This document details the shaping procedure needed to display text runs in the Sinhala script.
Table of Contents
The Sinhala script belongs to the Indic family, and follows the same general patterns as the other Indic scripts. More specifically, it belongs to the South Indic subgroup.
The Sinhala script is used to write multiple languages, most commonly Sinhalese and Pali. In addition, Sanskrit may be written in Sinhala, so Sinhala script runs may include glyphs from the Vedic Extensions block of Unicode.
Unlike many other Indic scripts, there is only one extant Sinhala
script tag defined in OpenType, <sinh>.
OpenType shaping uses a standard set of terms for Indic scripts. The terms used colloquially in any particular language may vary, however, potentially causing confusion.
Halant and Virama are both standard terms for the below-base "vowel-killer" sign. Unicode documents use the term "virama" most frequently, while OpenType documents use the term "halant" most frequently. In the Sinhalese language, this sign is known as the al-lakuna or hal kirīma.
Chandrabindu (or simply Bindu) is the standard term for the diacritical mark indicating that the preceding vowel should be nasalized.
The term base consonant is also critical to Indic shaping. The base consonant of a syllable is the consonant that carries the syllable's vowel sound, either the inherent vowel (for an unmarked base consonant) or a dependent vowel (with the addition of a matra).
A syllable's base consonant is generally rendered in its full form (although it may form ligatures), while other consonants in the syllable frequently take on secondary forms. Different GSUB substitutions may apply to a script's pre-base and post-base consonants. Some of these substitutions create above-base or below-base forms. The Reph form of the consonant "Ra" is an example. In the Sinhalese language, the Reph form is known as repaya.
Syllables may also begin with an independent vowel instead of a consonant. In these syllables, the independent vowel is rendered in full-letter form, not as a matra, and the independent vowel serves as the syllable base, similar to a base consonant.
Where possible, using the standard terminology is preferred, as the use of a language-specific term necessitates choosing one language over all of the others that share a common script.
Shaping Sinhala text depends on the shaping engine correctly classifying each glyph in the run. As with most other scripts, the classifications must distinguish between consonants, vowels (independent and dependent), numerals, punctuation, and various types of diacritical mark.
For most codepoints, the General Category property defined in the Unicode
standard is correct, but it is not sufficient to fully capture the
expected shaping behavior (such as glyph reordering). Therefore,
Sinhala glyphs must additionally be classified by how they are treated
when shaping a run of text.
The shaping classes listed in the tables that follow are defined so that they capture the positioning rules used by Indic scripts.
For most codepoints, the Shaping class is synonymous with the Indic Syllabic Category defined in Unicode. However, there are some
distinctions, where the defined category does not fully capture the
behavior of the character in the shaping process.
Several of the diacritic and syllable-modifying marks behave according
to their own rules and, thus, have a special class. These include
BINDU, VISARGA, AVAGRAHA, NUKTA, and VIRAMA. Some
less-common marks behave according to rules that are similar to these
common marks, and are therefore classified with the corresponding
common mark. The Vedic Extensions also include a CANTILLATION
class for tone marks.
Letters generally fall into the classes CONSONANT,
VOWEL_INDEPENDENT, and VOWEL_DEPENDENT. These classes help the
shaping engine parse and identify key positions in a syllable. For
example, Unicode categorizes dependent vowels as Mark [Mn], but the
shaping engine must be able to distinguish between dependent vowels
and diacritical marks (which are categorized as Mark [Mn]).
Other characters, such as symbols and miscellaneous letters (for example, letter-like symbols that only occur as standalone entities and do not occur within syllables), need no special attention from the shaping engine, so they are not assigned a shaping class.
Numbers are classified as NUMBER, even though they evoke no special
behavior from the Indic shaping rules, because there are OpenType features that
might affect how the respective glyphs are drawn, such as tnum,
which specifies the usage of tabular-width numerals, and sups, which
replaces the default glyphs with superscript variants.
Marks and dependent vowels are further labeled with a mark-placement subclass, which indicates where the glyph will be placed with respect to the base character to which it is attached. The actual position of the glyphs is determined by the lookups found in the font's GPOS table, however, the shaping rules for Indic scripts require that the shaping engine be able to identify marks by their general position.
For example, left-side dependent vowels (matras), classified
with LEFT_POSITION, must frequently be reordered, with the final
position determined by whether or not other letters in the syllable
have formed ligatures or combined into conjunct forms. Therefore, the
LEFT_POSITION subclass of the character must be tracked throughout
the shaping process.
There are four basic mark-placement subclasses for dependent vowels (matras). Each corresponds to the visual position of the matra with respect to the syllable base to which it is attached:
LEFT_POSITIONmatras are positioned to the left of the syllable base.RIGHT_POSITIONmatras are positioned to the right of the syllable base.TOP_POSITIONmatras are positioned above the syllable base.BOTTOM_POSITIONmatras are positioned below syllable base.
These positions may also be referred to elsewhere in shaping documents as:
- Pre-base matras
- Post-base matras
- Above-base matras
- Below-base matras
respectively. The LEFT, RIGHT, TOP, and BOTTOM designations
corresponds to Unicode's preferred terminology. The Pre, Post,
Above, and Below terminology is used in the official descriptions
of OpenType GSUB and GPOS features. Shaping engines may, internally,
use whichever terminology is preferred.
In addition, dependent-vowel codepoints that are composed of multiple
components will be designated in character tables as having a compound
mark-placement subclass, such as TOP_AND_RIGHT or LEFT_AND_RIGHT.
However, these multi-part matras are decomposed into separate matra components during the shaping process. After the decomposition, each matra component will belong to exactly one of the four basic mark-placement subclasses.
