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Font Feature Registry
This document describes the feature registry from several points
of view, including the font designer, the user, and the application
developer. For more information on presenting features to users,
please consult the Font Human Interface Guidelines.
The designer/user model
The All Typographic Features feature type allows the user to specify
whether or not any effects should occur. It is an all or nothing
bit--if off, all other specifications are ignored. This feature
might be turned off by a GX user who receives a document from
a non-GX user and wants to ensure that the document does not reflow.
The Ligatures feature type permits selection from different ranges
of ligatures. The Required selector should only be used for those
ligatures that are linguistically required (such as occur in Arabic).
The Common selector can be used for the ligatures that usually
appear in well-set text. The Rare selector can be used to include
ligatures in a font that are less common than those included in
the Common category. The Logos selector can be used to include
ligatures representing logos (e.g. trademarks). The Rebus Pictures
selector can be used to include ligatures that are pictures representing
words or syllables. The Diphthong Ligatures setting is used to
specify automatic creation of ligatures like Æ.
The Cursive Connection feature type is used for cursively-connected
scripts. It is required for Arabic, but may be used for other
scripts as well. The Unconnected selector disables cursive connection
altogether. The Partially Connected selector can be used to select
pre-drawn letterforms that connect in a non-contextual manner
(look at the "Swing" font, for example). The Cursive selector
is used to select full, contextual connection of letterforms.
This setting will be the default in Arabic fonts.
The Letter Case feature type is used to specify changes to the
case of letters (in scripts where case has meaning), in either
a contextual or a non-contextual manner. The Upper & Lower Case
selector is the default setting and leaves letters in whichever
case the user has specified. The All Caps and All Lower Case selectors
convert all letters (non-contextually) to the specified form.
The Small Caps selector specifies conversion of lower-case letters
to small caps forms. The Initial Caps selectors--the only contextual
selectors in this group--specify conversion of lower-case letters
at the beginnings of words into upper-case.
The Vertical Substitution feature type can be used to specify
that glyphs need to change their appearance in vertical runs of
text.
The Linguistic Rearrangement feature type specifies whether linguistic
rearrangement of glyphs (such as happens in South Asian scripts)
should happen. It is on by default for fonts representing these
scripts.
The Number Spacing feature type specifies a choice for the appearance
of digits. Monospaced Numbers are of uniform width and are useful
for displaying in columns, while Proportional Numbers may have
individual widths.
The Smart Swashes feature type controls contextual swash substitution.
The first 4 selectors control swashes that get substituted at
the beginnings or ends of words or whole lines. The Archaic Non-Finals
selector specifies forms that are used at the beginning or middle
of words--archaic s can be specified this way.
The Diacritics feature type allows control over how diacritics
(i.e. accent marks or applied vowels) appear in text. The default
selector is Show Diacritics. The Hide Diacritics selector means
to not show the marks. This is useful in Arabic children's book
text, where the same text can be shown with vowels for one reading
audience and without vowels for another. The Decompose Diacritics
selector means to not attach the marks, but rather to place them
in-line with the rest of the text.
The Vertical Position feature type controls things like superscripts
and subscripts. The Superiors and Inferiors selectors are used
to map to alternate glyph forms, positioned differently with respect
to the baseline. The Ordinals selector is used to contextually
change attached ordinal marks into superior forms.
The Fractions feature type controls the selection and/or generation
of fractions. The No Fractions selector specifies that fractions
should not be formed automatically. The Vertical Fractions selector
will form vertical (pre-drawn) fractions present in the font.
The Diagonal Fractions selector acts as the Vertical Fractions
selector does, but in addition fractions will be synthesized using
superiors and inferiors.
The Overlapping Characters feature type permits the user to prevent
the collision of long tails on characters with other characters.
The Typographic Extras feature type represents a collection of
effects that are associated with fine typography. The Hyphens
to Em Dash selector causes two adjacent hyphens to change into
a single em dash. The Hyphen to En Dash selector changes the hyphen
in a space-hyphen-space or digit-hyphen-digit group into an en
dash. The Unslashed Zero selector makes sure the unslashed zero
is always used (even if the font designates the slashed zero as
default), of course assuming the unslashed zero actually exists
in the font. The Form Interrobang option maps the strings `?!'
or `!?' to the interrobang (if present in the font). The Smart
Quotes selector contextually replaces straight quotes with curly
quotes.
