feature-breakdown.md

A breakdown of advanced font feature implementation issues

The OpenType, AAT, and Graphite font features present 3 implementation questions for application developers:

• Which features should be enabled by default if they are present in the font
• Which features should be activated automatically by context
• Which features should be explicitly presented to the user in the user interface (see UI)

Defaults

Adobe, Microsoft, and FontLab's Adam Twardoch each have a set of recommendations as to which features ought to be activated by default. They are indicated with "on" against a green background in the following table.

Where features are marked with "shaper," the recommendation is to leave activation and usage of that feature up to the script-shaping library (e.g., HarfBuzz). Features marked "dependent" are dependencies of other, higher-level features and are not activated separately. Featured marked "contextual" depend on the context of the text or document to determine whether they should be enabled.

The three lists agree on most features; those where there are conflicts are discussed below in the conflicts section.

Tag	Feature	Default: Adobe	Default: MS	Default: Twardoch	UI: Twardoch
aalt	Access All Alternates	off	off	off	special
abvf	Above-base Forms	shaper	shaper	shaper
abvm	Above-base Mark Positioning	shaper	shaper	shaper
abvs	Above-base Substitutions	shaper	shaper	shaper
afrc	Alternative Fractions	off	off	off	yes
akhn	Akhands	shaper	shaper	shaper
altv	Alternate Vertical Metrics [deprecated]			off
blwf	Below-base Forms	shaper	shaper	shaper
blwm	Below-base Mark Positioning	shaper	shaper	shaper
blws	Below-base Substitutions	shaper	shaper	shaper
calt	Contextual Alternates	on	on	on	yes
case	Case-Sensitive Forms	contextual	contextual	off	yes
ccmp	Glyph Composition / Decomposition	on	on	on
cfar	Conjunct Form After Ro		shaper	shaper
cjct	Conjunct Forms		shaper	shaper
clig	Contextual Ligatures	on	on	on	yes
cpct	Centered CJK Punctuation		off
cpsp	Capital Spacing	on	on	off	yes
crcy	Currency [deprecated]			off
cswh	Contextual Swash	off	off	off	yes
curs	Cursive Positioning	user	user	on
cv01-99	Character Variants		off	off	yes
c2pc	Petite Capitals From Capitals	off	off	off	yes
c2sc	Small Capitals From Capitals	off	off	off	yes
dflt	Default processing [deprecated]			off
dist	Distances	shaper	shaper	shaper
dlig	Discretionary Ligatures	off	off	off	yes
dnom	Denominators	dependent	dependent	off	yes
dpng	Dipthongs [deprecated]			off
dtls	Dotless forms		dependent	shaper
expt	Expert Forms	user	user	off	yes
falt	Final Glyph on Line Alternates	user	user	off	yes
fin2	Terminal Forms #2	shaper	shaper	shaper
fin3	Terminal Forms #3	shaper	shaper	shaper
fina	Terminal Forms	on	on	shaper	special
flac	Flattened Ascent Forms		contextual	shaper
frac	Fractions	off	off	off	yes
fwid	Full Widths	off	off	off	yes
half	Half Forms	shaper	shaper	shaper
haln	Halant Forms	shaper	shaper	shaper
halt	Alternate Half Widths	off	off	off	yes
hist	Historical Forms	off	off	off	yes
hkna	Horizontal Kana Alternates	of	off	contextual	special
hlig	Historical Ligatures	off	off	off	yes
hngl	Hangul	off	off	off	yes
hojo	Hojo Kanji Forms (JIS X 0212-1990 Kanji Forms)	off	off	off	yes
hwid	Half Widths	off	off	off	yes
init	Initial Forms	on	on	shaper	special
isol	Isolated Forms	on	on	shaper	special
ital	Italics	integrated	integrated	off	yes
jajp	Japanese Forms [deprecated]			off
jalt	Justification Alternates	user	user	off	yes
jp78	JIS78 Forms	off	off	off	yes
jp83	JIS83 Forms	off	off	off	yes
jp90	JIS90 Forms	off	off	off	yes
jp03	JIS03 Forms [deprecated]			off
jp04	JIS2004 Forms	off	off	off	yes
kern	Kerning	on	on	on
kokr	Korean Forms [deprecated]			off
lfbd	Left Bounds	dependent	dependent	on
liga	Standard Ligatures	on	on	on	yes
ljmo	Leading Jamo Forms	shaper	shaper	shaper
lnum	Lining Figures	off	off	off	yes
locl	Localized Forms	on	on	on	special
ltra	Left-to-right alternates		?	bidi
ltrm	Left-to-right mirrored forms		?	bidi
mark	Mark Positioning	shaper	shaper	on
med2	Medial Forms #2		shaper	on
medi	Medial Forms	on	on	shaper	special
mgrk	Mathematical Greek	off	off	off	yes
mkmk	Mark to Mark Positioning	on	on	on
mset	Mark Positioning via Substitution	shaper	shaper	on
nalt	Alternate Annotation Forms	off	off	off	yes
nlck	NLC Kanji Forms	off	off	off	yes
nukt	Nukta Forms	shaper	shaper	shaper
numr	Numerators	dependent	dependent	off	yes
onum	Oldstyle Figures	off	off	off	yes
opbd	Optical Bounds	on	on	on	special
ordn	Ordinals	off	off	off	yes
ornm	Ornaments	off	off	off	yes
palt	Proportional Alternate Widths	off	off	off	yes
pcap	Petite Capitals	off	off	off	yes
pkna	Proportional Kana		off	off	yes
pnum	Proportional Figures	off	off	off	yes
pref	Pre-Base Forms	shaper	shaper	shaper
pres	Pre-base Substitutions	shaper	shaper	shaper
pstf	Post-base Forms	shaper	shaper	shaper
psts	Post-base Substitutions	shaper	shaper	shaper
pwid	Proportional Widths	user	user	off	yes
qwid	Quarter Widths	off	off	off	yes
rand	Randomize	user / on	user / on	off	yes
rclt	Required Contextual Alternates		on	on
rkrf	Rakar Forms		shaper	shaper
rlig	Required Ligatures	on	on	on
rphf	Reph Forms	shaper	shaper	shaper
rtbd	Right Bounds	dependent	dependent	on
rtla	Right-to-left alternates	?	?	shaper
rtlm	Right-to-left mirrored forms		?
ruby	Ruby Notation Forms	off	off	off	yes
salt	Stylistic Alternates	off	off	off	yes
sinf	Scientific Inferiors	off	off	off	yes
size	Optical size	on	on	on	special
smcp	Small Capitals	off	off	off	yes
smpl	Simplified Forms	off	off	off	yes
ss01-20	Stylistic Sets 1 to 20	off	off	off	yes
ssty	Math Script Style Alternates		dependent	shaper
stch	Stretching Glyph Decomposition		on
subs	Subscript	off	off	off	yes
sups	Superscript	off	off	off	yes
swsh	Swash	off	off	off	yes
titl	Titling	off	off	off	yes
tjmo	Trailing Jamo Forms	shaper	shaper	shaper
tnam	Traditional Name Forms	off	off	off	yes
tnum	Tabular Figures	off	off	off	yes
trad	Traditional Forms	off	off	off	yes
twid	Third Widths	off	off	off	yes
unic	Unicase	off	off	off	yes
valt	Alternate Vertical Metrics	contextual	contextual	contextual	special
vatu	Vattu Variants	shaper	shaper	shaper
vert	Vertical Writing	contextual	contextual	contextual
vhal	Alternate Vertical Half Metrics	off	off	off	yes
vivn	Vietnamese Forms [deprecated]			off
vjmo	Vowel Jamo Forms	shaper	shaper	shaper
vkna	Vertical Kana Alternates	off	off	contextual	special
vkrn	Vertical Kerning	contextual	contextual	contextual	special
vpal	Proportional Alternate Vertical Metrics	off	off	off	yes
vrt2	Vertical Alternates and Rotation	contextual	contextual	contextual	special
zero	Slashed Zero	user	user	off	yes
zhcn	Simplified Chinese Forms [deprecated]			off
zntw	Traditional Chinese Forms [deprecated]			off

