The features listed below have been defined to create the basic forms for the languages that are supported on Tamil systems. Regardless of the model an application chooses for supporting layout of complex scripts, the shaping engine requires a fixed order for executing features within a run of text to consistently obtain the proper basic form.
The features of the basic shaping forms are applied one at a time to the cluster or portion of the cluster. The result impacts the analysis in terms of the conjoining behavior and final reordering. The features of the presentation forms are applied next, to the entire cluster simultaneously. Mandatory features must always be applied; the discretionary presentation-forms features listed should be applied by default, but can be suppressed by a client (normally at the discretion of the user).
The order of the lookups within each feature is also very important. For more information on lookups and defining features in OpenType fonts, see the Encoding section of the OpenType Font Development document.
OpenType features used for Tamil scripts, applied in the following order:
|Feature||Feature function||Layout operation|
|locl||Localization form substitution||GSUB|
|Basic shaping forms:|
|nukt||Nukta form substitution||GSUB|
|akhn||Akhand ligature substitution||GSUB|
|rphf||Reph form substitution||GSUB|
|pref||Pre-base form substitution||GSUB|
|Mandatory presentation forms:|
|haln||Halant form substitution||GSUB|
|Discretionary presentation forms:|
|abvm||Above-base mark positioning||GPOS|
|blwm||Below-base mark positioning||GPOS|
|[GSUB = glyph substitution, GPOS = glyph positioning]|
Many of the registered features described and illustrated in this document are based on the Microsoft OpenType font Latha. 'Latha' contains layout information and glyphs to support all of the required features for the Tamil script and language systems supported.
The illustrations in the following examples show the result of that particular feature being applied. Features must be written to match glyph sequences after re-ordering has occurred. Note that the input context for a feature may be the result of a previous feature having already been applied.
Feature Tag: "locl"
This feature is used in association with OpenType language system tags to trigger lookups that will select alternate glyphs needed for language-specific typographic conventions. The 'locl' should not be used in association with the default language system, but only used with other language system tags. See the Appendix of this document for language system tags associated with the Tamil script.
Feature Tag: "nukt"
The nukta alters the way a preceding consonant or vowel is pronounced. The most common nukta forms have been defined as separate characters in Unicode with their own code points. All consonants, as well as akhand forms should have an associated nukta form.
Note - Rather than using substitution, nukta forms can also be created by positioning the nukta as a below-base mark on the base glyph using the 'blwm' positioning feature
The input context for the nukt feature always consists of the full form of the consonant. The half form of nukta consonants will be substituted using the half feature.
Feature Tag: "akhn"
An akhand is a required consonant ligatures that may appear anywhere in the syllable, and may or may not involve the base glyph. Akhand ligatures have the highest priority and are formed first; some languages include them in their alphabets.
The input context for the akhand feature always consists of the full form of the consonant. The half forms of Akhand ligatures will be called later in the half feature.
Because the akhand feature is applied early in the sequence of features and is applied over the entire cluster, it can also be used to create certain forms that must take priority in particular contexts over forms that would be created during subsequent feature application.
Feature Tag: "rphf"
Applying this feature substitutes the Reph glyph. If the first consonant of the cluster consists of the full form of Ra + Halant, this feature substitutes the combining-mark form of Reph. In addition, the position of the Reph glyph is adjusted with the 'abvm' GPOS feature.
The input context for the Reph feature always consists of the full form of Ra + Halant.
Pre-base form of consonant
Feature Tag: "pref"
This feature substitutes the pre-base forms of Consonants.
Half form of consonant
Feature Tag: "half"
Applying this feature substitutes half forms - forms of consonants used in the pre-base position. Consonants that have a half form should be listed in the 'half' feature. Some scripts, like Devanagari have distinctly shaped half forms for most of the consonants however, if a consonant does not have a distinct shape for the half form and does not form any ligature, it will be displayed with an explicit Virama (same shape as the halant form).
Note - the result of listing a consonant in the half feature (whether it has a true half form or not) will affect the re-ordering (and positioning) of the reph and pre-pended matras. See illustration in the Introduction section of this document.
This feature is applied to all consonants preceding the 'main' consonant.
Note - While Tamil typically does not use half forms, this feature is made available for typographic preference.
After the glyphs have been reordered, the presentation lookups are applied to provide the best typographic rendering of the text. The features of the presentation forms are applied to the entire cluster simultaneously, executing lookups within each feature in the order that they are specified in the font.
The pres, abvs, blws, psts and haln features are all mandatory for software implementations: they are required for correct script behaviour and none should ever be treated as discretionary. Because of this and because they are all applied simultaneously over entire clusters, they are not functionally different: a set of lookups could be divided between these features or grouped together under one of them with no difference in effect. These multiple features are provided, however, as an aid to the font developer for organizing lookups based on the combinations of glyphs they apply to. There are no specific requirements on how each should be used; the examples provided below illustrate typical usage, however.
Feature Tag: "pres"
This feature is used to substitute pre-base consonant conjuncts made with half forms, the type most common in Devanagari. The resulting conjunct can be in full or half form.
This feature can also be used to select variant forms of Matras, or pre-composed ligatures of Matras with certain bases.
Feature Tag: "abvs"
This feature is used for glyph substitutions involving above-base marks. Such substitutions might be used to select contextual forms of marks, to create mark-mark ligatures, or to create mark-base ligatures. Specific context-dependent forms or below-base consonants are handled by this lookup as well.
Feature Tag: "blws"
This feature is used for glyph substitutions involving below-base marks or consonants. Such substitutions can be used to create conjuncts of base glyphs with below-base consonants, below mark ligatures or below mark-base ligatures. Specific context-dependent forms are handled by this lookup as well.
Feature Tag: "psts"
This feature is used to substitute post-base consonants or matras. Such substitutions can be used to create conjuncts of base glyphs with post-base consonants or post-base matra ligatures. It can also be used to specify contextual alternates of post-base forms.
Halant form of consonants
Feature Tag: "haln"
This feature is used to substitute a pre-composed halant form of a base (or conjunct base) glyph in syllables ending with a halant. (Rather than using substitution, halant forms can also be created by positioning the halant as a below-base mark on the base glyph using the 'blwm' positioning feature.)
This feature is applied only on the base glyph if the syllable ends with a halant, or in the case of non-final consonants that do not take a half form and do not form a conjunct ligature with the following consonant.
Feature Tag: "calt"
Unlike the previous presentation lookups, the 'calt' feature is optional and is used to substitute discretionary contextual alternates. It is important to note that an application may allow users to turn off this feature, therefore should not be used for any obligatory Tamil typography.
Feature Tag: "dist"
This feature covers positioning lookups that adjust distances between glyphs, such as kerning between pre- and post-base elements and the base glyph. Note; the feature 'dist' can be used in the same way as the 'kern' feature. The advantage of using the 'dist' feature is that it does not rely on the application to enable kerning.
Feature Tag: "abvm"
This feature positions all above-base marks on the base glyph or the post-base matra. The best method for encoding this feature in an OpenType font is to use a chaining context positioning lookup that triggers mark-to-base and mark-to-mark attachments for above-base marks.
Feature Tag: "blwm"
This feature positions all below-base marks on the base glyph. The best method for encoding this feature in an OpenType font is to use a chaining context positioning lookup that triggers mark-to-base and mark-to-mark attachments for below-base marks.
Examples of Tamil syllables
Complex Tamil syllable formation is possible using the wide range of features available in OpenType. The following examples show how the shaping engine applies the OpenType features, one at a time to the input string. These combinations do not necessarily represent actual syllables or words, but are meant to illustrate the various OpenType features in a Tamil font.