6.3 OpenType
Links to the OpenType specification, as well as other font specifications, can be found on ScriptSource.
Lookup Orders
The OpenType specification suggests that software should:
assemble all lookups from the set of chosen features, and apply the lookups in the order given in the LookupList table.
However in actual implementations this isn’t how it is usually done. Instead, features are divided into sets, and the sets are processed in a specific order. Within a set, all the lookups associated with the included features are executed (in lookup order) before moving on to the next set of features.
It is important, therefore, that font developers know how features are divided into sets, and the order in which these sets are executed. To complicate matters, this is:
- not always consistent among shaping engines
- script-specific, and
- not well documented.
The most useful information is gleaned by reading the Harfbuzz code1 since (a) the code is opensource and thus accessible, and (b) Harfbuzz tries to be compatible with Microsoft’s Uniscribe.
The following tables attempt to document what the Harfbuzz code indicates for GSUB execution. Each row represents a set of features, each feature identified by its 4-character tag.
| Default shaper | |
|---|---|
| GSUB | rvrn |
| ltra ltrm frac numr dnom rand ccmp locl rlig calt clig liga rclt userfeatures | |
| GPOS | mark mkmk curs kern userfeatures |
| USE | |
|---|---|
| GSUB | rvrn |
| syllable analysis | ltra ltrm rtla rtlm frac numr dnom rand trak HARF |
| locl ccmp nukt akhn | |
| rphf | |
| pref | |
| rkrf abvr blwf half pstf vatu cjct | |
| insert dotted circles, reorder | isol init med fina |
| abvs blws haln pres psts BUZZ ccmp locl rlig calt clig liga rclt vert userfeatures | |
| GPOS | dist abvm blwm mark mkmk curs kern userfeatures |
| Arabic | Syriac | |
|---|---|---|
| GSUB | rvrn | rvrn |
| rtla rtlm frac numr dnom rand | rtla rtlm frac numr dnom rand | |
| stch | ||
| ccmp locl | ccmp locl | |
| isol | isol | |
| fina | fina | |
| fin2 | ||
| fin3 | ||
| medi | medi | |
| med2 | ||
| init | init | |
| rlig | rlig | |
| rclt calt | rclt calt | |
| mset rlig clig liga userfeatures | mset rlig clig liga userfeatures | |
| GPOS | mark mkmk curs kern | mark mkmk curs kern |
| Indic | Khmer | |
|---|---|---|
| GSUB | rvrn | rvrn |
| ltra ltrm frac numr dnom rand | ltra ltrm frac numr dnom rand | |
| ccmp locl | ccmp locl | |
| nukt | ||
| akhn | ||
| rphf | ||
| rkrf | ||
| pref | pref blwf abvf | |
| blwf | ||
| abvf | ||
| half | ||
| pstf | ||
| vatu | ||
| cjct | ||
| cfar | ||
| init pres abvs blws psts haln rlig calt clig rclt | pres abvs blws psts rlig calt clig rclt userfeatures | |
| userfeatures | ||
| GPOS | curs kern dist abvm blwm | curs dist abvm blwm |
where userfeatures includes everything else that the user (or CSS) might have requested, such as dlig, ssxx, cvxx, smcp, onum, etc.
Language Tags
(This section has not yet been written.)
Feature Tags
The OpenType specification provides for Layout Feature tables, each of which is identified by a 4-character tag.
(This rest of this section has not yet been written.)
Stylistic Sets and Character Variants
Tags for Layout Feature tables which are in the range “ss01” to “ss20” are registered as Stylistic Set 1 through 20. Tags in the range “cv01” to “cv99” are registered as Character Variant 1 through 99.
When should one use Stylistic Sets and when should one use Character Variants?
According to the OpenType spec, Character Variants (cvxx tags) should be preferred when only one character or very closely related characters are affected. Examples include:
- matching lower and upper case characters
- a lower case character and its small capital form
- all characters that use a particular diacritic, where the diacritic has two forms
Stylistic Sets (ssxx tags) are preferred when systematically related changes affect more distinct characters. Examples include:
- ‘barred b’, ‘barred d’, and ‘barred g’ characters with different placements for the bar
- ‘a’ and ‘g’ characters with literacy forms
Having said that, there are some technical and practical distinctions that may require violating these guidelines:
- The cvxx tags can utilize GSUB “alternate” (type 3) lookups, thus allowing a feature to have more than on/off values.
- There are only 20 registered ssxx features, but 99 cvxx features.
- Some apps (MS Word for example) assume that users would need to turn on only one ssxx feature at a time.
- Not as many apps support cvxx (yet).
GSUB only
Note that only substitution type lookups are allowed in Stylistic Set and Character Variant features and the features must be in the GSUB table. There may be cases where variant positioning behavior is needed, such as a feature that controls whether a particular diacritic is drawn touching its base or separated from it. While it may seem reasonable to use positioning type lookups and place the features in the GPOS table, it is unlikely that rendering engines will actually process such features.
Mark classes and filter sets
(This section has not yet been written.)