The Mathematical Extras feature type represents a collection of
effects useful in setting figures and mathematics. The Hyphen
To Minus selector changes the hyphen in a space-hyphen-space or
digit-hyphen-digit group into a minus (note this overrides the
effect of the Hyphen to En Dash selector in the Typographic Extras
feature type). The Asterisk to Multiply selector changes the *
in a space-*-space group into a multiplication sign. The Slash
to Divide selector changes the / in a space-/-space group into
a stacked divide glyph. The Inequality Ligatures selector changes
the >= sequence into a stacked >= glyph and the <= sequence into
a stacked <= glyph. The Exponents selector takes digits immediately
following a ^ and changes them into their superior forms.
The Ornament Sets feature type specifies non-letter ornament sets
of glyphs. For example, fleurons or decorative borders could be
represented by this feature type. The None selector means to do
no simple ornament mapping. The Dingbats selector chooses miscellaneous
symbols such as arrows, stars, and pointing hands used for occasional
emphasis in display. The Pi Characters selector chooses a set
of related symbols designed for a particular purpose (e.g. cartography
or musical notation) which do not make up a formal alphabet. The
Fleurons selector chooses ornaments in the shape of flowers, vine
leaves, and so on. The Decorative Borders selector chooses glyphs
designed to be used in interlocking or repeating patterns as borders
for text. The International Symbols selector chooses glyphs representing
standard, internationally-recognizable icons (for example, the
circle with the bar through it representing "no..."). The Math
Symbols selector chooses special symbols used in setting mathematics
or logic text.
The radio button selectors for this type simply select different
sets of glyphs in a font. For example, a font with twenty ampersands
could place them in twenty selectors under this feature type.
Use of the Style Options feature type is, however, preferred when
feasible. The first setting for this feature should be "No Alternates".
The Design Complexity feature type controls the overall appearance
of the font. It can be used to allow a single font to contain plain glyphs, italic glyphs, calligraphic chancery
glyphs, etc. Design Level 1 should be available for any font which
utilizes this feature type, and it should generally be the default
setting. The remaining design levels progress from simplest to
most complex.
The Style Options feature type allows the font designer to group
together collections of non-contextual substitutions into named
sets. If plain text is desired, the No Style Options feature selector
should be chosen. The Display Text selector chooses glyphs designed
to emphasize the unique appearance of the design at display sizes,
typically above 24 point. The Engraved Text selector chooses glyphs
with contrasting strokes parallel to the main strokes, particularly
those designed to look like they're engraved in stone. The Illuminated
Caps selector chooses capital letter glyphs with decoration filling
the white space surrounding the glyph, in the manner used by medieval
scribes. The Titling Caps selector specifies conversion of capital
letters to a special titling form. The Tall Caps selector specifies
conversion of certain capital letters into taller forms.
The Character Shape feature type is primarily for use with Chinese
fonts to specify the use of the Traditional or Simplified character
forms. In unusual cases in which no other feature type seems appropriate,
Character Shape might be used with other, possibly Roman, fonts
to include multiple forms of the same character (e.g., two different
forms of "A", the simplified version of which might not have a
crossbar).
Number Case is independent of Letter Case. Lower Case Numbers
(also called "traditional" or "old style") are digits which may
descend below the baseline, as opposed to Upper Case Numbers (also
called "lining"), which do not descend below the baseline.
The Text Spacing feature type is used to select between the proportional
and monospaced forms of letters in a font. Note that it is separate
from the Number Spacing type, which just controls digits.