For most mark and dependent-vowel codepoints, the mark-placement
subclass is synonymous with the Indic Positional Category defined
in Unicode. However, there are some distinctions, where the defined
category does not fully capture the behavior of the character in the
shaping process.
Separate character tables are provided for the Sinhala, Sinhala
Archaic Numbers, and Vedic Extensions block as well as for other
miscellaneous characters that are used in <sinh> text runs:
- Sinhala character table
- Sinhala Archaic Numbers character table
- Vedic Extensions character table
- Miscellaneous character table
The tables list each codepoint along with its Unicode general category, its shaping class, and its mark-placement subclass. The codepoint's Unicode name and an example glyph are also provided.
For example:
| Codepoint | Unicode category | Shaping class | Mark-placement subclass | Glyph |
|---|---|---|---|---|
U+0D82 |
Mark [Mn] | BINDU | RIGHT_POSITION | ං Anusvara |
U+0D9A |
Letter | CONSONANT | null | ක Ka |
Codepoints with no assigned meaning are designated as unassigned in the Unicode category column.
Assigned codepoints with a null in the Shaping class column evoke no special behavior from the shaping engine.
The Mark-placement subclass column indicates mark-placement positioning for codepoints in the Mark category. Assigned, non-mark codepoints have a null in this column and evoke no special mark-placement behavior. Marks tagged with [Mn] in the Unicode category column are categorized as non-spacing; marks tagged with [Mc] are categorized as spacing-combining.
Some codepoints in the tables use a Shaping class that differs from the codepoint's Unicode General Category. The Shaping class takes precedence during OpenType shaping, as it captures more specific, script-aware behavior.
Other important characters that may be encountered when shaping runs
of Sinhala text include the dotted-circle placeholder (U+25CC), the
zero-width joiner (U+200D) and zero-width non-joiner (U+200C), and
the no-break space (U+00A0).
Each of these is of particular importance to shaping engines, because these codepoints interact with the shaping engine, the text run, and the active font, either to mediate non-default shaping behavior or to relay information about the current shaping process.
The dotted-circle placeholder is frequently used when displaying a dependent vowel (matra) or a combining mark in isolation. Real-world text syllables may also use other characters, such as hyphens or dashes, in a similar placeholder fashion; shaping engines should cope with this situation gracefully.
Dotted-circle placeholder characters (like any Unicode codepoint) can appear anywhere in text input sequences and should be rendered normally. GPOS positioning lookups should attach mark glyphs to dotted circles as they would to other non-mark characters. As visible glyphs, dotted circles can also be involved in GSUB substitutions.
In addition to the default input-text handling process, shaping engines may also insert dotted-circle placeholders into the text sequence. Dotted-circle insertions are required when a non-spacing mark or dependent sign is formed with no base character present.
This requirement covers:
-
Dependent signs that are assigned their own individual Unicode codepoints (such as most dependent-vowel marks or matras)
-
Dependent signs that are formed only by specific sequences of other codepoints (such as "Reph")
In other Indic scripts, the zero-width joiner (ZWJ) is used to prevent the formation of conjuncts and to suppress the formation of "Reph".
Sinhala, however, differs considerably in its use of "ZWJ".
- In
<sinh>text, "Reph" is only formed by the use of an explicit "Ra,Halant,ZWJ" sequence. - In
<sinh>text, the sequence "Consonant_1,Halant,ZWJ,Consonant_2" is used to specify the subjoined form of "Consonant_2".
The zero-width non-joiner (ZWNJ) is not used in shaping runs of Sinhala text. The ZWNJ is referenced below in various regular expressions and shaping rules, however, because it is used by other Indic scripts.
The ZWJ and ZWNJ characters are, by definition, non-printing control characters and have the Default_Ignorable property in the Unicode Character Database. In standard text-display scenarios, their function is to signal a request from the user to the shaping engine for some particular non-default behavior. As such, they are not rendered visually.
Note: Naturally, there are special circumstances where a user or document might need to request that a ZWJ or ZWNJ be rendered visually, such as when illustrating the OpenType shaping process, or displaying Unicode tables.
Because the ZWJ and ZWNJ are non-printing control characters, they can be ignored by any portion of a software text-handling stack not involved in the shaping operations that the ZWJ and ZWNJ are designed to interface with. For example, spell-checking or collation functions will typically ignore ZWJ and ZWNJ.
Similarly, the ZWJ and ZWNJ should be ignored by the shaping engine when matching sequences of codepoints against the backtrack and lookahead sequences of a font's GSUB or GPOS lookups.
For example:
- A lookup that substitutes an alternate version of a dependent-vowel (matra) glyph when it is preceded by "Ka,Halant,Tta" should still be applied if the dependent-vowel codepoint is preceded by "Ka,Halant,ZWJ,Tta" in the text run.
The no-break space (NBSP) is primarily used to display those codepoints that are defined as non-spacing (marks, dependent vowels (matras), below-base consonant forms, and post-base consonant forms) in an isolated context, as an alternative to displaying them superimposed on the dotted-circle placeholder. These sequences will match "NBSP,ZWJ,Halant,Consonant", "NBSP,mark", or "NBSP,matra".
Processing a run of <sinh> text involves six top-level stages:
- Identifying syllables and other sequences
- Initial reordering
- Applying the basic substitution features from GSUB
- Final reordering
- Applying all remaining substitution features from GSUB
- Applying all remaining positioning features from GPOS
As with other Indic scripts, the initial reordering stage and the final reordering stage each involve applying a set of several script-specific rules. The basic substitution features must be applied to the run in a specific order. The remaining substitution features in stage five, however, do not have a mandatory order.
Indic scripts follow many of the same shaping patterns, but they differ in a few critical characteristics that the shaping engine must track. These include:
-
The position of the base consonant in a syllable.
