This document details the shaping procedure needed to display text runs in the Myanmar script.
Table of Contents
- General information
- Terminology
- Glyph classification
- The
<mym2>shaping model - The
<mymr>shaping model
The Myanmar or Burmese script is a descendant of the Brahmi script, and follows many of the same general patterns found in Indic scripts. However, Myanmar incorporates enough distinctions of its own that it is generally not advisable to attempt supporting it in a general-purpose Indic shaping engine.
For example, Myanmar script includes a "Reph"-like feature known as "Kinzi" although, unlike "Reph", a "Kinzi" may be formed by any of several initial consonants. Also, notably, real-world texts written in Myanmar script often do not use inter-word spaces, which may make the process of syllable identification substantially different from processing Indic scripts.
The Myanmar script is used to write multiple languages, most commonly Burmese, Mon, Karen, Kayah, Shan, Palaung, and Pali. In addition, Sanskrit may be written in Myanmar, so Myanmar script runs may include glyphs from the Vedic Extensions block of Unicode.
There are two extant Myanmar script tags defined in OpenType, <mymr>
and <mym2>. The older script tag, <mymr>, was deprecated in 2005.
Therefore, new fonts should be engineered to work with the <mym2>
shaping model. However, if a font is encountered that supports only
<mymr>, the shaping engine should deal with it gracefully.
OpenType shaping uses a standard set of terms for Brahmi-derived and Indic scripts. The terms used colloquially in any particular language may vary, however, potentially causing confusion.
Matra is the standard term for a dependent vowel sign. Syllables in Myanmar script can include sequences of multiple vowels and, therefore, multiple matras.
Halant and Virama are both standard terms for the below-base "vowel-killer" mark. Unicode documents use the term "virama" most frequently, while OpenType documents use the term "halant" most frequently.
Asat is the term for the "pure killer" character in Myanmar. An asat after a consonant serves a similar function as a halant by suppressing the inherent vowel of the consonant, but the asat is rendered visually, either as an above-base mark or in a substitution form triggered by an adjacent codepoint.
An asat may be placed following a consonant to denote that the consonant is doubled. An asat may also be followed by a halant, a sequence that is used to trigger the "Kinzi" special form.
Chandrabindu (or simply Bindu) is the standard term for the diacritical mark indicating that the preceding vowel should be nasalized. Myanmar script does not use a chandrabindu; however, the BINDU category is used for other marks during the syllable-identification stage in order to maintain compatibility with other scripts.
Tone markers are an important part of languages written in Myanmar
script. These markers may be either spacing-combining ([Mc]) or
non-spacing ([Mn]). Several tone markers may be used within a single
syllable.
The term base consonant is also critical to Myanmar 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 Kinzi form of certain consonants is an example, akin to the "Reph" form of "Ra" in many Indic scripts.
Many Myanmar letters may be followed by a Variation Selector codepoint in order to request the dotted form of the corresponding glyph, which is preferred for some languages written with Myanmar script. Fonts are not required to include the dotted-form variants; when they are absent from the active font, the default form of the corresponding letter will be used instead.
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 Myanmar 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,
Myanmar 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 Myanmar script.
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 and VISARGA. 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.
Myanmar's "halant" codepoint is classified as INVISIBLE_STACKER,
rather than the more common VIRAMA. This is to indicate that, unlike
the "halant"/"virama" characters in several other scripts, the Myanmar
"halant" is never rendered visually as a glyph.
Myanmar's "Asat" codepoint, however, is rendered visually when it
appears in a syllable. The "Asat" behaves differently than Indic
"halant", however. It can be used to kill a consonant's inherent vowel
sound, but it is not used between consonants to indicate the formation
of a conjunct or a subjoined form. The "Asat" is classified as
PURE_KILLER.
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]).
Myanmar uses one subclass of consonant, CONSONANT_MEDIAL. This
subclass is used for special non-base variants of several consonants that
serve to modify the syllable's vowel sound. These medial consonants
are rendered as non-spacing marks attached to the base consonant.
Note: The medial "Ra" is reordered to pre-base-consonant position. The other medial consonants do not require reordering.
Note: The medial consonants are encoded in separate codepoints, distinguishing them from the standard (non-medial) variant of the corresponding consonant.
In addition, the Myanmar and Myanmar Extended Unicode blocks include
several codepoints classified as CONSONANT_PLACEHOLDER. These
codepoints are used in verbal transcriptions to take tone
marks. However, these glyphs are not consonants in the true sense and
are unlikely to occur within normal words.
