is no longer as applicable as it once was. I knew enough about Indic or Arabic typesetting I would provide an exampleįor those scripts. Why do I provide an example which might not work? It’s the best I can do. “Contextual Alternative” (‘calt’) feature for the latin script. The following example may not work! The font tables produced by it are allĬorrect, but early implementations of OpenType did not support decided the Let us call this second set of letters the “high” lettersĪnd name them “a.high,” “b.high” and so forth. That joins (on the left) at the baseline (the default variant) and one which Thus we need two variants for each glyph, one “v” and “w” join their following letter near the x-height, while all other With, the problem of typesetting a latin script font where the letters “b,” “o,” Instead of an Indic or Arabic example, let us take something I’m more familiar Substitution, later of contextual ligatures. First I shall provide an example of a contextual Specified by a state machine – a mini program which parses and transforms theĬonditional features may involve substitutions, ligatures or kerning (and some Substitutions it defines will be applied. In OpenType a context is specified by a set of patterns thatĪre tested against the glyph stream of a document. Which only take place in a given context and are essential for typesetting IndicĪnd Arabic scripts. OpenType and Apple fonts both provide contextual features.
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