-
The final position of "Reph".
-
Whether "Reph" must be requested explicitly or if it is formed by a specific, implicit sequence.
-
Whether the below-base forms feature is applied only to consonants before the syllable base, only to consonants after the base consonant, or to both.
-
The ordering positions for dependent vowels (matras). Specifically, right-side, above-base, and below-base matras follow different rules in different scripts. All Indic scripts position left-side matras in the same manner, in the ordering position
POS_PREBASE_MATRA.
With regard to these common variations, Sinhala's specific shaping characteristics include:
-
BASE_POS_LAST_SINHALA= The base consonant of a syllable is the last consonant, not counting any special final-consonant forms. However, the algorithm used for locating the base consonant in<sinh>text differs from that used by otherBASE_POS_LASTscripts. -
REPH_POS_AFTER_MAIN= "Reph" is ordered after the base consonant or syllable base. -
REPH_MODE_EXPLICIT= "Reph" is formed by an initial "Ra,Halant,ZWJ" sequence. -
BLWF_MODE_PRE_AND_POST= The below-forms feature is applied both to pre-base consonants and to post-base consonants. -
MATRA_POS_TOP=POS_AFTER_SUBJOINED= Above-base matras are ordered after subjoined (i.e., below-base) consonant forms. -
MATRA_POS_RIGHT=POS_AFTER_SUBJOINED= Right-side matras are ordered after subjoined (i.e., below-base) consonant forms. -
MATRA_POS_BOTTOM=POS_AFTER_SUBJOINED= Below-base matras are ordered after all subjoined (i.e., below-base) consonant forms.
These characteristics determine how the shaping engine must reorder certain glyphs, how base consonants are determined, and how "Reph" should be encoded within a run of text.
A syllable in Sinhala consists of a valid orthographic sequence that may be followed by a "tail" of modifier signs.
Note: The Sinhala Unicode block enumerates two modifier signs, "Anusvara" (
U+0D82) and "Visarga" (U+0D83). In addition, Sanskrit text written in Sinhala may include additional signs from Vedic Extensions block.
Each syllable contains exactly one vowel sound. Valid syllables may begin with either a consonant or an independent vowel.
If the syllable begins with a consonant, then the consonant that provides the vowel sound is referred to as the "base" consonant. If the syllable begins with an independent vowel, that independent vowel is the syllable's only vowel sound and serves as the "base".
Note: A consonant that is not accompanied by a dependent vowel (matra) sign carries the script's inherent vowel sound. This vowel sound is changed by a dependent vowel (matra) sign following the consonant.
From the shaping engine's perspective, the main distinction between a syllable with a base consonant and a syllable with an independent-vowel base is that a syllable with an independent-vowel base is less likely to include additional consonants in special forms and less likely to include dependent vowel signs (matras). Therefore, in the common case, vowel-based syllables may involve less reordering, substitution feature applications, and other processing than consonant-based syllables.
In some languages and orthographies, vowel-based syllables are not permitted to include additional consonants or matras, and certain GSUB substitution features do not occur. However, there are often known exceptions, and real-world text makes no such guarantees.
Note: Shaping engines may choose to treat independent-vowel bases like base consonants for the sake of simplicity or code reuse.
However, implementations that take this approach should note that removing the distinction between base consonants and independent-vowel bases entirely may have unintended consequences. Making guarantees about the correctness of the results or about language-specific tests is out of scope for this document.
Generally speaking, the base consonant is the final consonant of the
syllable that does not take on a subjoined form, and its vowel sound
designates the end of the syllable. This rule is synonymous with the
BASE_POS_LAST_SINHALA characteristic mentioned earlier.
Valid consonant-based syllables may include one or more additional consonants that precede the base consonant. Each of these other, pre-base consonants will be followed by the "Halant" mark, which indicates that they carry no vowel. They affect pronunciation by combining with the base consonant (e.g., "str", "pl") but they do not add a vowel sound.
As with other Indic scripts, the consonant "Ra" receives special treatment; in many circumstances it is replaced by a combining mark-like form.
- A "Ra,Halant,ZWJ" sequence at the beginning of a syllable
is replaced with an above-base mark called "Reph" (unless the "Ra"
is the only consonant in the syllable).
This rule is synonymous with the
REPH_MODE_EXPLICITcharacteristic mentioned earlier.
In addition, the subjoined form of a post-base-consonant "Ra" can be explicitly requested with a "Halant,ZWJ,Ra" sequence. This form is called "Rakaaraansaya".
"Reph" characters must be reordered after the syllable-identification stage is complete. "Rakaaraansaya" is not reordered.
In addition to valid syllables, standalone sequences may occur, such as when an isolated codepoint is shown in example text.
Note: Foreign loanwords, when written in the Sinhala script, may not adhere to the syllable-formation rules described above. In particular, it is not uncommon to encounter foreign loanwords that contain a word-final suffix of consonants.
Nevertheless, such word-final suffixes will be correctly matched by the regular expressions listed below. These loanwords are pronounced different, which raises issues for potential readers, but the character sequences do not affect the shaping process.
Syllables should be identified by examining the run and matching glyphs, based on their categorization, using regular expressions.
The following general-purpose Indic-shaping regular expressions can be used to match Sinhala syllables.
The regular expressions utilize the shaping classes from the tables above. For the purpose of syllable identification, more general classes can be used, as defined in the following table. This simplifies the resulting expressions.