Other characters, such as symbols, 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. 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 base consonant to which it is attached:
LEFT_POSITIONmatras are positioned to the left of the base consonant.RIGHT_POSITIONmatras are positioned to the right of the base consonant.TOP_POSITIONmatras are positioned above the base consonant.BOTTOM_POSITIONmatras are positioned below base consonant.
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.
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.
Many Myanmar letters may be followed by a Variation Selector
codepoint in order to request the "dotted form" of the
corresponding glyph. These variations are defined in Unicode's
Standardized Variants document; only the codepoints listed in that
document support substitution via variation selectors. At present,
only "Variation Selector 1" (U+FE00) is used with Myanmar.
If the active font does not include glyphs representing the requested variant of the letter preceding the variation selector, then the shaping engine must treat the variation selector codepoint as invisible and ignorable and use the default version of the preceding letter.
Separate character tables are provided for the Myanmar and Vedic
Extensions blocks as well as for other miscellaneous characters that
are used in <mym2> text runs:
- Myanmar character table
- Myanmar Extended-A character table
- Myanmar Extended-B 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+1000 |
Letter | CONSONANT | null | က Ka |
U+1036 |
Mark [Mn] | BINDU | TOP_POSITION | ံ Anusvara |
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 Myanmar text include the dotted-circle placeholder (U+25CC),
the no-break space (U+00A0), and the zero-width space (U+200B).
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.
The no-break space 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".
The zero-width space may be used between words — even though no visual word spacing results — in order to indicate word breaks within a text that can be used by line-breaking algorithms in a higher-level typesetting environment.
Processing a run of <mym2> text involves five top-level stages:
- Identifying syllables and other sequences
- Initial reordering
- Applying the basic substitution features from GSUB
- Applying all remaining substitution features from GSUB
- Applying all remaining positioning features from GPOS
As with other Brahmi-derived and 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 four, however, do not have a mandatory order.
Myanmar exhibits many of the same shaping patterns found in Indic scripts, but it differs in a few critical characteristics. With regard to these common variations, Myanmar's specific shaping characteristics include:
- The first consonant of a syllable is always the base consonant, excluding a consonant that is part of an initial "Kinzi"-forming sequence (if it is present).
Note: For comparison with the General Indic shaping model, this characteristic would correspond to
BASE_POS_FIRST.
- "Kinzi" is always encoded as a syllable-initial sequence, but it is reordered. The final position of "Kinzi" is immediately after the base consonant.
Note: For comparison with the General Indic shaping model, the Kinzi -encoding characteristic would correspond to
REPH_MODE_EXPLICIT, and the reordering characteristic would correspond toPOS_AFTER_MAIN.
- The below-base forms feature is applied only to consonants after the base consonant.
Note: For comparison with the General Indic shaping model, this characteristic would correspond to
BLWF_MODE_POST_ONLY.
-
Medial Ra is reordered to pre-base position.
-
Pre-base matras are reordered to the beginning of the syllable. Multiple pre-base matras can occur; any such sequences must be moved together, as a block, at the reordering stage.
Note: For comparison with the General Indic shaping model, this characteristic is distinct to Mynanmar script. Indic scripts apply different reordering rules to pre-base matras that depend on the contents of the syllable.
-
The ordering positions for right-side and above-base matras is the same. All are reordered to immediately after all subjoined consonants.
-
Below-base matras are reordered to immediately before any right-side and above-base matras.
Note: For comparison with the General Indic shaping model, this characteristic would correspond to
MATRA_POS_TOP,MATRA_POS_RIGHTtaking the ordering positionPOS_AFTER_SUBJOINED, andMATRA_POS_BOTTOMtaking the ordering positionPOS_BELOWBASE_CONSONANT.
A syllable in Myanmar consists of a valid orthographic sequence that may be followed by a "tail" of modifier signs.
Note: The Myanmar Unicode block enumerates two modifier signs, "Anusvara" (
U+1036) and "Visarga" (U+1038). There are also twenty-one tone markers in the Myanmar and Myanmar Extended-A blocks. In addition, Sanskrit text written in Myanmar may include additional signs from Vedic Extensions block.
Because texts written in Myanmar script do not generally employ inter-word spaces, however, shaping engines must rely on syllable-identification algorithms to recognize word-boundary patterns — distinguishing numeric sequences, symbols, punctuation, and other miscellaneous script characters from syllables within words.