Conflicts between recommendations

There are 22 features where the various specification recommendations disagree about the correct default state, not counting those instances where one or more of the documents is silent on the question.

• altv Alternate Vertical Metrics [deprecated]
• case Case-Sensitive Forms
• curs Cursive Positioning
• expt Expert Forms
• falt Final Glyph on Line Alternates
• fina Terminal Forms
• hkna Horizontal Kana Alternates
• init Initial Forms
• isol Isolated Forms
• ital Italics
• jalt Justification Alternates
• lfbd Left Bounds
• mark Mark Positioning
• med2 Medial Forms #2
• medi Medial Forms
• mset Mark Positioning via Substitution
• pwid Proportional Widths
• rand Randomize
• rtbd Right Bounds
• ssty Math Script Style Alternates
• zero Slashed Zero

Several others Twardoch recommends off-by-default or leaving the decision up to the shaper, while Adode and Microsoft say the feature should be automatically enabled by context, since it is dependent on another feature:

• dnom Denominators
• dtls Dotless forms
• numr Numerators

UI exposure

Twardoch makes a point of highlighting those features which should be accessible through the user interface; this is noted in the right-most column. Note also that he designates some features as "special" indicating that the UI access involves more than a simple toggle or checkbox.

Adobe and Microsoft also advise allowing application-wide user preferences to determine the state of seven features:

• curs Cursive Positioning
• expt Expert Forms
• falt Final Glyph on Line Alternates
• jalt Justification Alternates
• pwid Proportional Widths
• rand Randomize
• zero Slashed Zero

Finally, Adobe and Microsoft advise that usage of the "Italics" feature, ital, be integrated into an application's existing UI for selecting roman/italic variants of a font. Twardoch does not mark ital as a special case.