_ra_ = The consonant "Ra"
_consonant_ = ( `CONSONANT` | `CONSONANT_DEAD` ) - _ra_
_vowel_ = `VOWEL_INDEPENDENT`
_nukta_ = `NUKTA`
_halant_ = `VIRAMA`
_zwj_ = `JOINER`
_zwnj_ = `NON_JOINER`
_matra_ = `VOWEL_DEPENDENT` | `PURE_KILLER`
_syllablemodifier_ = `SYLLABLE_MODIFIER` | `BINDU` | `VISARGA` | `GEMINATION_MARK`
_vedicsign_ = `CANTILLATION`
_placeholder_ = `PLACEHOLDER` | `CONSONANT_PLACEHOLDER` | `NUMBER`
_dottedcircle_ = `DOTTED_CIRCLE`
_repha_ = `CONSONANT_PRE_REPHA`
_consonantmedial_ = `CONSONANT_MEDIAL`
_symbol_ = `SYMBOL` | `AVAGRAHA`
_consonantwithstacker_ = `CONSONANT_WITH_STACKER`
_other_ = `OTHER`| `MODIFYING_LETTER`Note: the ra identification class is mutually exclusive with the consonant class. The union of the consonant and ra classes is used in the regular expression elements below in order to correctly identify "Ra" characters that do not trigger "Reph" or "Rakaar" shaping behavior.
Note, also, that the cantillation mark "combining Ra" in the Devanagari Extended block does not belong to the ra identification class, and that the other "combining consonant" cantillation marks in the Devanagari Extended block do not belong to the consonant identification class.
Note: The placeholder identification class includes codepoints that are often used in place of vowels or consonants when a document needs to display a matra, mark, or special form in isolation or in another context beyond a standard syllable. Examples of placeholder codepoints include hyphens and non-breaking spaces. Sequences that utilize this approach should be identified as "standalone" syllables.
The placeholder identification class also includes numerals, which are commonly used as word substitutes within normal text. Examples include ordinals (e.g., "4th").
Note: The other identification class includes codepoints that do not interact with adjacent characters for shaping purposes. Even though some of these codepoints (such as
MODIFYING_LETTER) can occur within words, they evoke no behavior from the shaping engine and do not factor into the regular expressions that follow. Therefore, the shaping engine may choose to ignore them during syllable identification; they are listed here for completeness.
These identification classes form the bases of the following regular expression elements:
C = _consonant_ | _ra_
Z = _zwj_ | _zwnj_
REPH = (_ra_ _halant_) | _repha_
CN = C _zwj_? _nukta_?
FORCED_RAKAR = _zwj_ _halant_ _zwj_ _ra_
S = _symbol_ _nukta_?
MATRA_GROUP = Z{0,3} _matra_ _nukta_? (_halant_ | FORCED_RAKAR)?
SYLLABLE_TAIL = (Z? _syllablemodifier_ _syllablemodifier_? _zwnj_?)? _vedicsign_{0,3}
HALANT_GROUP = Z? _halant_ (_zwj_ _nukta_?)?
FINAL_HALANT_GROUP = HALANT_GROUP | (_halant_ _zwnj_)
MEDIAL_GROUP = _consonantmedial_?
HALANT_OR_MATRA_GROUP = FINAL_HALANT_GROUP | ((_halant_ _zwj_)? MATRA_GROUP*)Note: Practically speaking, shaping engines are highly unlikely to encounter more than 4 sequential
(MATRA_GROUP)instances in any real-word syllables. Thus, implementations may choose to limit occurrences by limiting the above expressions to a finite length, such as(MATRA_GROUP){0,4}.
Using the above elements, the following regular expressions define the possible syllable types:
A consonant-based syllable will match the expression:
(_repha_|_consonantwithstacker_)? (CN HALANT_GROUP)* CN MEDIAL_GROUP HALANT_OR_MATRA_GROUP SYLLABLE_TAILNote: Practically speaking, shaping engines are highly unlikely to encounter more than 4 sequential
(CN HALANT_GROUP)instances in any real-word syllables. Thus, implementations may choose to limit occurrences by limiting the above expressions to a finite length, such as(CN HALANT_GROUP){0,4}.
A vowel-based syllable will match the expression:
REPH? _vowel_ _nukta_? (_zwj_ | (HALANT_GROUP CN)* MEDIAL_GROUP HALANT_OR_MATRA_GROUP SYLLABLE_TAIL)Note: Practically speaking, shaping engines are highly unlikely to encounter more than 4 sequential
(HALANT_GROUP CN)instances in any real-word syllables. Thus, implementations may choose to limit occurrences by limiting the above expressions to a finite length, such as(HALANT_GROUP CN){0,4}.
A standalone syllable will match the expression:
((_repha_|_consonantwithstacker_)? _placeholder_ | REPH? _dottedcircle_) _nukta_? (HALANT_GROUP CN)* MEDIAL_GROUP HALANT_OR_MATRA_GROUP SYLLABLE_TAILNote: Practically speaking, shaping engines are highly unlikely to encounter more than 4 sequential
(HALANT_GROUP CN)instances in any real-word syllables. Thus, implementations may choose to limit occurrences by limiting the above expressions to a finite length, such as(HALANT_GROUP CN){0,4}.
Note: Although they are labeled as "standalone syllables" here, many sequences that match the standalone regular expression above are instances where a document needs to display a matra, combining mark, or special form in isolation. Such sequences might not have any significance with regard to the definition of syllables used in the language or orthography of the text.
A symbol-based syllable will match the expression:
S SYLLABLE_TAILA broken syllable will match the expression:
REPH? _nukta_? (HALANT_GROUP CN)* MEDIAL_GROUP HALANT_OR_MATRA_GROUP SYLLABLE_TAILNote: Practically speaking, shaping engines are highly unlikely to encounter more than 4 sequential
(HALANT_GROUP CN)instances in any real-word syllables. Thus, implementations may choose to limit occurrences by limiting the above expressions to a finite length, such as(HALANT_GROUP CN){0,4}.