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 vowel is the syllable's only vowel sound and, by definition, there is no "base" consonant.
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.
Generally speaking, the base consonant is the first consonant of the syllable and its vowel sound designates the end of the syllable. The exception to this rule is consonants that are part of a "Kinzi"-triggering sequence.
Post-base consonants in a valid syllable will be preceded by "Halant" marks.
BaseC Halant Post-baseC
The algorithm for correctly identifying the base consonant includes a test to recognize these sequences and not mis-identify the base consonant.
Medial consonants, if they occur, will not be preceded by a "Halant". This is because medial consonants in Myanmar are used to modify the vowel sound of the syllable.
Note: in the Myanmar script, all medial consonants have their own distinct codepoints. Therefore, they can be identified by codepoint alone, and there is no need for a text run to identify them using any special sequences.
As with other Brahmi-derived and Indic scripts, the consonant "Ra" receives special treatment.
-
A "Medial Ra" (
U+103C) must be reordered to a position immediately before the syllable's base consonant.Note, however, that "Medial Ra" is a separate codepoint from the standard "Ra" (
U+101B). -
A syllable-initial "Ra" may also be part of a "Kinzi"-triggering sequence.
Notably, however, although "Ra" alone will take on the "Reph" form in Indic script sequences, the Myanmar script's "Kinzi" feature can be triggered for three consonants, depending on the language in use: "Ra" (
U+101B), "Nga" (U+1004), and "Mon Nga" (U+105A). In each case, the "Kinzi" form is triggered by an explicit sequence: "consonant,Asat,Halant".There are, therefore, exactly three "Kinzi"-forming sequences to test for:
- "Ra,Asat,Halant"
- "Nga,Asat,Halant"
- "Mon Nga,Asat,Halant"
In the Myanmar (or Burmese) language, "Nga" is the only "Kinzi"-forming consonant. "Mon Nga" can form a "Kinzi" in the Mon language, and "Ra" can form a "Kinzi" in Sanskrit written with the Myanmar script.
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 Myanmar script, may not adhere to the syllable-formation rules described above.
Syllables should be identified by examining the run and matching glyphs, based on their categorization, using regular expressions.
The following regular expressions can be used to match Myanmar-script 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_ = "Ra" | "Nga" | "Mon Nga"
_consonant_ = `CONSONANT` | `CONSONANT_PLACEHOLDER` - _ra_
_vowel_ = `VOWEL_INDEPENDENT`
_halant_ = `INVISIBLE_STACKER`
_asat_ = "Asat"
_a_ = "Anusvara" | "Sign Ai"
_db_ = "Dot Below"
_zwj_ = `JOINER`
_zwnj_ = `NON_JOINER`
_mh_ = "Medial Ha" | "Mon Medial La"
_mr_ = "Medial Ra"
_mw_ = "Medial Wa" | "Shan Medial Wa"
_my_ = "Medial Ya" | "Mon Medial Na" | "Mon Medial Ma"
_d_ = `NUMBER`
_pt_ = "Tone Sgaw Karen Hathi" | "Tone Sgaw Karen Ke Pho" |
"Western Pwo Karen Tone 1" | "Western Pwo Karen Tone
2" | "Western Pwo Karen Tone 3" | "Western Pwo Karen
Tone 4" | "Western Pwo Karen Tone 5" | "Pao Karen
Tone"
_sm_ = "Visarga" | "Shan Tone 2" | "Shan Tone 3" | "Shan
Tone 5" | "Shan Tone 6" | "Shan Council Tone 2" |
"Shan Council Tone 3" | "Shan Council Emphatic Tone"
| "Rumai Palaung Tone 5" | "Khamti Tone 1" | "Khamti
Tone 3" | "Aiton A"
_punc_ = "Little Section" | "Section"
_matrapre_ = `MATRA` & `LEFT_POSITION`
_matrapost_ = `MATRA` &`RIGHT_POSITION`
_matraabove_ = `MATRA` & `TOP_POSITION` - _a_
_matrabelow_ = `MATRA` & `BOTTOM_POSITION`
_gb_ = U+002D | U+00A0 | U+00D7 | U+2012 | U+2013 | U+2014 |
U+2015 | U+2022 | U+25CC | U+25FB | U+25FC | U+25FD |
U+25FE
_cs_ = `CONSONANT_WITH_STACKER`
_v_ = `VISARGA`
_vs_ = "Variation Selector"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", "Nga", and "Mon Nga" characters that do not trigger "Kinzi" forms.