The application interface
This section gives the C language interface that applications
use to support the feature types and selectors listed above.
enum {
allTypographicFeaturesType = 0,
allTypeFeaturesOnSelector = 0,
allTypeFeaturesOffSelector = 1,
ligaturesType = 1,
requiredLigaturesOnSelector = 0,
requiredLigaturesOffSelector = 1,
commonLigaturesOnSelector = 2,
commonLigaturesOffSelector = 3,
rareLigaturesOnSelector = 4,
rareLigaturesOffSelector = 5,
logosOnSelector = 6,
logosOffSelector = 7,
rebusPicturesOnSelector = 8,
rebusPicturesOffSelector = 9,
diphthongLigaturesOnSelector = 10,
diphthongLigaturesOffSelector = 11,
cursiveConnectionType = 2,
unconnectedSelector = 0,
partiallyConnectedSelector = 1,
cursiveSelector = 2,
letterCaseType = 3,
upperAndLowerCaseSelector = 0,
allCapsSelector = 1,
allLowerCaseSelector = 2,
smallCapsSelector = 3,
initialCapsSelector = 4,
initialCapsAndSmallCapsSelector = 5,
verticalSubstitutionType = 4,
substituteVerticalFormsOnSelector = 0,
substituteVerticalFormsOffSelector = 1,
linguisticRearrangementType = 5,
linguisticRearrangementOnSelector = 0,
linguisticRearrangementOffSelector = 1,
numberSpacingType = 6,
monospacedNumbersSelector = 0,
proportionalNumbersSelector = 1,
appleReserved1Type = 7,
smartSwashType = 8,
wordInitialSwashesOnSelector = 0,
wordInitialSwashesOffSelector = 1,
wordFinalSwashesOnSelector = 2,
wordFinalSwashesOffSelector = 3,
lineInitialSwashesOnSelector = 4,
lineInitialSwashesOffSelector = 5,
lineFinalSwashesOnSelector = 6,
lineFinalSwashesOffSelector = 7,
nonFinalSwashesOnSelector = 8,
nonFinalSwashesOffSelector = 9,
diacriticsType = 9,
showDiacriticsSelector = 0,
hideDiacriticsSelector = 1,
decomposeDiacriticsSelector = 2,
verticalPositionType = 10,
normalPositionSelector = 0,
superiorsSelector = 1,
inferiorsSelector = 2,
ordinalsSelector = 3,
fractionsType = 11,
noFractionsSelector = 0,
verticalFractionsSelector = 1,
diagonalFractionsSelector = 2,
appleReserved2Type = 12,
overlappingCharactersType = 13,
preventOverlapOnSelector = 0,
preventOverlapOffSelector = 1,
typographicExtrasType = 14,
hyphensToEmDashOnSelector = 0,
hyphensToEmDashOffSelector = 1,
hyphenToEnDashOnSelector = 2,
hyphenToEnDashOffSelector = 3,
unslashedZeroOnSelector = 4,
unslashedZeroOffSelector = 5,
formInterrobangOnSelector = 6,
formInterrobangOffSelector = 7,
smartQuotesOnSelector = 8,
smartQuotesOffSelector = 9,
mathematicalExtrasType = 15,
hyphenToMinusOnSelector = 0,
hyphenToMinusOffSelector = 1,
asteriskToMultiplyOnSelector = 2,
asteriskToMultiplyOffSelector = 3,
slashToDivideOnSelector = 4,
slashToDivideOffSelector = 5,
inequalityLigaturesOnSelector = 6,
inequalityLigaturesOffSelector = 7,
exponentsOnSelector = 8,
exponentsOffSelector = 9,
ornamentSetsType = 16,
noOrnamentsSelector = 0,
dingbatsSelector = 1,
piCharactersSelector = 2,
fleuronsSelector = 3,
decorativeBordersSelector = 4,
internationalSymbolsSelector = 5,
mathSymbolsSelector = 6,
characterAlternativesType = 17,
noAlternatesSelector = 0,
designComplexityType = 18,
designLevel1Selector = 0,
designLevel2Selector = 1,
designLevel3Selector = 2,
designLevel4Selector = 3,
designLevel5Selector = 4,
styleOptionsType = 19,
noStyleOptionsSelector = 0,
displayTextSelector = 1,
engravedTextSelector = 2,
illuminatedCapsSelector = 3,
titlingCapsSelector = 4,
tallCapsSelector = 5,
characterShapeType = 20,
traditionalCharactersSelector = 0,
simplifiedCharactersSelector = 1,
numberCaseType = 21,
lowerCaseNumbersSelector = 0,
upperCaseNumbersSelector = 1,
textSpacingType = 22,
proportionalTextSelector = 0,
monospacedTextSelector = 1,
lastFeatureType = -1
};
The Apple Fonts Group (applefonts@apple.com)
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