The primary problem involved in shaping broken syllables is the lack of a syllable base (either a base consonant or an independent vowel). Without a syllable base, the shaping engine cannot perform GPOS positioning and other contextual operations that are required later in the shaping process.
To make up for this limitation, shaping engines should insert a
dotted-circle placeholder (U+25CC) character into the text stream
where the missing syllable base was expected to occur. This
placeholder allows the shaping process to proceed on a best-effort
basis at handling the broken-syllable sequence, but making guarantees
about the orthographic correctness or preferred appearance of the
final result is out of scope for this document.
Shaping engines can perform this dotted-circle insertion at any point after the broken syllable has been recognized and before GSUB features are applied. However, the best results will likely be attained by performing the insertion immediately, before proceeding to stage 2. This will enable the maximum number of GSUB and GPOS features in the active font to be correctly applied to the text run by ensuring that all reordering, tagging, and sorting algorithms are executed as usual.
Note: In software stacks where other text-handling operations, such as Unicode normalization and localization, are performed before the text run is passed to the shaping engine, there is a potential for the dotted-circle insertion to cause unexpected effects.
For example, if a
ccmporloclfeature substitutes the default dotted-circle placeholder glyph with a variant glyph of a different size or weight for the (U+25CC) codepoint, then any shaping engine which relies on another software component to handle that functionality must take additional care to ensure consistency.
The expressions above use state-machine syntax from the Ragel state-machine compiler. The operators represent:
a* = zero or more copies of a
b+ = one or more copies of b
c? = optional instance of c
d{n} = exactly n copies of d
d{,n} = zero to n copies of d
d{n,} = n or more copies of d
d{n,m} = n to m copies of d
!e = not e
^f = character-level not f
g.h = concatenation of g and h
i|j = i or j
( ) = grouping of expression elementsAfter the syllables have been identified, each of the subsequent shaping stages occurs on a per-syllable basis.
The initial reordering stage is used to relocate glyphs from the phonetic order in which they occur in a run of text to the orthographic order in which they are presented visually.
Note: Primarily, this means moving dependent-vowel (matra) glyphs, "Ra,Halant,ZWJ" glyph sequences, and other consonants that take special treatment in some circumstances. "Ya" may take on special forms, depending on its position in the syllable.
These reordering moves are mandatory. The final-reordering stage may make additional moves, depending on the text and on the features implemented in the active font.
The syllable should be processed by tagging each glyph with its intended position based on its ordering category. After all glyphs have been tagged, the entire syllable should be sorted in stable order, so that glyphs of the same ordering category remain in the same relative position with respect to each other.
The final sort order of the ordering categories should be:
POS_RA_TO_BECOME_REPH
POS_PREBASE_MATRA
POS_PREBASE_CONSONANT
POS_SYLLABLE_BASE
POS_AFTER_MAIN
POS_ABOVEBASE_CONSONANT
POS_BEFORE_SUBJOINED
POS_BELOWBASE_CONSONANT
POS_AFTER_SUBJOINED
POS_BEFORE_POST
POS_POSTBASE_CONSONANT
POS_AFTER_POST
POS_FINAL_CONSONANT
POS_SMVD
This sort order enumerates all of the possible final positions to which a codepoint might be reordered, across all of the Indic scripts. It includes some ordering categories not utilized in Sinhala.
The basic positions (left to right) are "Reph"
(POS_RA_TO_BECOME_REPH), dependent vowels (matras) and consonants
positioned before the base consonant or syllable base
(POS_PREBASE_MATRA and POS_PREBASE_CONSONANT), the base consonant
or syllable base (POS_SYLLABLE_BASE), above-base consonants
(POS_ABOVEBASE_CONSONANT), below-base consonants
(POS_BELOWBASE_CONSONANT), consonants positioned after the base
consonant or syllable base (POS_POSTBASE_CONSONANT), syllable-final
consonants (POS_FINAL_CONSONANT), and syllable-modifying or Vedic
signs (POS_SMVD).
In addition, several secondary positions are defined to handle various
reordering rules that deal with relative, rather than absolute,
positioning. POS_AFTER_MAIN means that a character must be
positioned immediately after the syllable base. POS_BEFORE_SUBJOINED
and POS_AFTER_SUBJOINED mean that a character must be positioned
before or after any below-base consonants, respectively. Similarly,
POS_BEFORE_POST and POS_AFTER_POST mean that a character must be
positioned before or after any post-base consonants, respectively.
For shaping-engine implementers, the names used for the ordering categories matter only in that they are unambiguous.
For a definition of the "base" consonant, refer to step 2.1, which follows.
The first step is to determine the base consonant of the syllable, if
there is one, and tag it as POS_SYLLABLE_BASE.
In a syllable that begins with an independent vowel, the independent
vowel will always serve as the syllable base, and it should be tagged
as POS_SYLLABLE_BASE. The shaping engine can then proceed to step 2.
In a standalone sequence or other syllable that begins with a placeholder
or dotted circle, the placeholder or dotted circle will always serve
as the syllable base, and it should be tagged as
POS_SYLLABLE_BASE. The shaping engine can then proceed to step 2.
In a syllable that begins with a consonant, the shaping engine must determine the base consonant by a script-specific algorithm.
Note: Shaping engines may choose to treat independent-vowel bases like base consonants for the sake of simplicity or code reuse.
However, implementations that take this approach should note that removing the distinction between base consonants and independent-vowel bases entirely may have unintended consequences. Making guarantees about the correctness of the results or about language-specific tests is out of scope for this document.