Note, also, that the
CONSONANT_PLACEHOLDERclass is unioned with theCONSONANTclass for the purpose of syllable identification, even those these two classes are treated separately in general.Note: The mh, mw, and my identification classes include several medial letters from the non-Burmese languages; they are grouped according to the medial consonants in Burmese that are the closest match in terms of shaping behavior.
Note: "Sign Ai" is classified as a, not as matraabove, in order to implement orthographically correct behavior.
Note: the gb identification class includes several "generic base" codepoints that are often used in real-world text runs to act as placeholders for missing letters.
Note: the tone marker codepoints are divided up between two identification classes, reflecting the differing orthographic rules they follow. The pt identification class constitutes the "Pwo tone" markers, while the sm identification class includes the remaining tone markers and other syllable modifiers.
These identification classes form the bases of the following regular expression elements:
C = _consonant_ | _ra_
Z = _zwj_ | _zwnj_
K = _ra_ _asat_ _halant_
Med = _my_? _mr_? _mw_? _mh_? _asat_?
Vmain = _matrapre_* _matraabove_* _matrabelow_* _a_* (_db_ _asat_?)?
Vpost = _matrapost_ _mh_? _asat_* _matraabove_* _a_* (_db_ _asat_?)?
Pwo = _pt_ _a_* _db_? _asat_?
Tcomplex= _asat_* Med Vmain Vpost* Pwo* _v_* Z?
Tail = _halant_ | TcomplexUsing the above elements, the following regular expressions define the possible syllable types:
A consonant-based syllable will match the expression:
(K | _cs_)? (C | _vowel_ | _d_ | _gb_) _vs_? (_halant_ (C | _vowel_) _vs_?)* TailThe 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 elementsA sequence that does not match any of these expressions should be regarded as broken. The shaping engine may make a best-effort attempt to shape the broken sequence, but making guarantees about the correctness or appearance of the final result is out of scope for this document.
After 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, "Kinzi"-forming sequences, and pre-base-reordering medial consonants.
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_PREBASE_MATRA
POS_PREBASE_CONSONANT
POS_BASE_CONSONANT
POS_AFTER_MAIN
POS_BEFORE_SUBJOINED
POS_BELOWBASE_CONSONANT
POS_AFTER_SUBJOINED
This sort order enumerates all of the possible final positions to which a codepoint might be reordered in Myanmar script.
The position names mimic those used in the General Indic shaping model, for ease of implementation. However, shaping engines are free to use any naming scheme they choose.
The basic positions (left to right) are dependent
vowels (matras) and consonants positioned before the base
consonant (POS_PREBASE_MATRA and POS_PREBASE_CONSONANT), the base
consonant (POS_BASE_CONSONANT), below-base consonants
(POS_BELOWBASE_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 base consonant. POS_BEFORE_SUBJOINED
and POS_AFTER_SUBJOINED mean that a character must be positioned
before or after any below-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_BASE_CONSONANT.
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.
Vowel-based syllables, standalone sequences, and broken text runs will not have base consonants.
The algorithm for determining the base consonant is
- Starting from the beginning of the syllable, move forwards until a
CONSONANTis found.- If the consonant is part of a "Kinzi" sequence, move to the next consonant.
- The consonant stopped at will be the base consonant.
Note: The algorithm considers only
CONSONANTclass consonants,
Second, all left-side dependent-vowel (matra) signs must be tagged to be
moved to the beginning of the syllable, with POS_PREBASE_MATRA.
All right-side and above-base dependent-vowel (matra)
signs are tagged POS_AFTER_SUBJOINED.
All below-base dependent-vowel (matra) signs are tagged
POS_BELOWBASE_CONSONANT.
For simplicity, shaping engines may choose to tag matras in an earlier text-processing step, using the information in the Mark-placement subclass column of the character tables. It is critical at this step, however, that all matras correctly tagged before proceeding to the next step.
Third, any ANUSVARA marks appearing immediately after a below-base
vowel sign must be tagged with POS_BEFORE_SUBJOINED, so that the
marks are reordered to a position immediately before the below-base
vowel signs.
Fourth, all pre-base-reordering consonants must be tagged with
POS_PREBASE_CONSONANT.
Myanmar has one pre-base-reordering consonant: "Medial Ra".