The base consonant is defined as the consonant in a consonant-based syllable that carries the syllable's vowel sound. That vowel sound will either be provided by the script's inherent vowel (in which case it is not written with a separate character) or the sound will be designated by the addition of a dependent-vowel (matra) sign.
Due to the different usage of ZWJ characters in <sinh> text runs, a
different algorithm is required for the shaper to identify the base
consonant of a syllable. The algorithm for determining the base
consonant in Sinhala is
- If the syllable starts with "Ra,Halant,ZWJ" and the syllable contains more than one consonant, exclude the starting "Ra" from the list of consonants to be considered.
- Starting from the end of the syllable, move backwards until a consonant is found.
- If the consonant is immediately preceded by a ZWJ, move to the previous consonant. If the consonant is not immediately preceded by a ZWJ, stop.
- If the consonant is the first consonant, stop.
- The consonant stopped at will be the base consonant.
Note: Unlike with many other Indic scripts, it is not necessary for the shaping engine to independently determine if any consonant has a post-base or below-base form in the active font. The use of a ZWJ character before a consonant in the search explicitly designates such a special form.
Second, any multi-part dependent vowels (matras) must be decomposed into their individual components.
Sinhala has four multi-part dependent vowels, "Ee" (U+0DDA), "O"
(U+0DDC), "Oo" (U+0DDD), and "Au" (U+0DDE). Each
has a canonical decomposition, so this step is unambiguous.
"Ee" (
U+0DDA) decomposes to "U+0DD9,U+0DCA""O" (
U+0DDC) decomposes to "U+0DD9,U+0DCF""Oo" (
U+0DDD) decomposes to "U+0DD9,U+0DCF,U+0DCA""Au" (
U+0DDE) decomposes to "U+0DD9,U+0DDF"
Because this decomposition is a character-level operation, the shaping engine may choose to perform it earlier, such as during an initial Unicode-normalization stage. However, all such decompositions must be completed before the shaping engine begins step three, below.
Note: The decomposition of "Oo" (
U+0DDD) is atypical; Unicode specifies that the codepoint decomposes to "O" (U+0DDC) followed byU+0DCA; the "O" codepoint is then decomposed to "U+0DD9,U+0DCF". Shaping engines must take care not to miss this second decomposition.
Note: For Sinhala, the
pstfsubstitution feature of GSUB is defined as replacing the entire multi-part matra with its right-side component.The Microsoft Uniscribe shaping engine historically supported this behavior -- in a sense, decomposing each matra into its left-side component followed by a duplicate of the original matra, then substituting the duplicated matra with the right-side matra component in stage 3, step 10, when the
pstffeature is applied.Fonts that were engineered to support this behavior might not include GPOS positioning rules for the right-side matra components, relying instead on the
pstfsubstitution to provide a suitable replacement. Shaping engines should do their best to deal gracefully with fonts that were developed only with this behavior in mind.
Third, all left-side dependent-vowel (matra) signs must be tagged to be
moved to the beginning of the syllable, with POS_PREBASE_MATRA.
Above-base, right-side, and below-base dependent-vowel (matra) signs
must be tagged with POS_AFTER_SUBJOINED.
Fourth, any subsequences of marks that include a "Nukta" and a "Halant" or Vedic sign must be reordered so that the "Nukta" appears first.
This means that the subsequence "Halant,Nukta" is reordered to "Nukta,Halant" and that the subsequence "Vedic_sign,Nukta" is reordered to "Nukta,_Vedic_sign".
For subsequences of affected marks that are longer than two, the reordering operation must be repeated until the "Nukta" is the first character in the subsequence. No other marks in the subsequence should be reordered.
This order is canonical in Unicode and is required so that "consonant,Nukta" substitution rules from GSUB will be correctly matched later in the shaping process.
Note: Nukta usage in Sinhala is rare.
Fifth, consonants that occur before the base consonant or syllable base must be tagged
with POS_PREBASE_CONSONANT.
Sixth, initial "Ra,Halant,ZWJ" sequences that will become "Reph"s must be tagged with
POS_RA_TO_BECOME_REPH.
Note: an initial "Ra,Halant,ZWJ" sequence will always become a "Reph" unless the "Ra" is the only consonant in the syllable.
Seventh, any non-base consonants that occur after a dependent vowel
(matra) sign must be tagged with POS_POSTBASE_CONSONANT.
In Sinhala, the only consonants that can appear in this position are
"Ra" and "Ya". A "Halant,ZWJ,Ya" sequence after the base consonant or syllable base will take on
the "Yansaya" form when the vatu feature is applied. A
"Halant,ZWJ,Ra" sequence after the base consonant or syllable base will take on
the "Rakaaraansaya" form when the vatu feature is applied.
Eighth, all marks must be tagged.
Note: In this step, joiner and non-joiner characters must also be tagged according to the same rules given for marks, even though these characters are not categorized as marks in Unicode.
Marks in the BINDU, VISARGA, AVAGRAHA, CANTILLATION,
SYLLABLE_MODIFIER, GEMINATION_MARK, and SYMBOL categories should
be tagged with POS_SMVD.
All "Nukta"s must be tagged with the same positioning tag as the preceding consonant, independent vowel, placeholder, or dotted circle.
All remaining marks (not in the POS_SMVD category and not "Nukta"s)
must be tagged with the same positioning tag as the closest non-mark
character the mark has affinity with, so that they move together
during the sorting step.
There are two possible cases: those marks before the syllable base and those marks after the syllable base. In addition, an exception is made for "Halant" marks that follow a left-side (pre-base) matra.
-
Initially, all remaining marks should be tagged with the same positioning tag as the closest preceding consonant.
-
For each consonant after the syllable base (such as post-base consonants, below-base consonants, or final consonants), all remaining marks located between that current consonant and any previous consonant should be tagged with the same positioning tag as the current (later) consonant.
In other words, all consonants preceding the syllable base "own" the marks that follow them, while all consonants after the syllable base "own" the marks that come before them. When a syllable does not have any consonants after the syllable base, the syllable base should "own" all the marks that follow it.
-
Finally, "Halant" marks that follow a left-side dependent vowel (matra) should not be tagged with the left-side matra's positioning tag. Instead, the "Halant" should be tagged with the positioning tag of the non-mark character preceding the left-side matra. This prevents the "Halant" mark from being moved with the left-side matra when the syllable is sorted.
With these steps completed, the syllable can be sorted into the final sort order as listed at the beginning of stage 2.
The glyphs in the syllable should be sorted in stable order, so that glyphs of the same ordering category remain in the same relative position with respect to each other.
With the initial reordering complete, those glyphs in the syllable that may have GSUB or GPOS features applied in stages 3, 5, and 6 should be flagged for each potential feature.
This flagging is preliminary; the set of potential features varies between different scripts and which features are supported varies between fonts. It is also possible that the application of one feature on a glyph sequence will perform a substitution that makes a later feature no longer applicable to the updated sequence.
Consequently, the flagging must be completed before shaping proceeds to the stages during which features are applied.
Some shaping features, such as locl, can potentially apply to any
glyphs. Therefore it is not necessary to maintain a separate flag for
these features in the bitmask (or other data structure) used to track
the flags -- although shaping engines may do so if desired.
The sequences to flag are summarized in the list below; a full description of each feature's function and interpretation is provided in GSUB and GPOS application stages that follow.
akhnshould match "Consonant,Halant,ZWJ,Consonant" and "Consonant,ZWJ,Halant,Consonant" sequencesrphfshould match initial "Ra,Halant,ZWJ" sequencespstfshould match "Matra" in post-base positionvatushould match "Halant,ZWJ,Ra" and "Halant,ZWJ,Va"
The basic-substitution stage applies mandatory substitution features using the rules in the font's GSUB table. In preparation for this stage, glyph sequences should be flagged for possible application of GSUB features in stage 2, step 10.
The order in which these substitutions must be performed is fixed for all Indic scripts:
locl
nukt (not used in Sinhala)
akhn
rphf
rkrf (not used in Sinhala)
pref (not used in Sinhala)
blwf (not used in Sinhala)
abvf (not used in Sinhala)
half (not used in Sinhala)
pstf
vatu
cjct (not used in Sinhala)
cfar (not used in Sinhala)
The locl feature replaces default glyphs with any language-specific
variants, based on examining the language setting of the text run.
Note: Strictly speaking, the use of localized-form substitutions is not part of the shaping process, but of the localization process, and could take place at an earlier point while handling the text run. However, shaping engines are expected to complete the application of the
loclfeature before applying the subsequent GSUB substitutions in the following steps.
This feature is not used in Sinhala.
In Sinhala, the akhn feature provides two substitution types.
- "Consonant,Halant,ZWJ,Consonant" sequences are used to specify a ligature.
- "Consonant,ZWJ,Halant,Consonant" sequences are used to specify "touching consonant" substitutions used in Pali and Sanskrit.
The rphf feature replaces initial "Ra,Halant,ZWJ" sequences with the
"Reph" glyph.
This feature is not used in Sinhala.
This feature is not used in Sinhala.
This feature is not used in Sinhala.
This feature is not used in Sinhala.
This feature is not used in Sinhala.
In Sinhala, the pstf feature replaces multi-part dependent vowels
(matras) with the right-side matra component of the canonical
decomposition.
Note: This substitution is possible because all multi-part dependent vowels in Sinhala use the same left-side matra component,
U+0DD9.The Microsoft Uniscribe shaping engine historically supported this behavior by handling the decomposition of multi-part dependent vowels in stage 2, step 2 differently for Sinhala -- in a sense, decomposing each matra into its left-side component followed by a duplicate of the original matra, then substituting the duplicated matra with the right-side matra component when the
pstffeature is applied.Shaping engines may, optionally, decompose multi-part dependent vowels in stage 2, step 2 into their canonical Unicode decompositions, as is done in other scripts, and substitute the decomposed right-side matra components at that point.
Doing so will negate the need to apply the
pstfsubstitution. However, fonts that were engineered to support the Uniscribe-supported behavior might not include GPOS positioning rules for the right-side matra components, relying instead on thepstfsubstitution to provide a suitable replacement. Shaping engines should do their best to deal gracefully with fonts that were developed only with this behavior in mind.
In Sinhala, the vatu feature replaces certain sequences with
ligatures using the subjoined forms of "Ra" or "Ya".
- The sequence "Consonant,Halant,ZWJ,Ra" triggers the "Rakaaraansaya" form of the consonant.
- The sequence "Consonant,Halant,ZWJ,Ya" triggers the "Yansaya" form of the consonant.
This feature is not used in Sinhala.
This feature is not used in Sinhala.
The final reordering stage repositions marks, dependent-vowel (matra) signs, and "Reph" glyphs to the appropriate location with respect to the base consonant or syllable base. Because multiple substitutions may have occurred during the application of the basic-shaping features in the preceding stage, these repositioning moves could not be performed during the initial reordering stage.
Like the initial reordering stage, the steps involved in this stage occur on a per-syllable basis.
The final reordering stage, like the initial reordering stage, begins with determining the syllable base of each syllable, following the same algorithm used in stage 2, step 1.
In a syllable that begins with an independent vowel, the independent vowel will always serve as the syllable base. In a standalone sequence or other syllable that begins with a placeholder or a dotted circle, the placeholder or dotted circle will always serve as the syllable base.
In a syllable that begins with a consonant, the shaping engine must repeat the base-consonant search algorithm used in stage 2, step 1.
The codepoint of the underlying base consonant or syllable base will not change between the search performed in stage 2, step 1, and the search repeated here. However, the application of GSUB shaping features in stage 3 means that several ligation and many-to-one substitutions may have taken place. The final glyph produced by that process may, therefore, be a conjunct or ligature form — in most cases, such a glyph will not have an assigned Unicode codepoint.
Pre-base dependent vowels (matras) that were reordered during the initial reordering stage must be moved to their final position. This position is defined as:
- after the last standalone "Halant" glyph that comes after the matra's starting position and also comes before the main consonant.
- If a zero-width joiner follows this last standalone "Halant", the final matra position is moved to after the joiner.
This means that the matra will move to the right of all explicit "consonant,Halant" subsequences, but will stop to the left of the base consonant or syllable base, all conjuncts or ligatures that contain the base consonant or syllable base, and all half forms.
Note: OpenType and Unicode both state that if the syllable includes a ZWJ immediately after the last "Halant", then the final matra position should be after the ZWJ.
However, there are several test sequences indicating that Microsoft's Uniscribe shaping engine did not follow this rule (in, at least, Devanagari and Bengali text), and in these circumstances Uniscribe instead makes the final matra position before the final "Consonant,Halant,ZWJ".
Subsequently, the HarfBuzz shaping engine has also followed the same pattern. If other shaping engine implementations prefer to maintain maximum compatibility with Uniscribe and HarfBuzz, then they should also follow suit.
Note: The Microsoft script-development specifications for OpenType shaping also state that if a zero-width non-joiner follows the last standalone "Halant", the final matra position is moved to after the non-joiner. However, it is unnecessary to test for this condition, because a "Halant,ZWNJ" subsequence is, by definition, the end of a syllable. Consequently, a "Halant,ZWNJ" cannot be followed by a pre-base dependent vowel.
"Reph" must be moved from the beginning of the syllable to its final
position. Because Sinhala incorporates the REPH_POS_AFTER_MAIN
shaping characteristic, this final position is immediately after the
syllable base.
The algorithm for finding the final "Reph" position is
- Move the "Reph" to the position immediately before
the first post-base matra, syllable modifier, or Vedic sign that
has a positioning tag after the script's "Reph" position in the
syllable sort order (as listed in stage
2). This will be the final "Reph"
position.
Note: Because Sinhala incorporates the
REPH_POS_AFTER_MAINshaping characteristic, this means any positioning tag ofPOS_ABOVEBASE_CONSONANTor later, although a post-base matra, syllable modifier, or Vedic sign would not typically be tagged withPOS_ABOVEBASE_CONSONANT. - If no other location has been located in the previous step, move the "Reph" to the end of the syllable.
Finally, if the final position of "Reph" or "Repha" occurs after a
"matra,Halant" subsequence, then "Reph"/"Repha" must be repositioned to the
left of "Halant", to allow for potential matching with abvs or
psts substitutions from GSUB.
Any pre-base-reordering consonants must be moved to immediately before the base consonant or syllable base.
Sinhala does not use pre-base-reordering consonants, so this step will
involve no work when processing <sinh> text. It is included here in order
to maintain compatibility with the other Indic scripts.
Any left-side dependent vowels (matras) that are at the start of a
word must be flagged for potential substitution by the init feature
of GSUB.
Sinhala does not use the init feature, so this step will
involve no work when processing <sinh> text. It is included here in
order to maintain compatibility with the other Indic scripts.
In this stage, the remaining substitution features from the GSUB table are applied. In preparation for this stage, glyph sequences should be flagged for possible application of GSUB features in stage 2, step 10.
The order in which these features are applied is not canonical; they should be applied in the order in which they appear in the GSUB table in the font.
init (not used in Sinhala)
pres
abvs
blws
psts
haln (not used in Sinhala)
The init feature is not used in Sinhala.
The pres feature replaces pre-base-consonant glyphs with special
presentations forms. This can include ligatures, "touching consonant" forms,
and stylistic variants of left-side dependent vowels (matras).
The abvs feature replaces above-base-consonant glyphs with special
presentation forms. This usually includes contextual variants of
above-base marks or contextually appropriate mark-and-base ligatures.
The blws feature replaces below-base-consonant glyphs with special
presentation forms. This usually includes replacing base consonants or
syllable bases
and attached below-base marks with contextual ligatures.
The psts feature replaces post-base-consonant glyphs with special
presentation forms. This usually includes replacing right-side
dependent vowels (matras) with stylistic variants or replacing
base-consonant/matra pairs with contextual ligatures.
The haln feature is not used in Sinhala.
Note: The
caltfeature, which allows for generalized application of contextual alternate substitutions, is usually applied at this point. However,caltis not mandatory for correct Sinhala shaping and may be disabled in the application by user preference.
In this stage, mark positioning, kerning, and other GPOS features are applied.
As with the preceding stage, the order in which these features are applied is not canonical; they should be applied in the order in which they appear in the GPOS table in the font.
dist
abvm
blwm
Note: The
kernfeature is usually applied at this stage, if it is present in the font. However,kern(likecalt, above) is not mandatory for shaping Sinhala text and may be disabled by user preference.
The dist feature adjusts the horizontal positioning of
glyphs. Unlike kern, adjustments made with dist do not require the
application or the user to enable any software kerning features, if
such features are optional.
The abvm feature positions above-base marks for attachment to base
characters. In Sinhala, this includes "Reph" in addition to
above-base dependent vowels (matras), diacritical marks, and Vedic signs.
The blwm feature positions below-base marks for attachment to base
characters. In Sinhala, this includes below-base dependent vowels
(matras) and diacritical marks.