Fifth, initial "Kinzi"-triggering sequences that will become "Kinzi"s
must be tagged with POS_AFTER_MAIN.
The sequences are:
- "Ra,Asat,Halant"
- "Nga,Asat,Halant"
- "Mon Nga,Asat,Halant"
In the Myanmar (or Burmese) language, "Nga" is the only "Kinzi"-forming consonant. "Mon Nga" can form a "Kinzi" in the Mon language, and "Ra" can form a "Kinzi" in Sanskrit written with the Myanmar script.
Sixth, any remaining non-base consonants that occur after the base
consonant must be tagged with POS_AFTER_MAIN. Full consonants (of
class CONSONANT) will be preceded by a "Halant" glyph. Medial
consonants (of class CONSONANT_MEDIAL) will not be preceded by a
"Halant" glyph.
Note: "Medial Ra" should have been tagged with
POS_PREBASE_CONSONANTin stage 2, step four, and must not be re-tagged in this step.
Seventh, all marks 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.
For all marks preceding the base consonant, the mark must be tagged with the same positioning tag as the closest preceding non-mark consonant.
For all marks occurring after the base consonant, the mark must be tagged with the same positioning tag as the closest subsequent consonant.
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.
With these steps completed, the syllable can be sorted into the final sort order.
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 tagged for possible application of GSUB features.
The order in which these substitutions must be performed is fixed:
locl
ccmp
rphf
pref
blwf
pstf
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.
The ccmp feature allows a font to substitute mark-and-base sequences
with a pre-composed glyph including the mark and the base, or to
substitute a single glyph into an equivalent decomposed sequence of glyphs.
If present, these composition and decomposition substitutions must be
performed before applying any other GSUB lookups, because
those lookups may be written to match only the ccmp-substituted
glyphs.
Note:
ccmpusage is uncommon in Myanmar fonts. Nevertheless, shaping engines must apply anyccmpsubstitutions if they are present in the active font.
The rphf feature replaces initial "Kinzi"-triggering sequences with
the "Kinzi" glyph. The sequences are:
- "Ra,Asat,Halant"
- "Nga,Asat,Halant"
- "Mon Nga,Asat,Halant"
In the Myanmar (or Burmese) language, "Nga" is the only "Kinzi"-forming consonant. "Mon Nga" can form a "Kinzi" in the Mon language, and "Ra" can form a "Kinzi" in Sanskrit written with the Myanmar script.
The pref feature replaces pre-base-consonant glyphs with
any special forms. In Myanmar, this can include variant forms for
"Medial Ra" or for the left-side matras "Sign E" (U+1031) or "Shan
Sign E" (U+1084)
The blwf feature replaces below-base-consonant glyphs with any
special forms. In Myanmar, this usually means replacing
post-base-consonant "Halant,Consonant" sequences with subjoined
forms of the consonant.
However, Myanmar includes several other below-base-consonant forms, including medial consonants and below-base dependent vowel (matra) signs.
The below-base forms feature is applied only to glyphs occurring after the base consonant.
The pstf feature replaces post-base-consonant glyphs with any
special forms.
Note:
pstfusage is uncommon in Myanmar fonts, because the script does not employ special post-base forms of consonants. Nevertheless, shaping engines should apply anypstfsubstitutions if they are present in the active font.
In this stage, the remaining substitution features from the GSUB table are applied. 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.
pres
abvs
blws
psts
liga
The pres feature replaces pre-base-consonant glyphs with special
presentations forms. In Myanmar, this can include stylistic variants
of left-side dependent vowels (matras) or of "Medial Ra".
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. In Myanmar, this can include contextual ligatures
involving below-base dependent vowel marks (matras), medial
consonants, or subjoined consonants.
The psts feature replaces post-base-consonant glyphs with special
presentation forms. This usually includes replacing right-side
dependent vowels (matras) with stylistic variants.
The liga feature substitutes standard, optional ligatures that are on
by default. Substitutions made by liga may be disabled by
application-level user interfaces.
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
mark
mkmk
Note: The
kernfeature is usually applied at this stage, if it is present in the font. However,kernis not mandatory for shaping Myanmar 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.
In Myanmar text, dist is typically used to adjust the space around a
pre-base-reordering "Medial Ra", because the "Medial Ra" codepoint is
classified as being of zero width, but is orthographically a glyph
that encloses the adjacent letter.
The abvm feature positions above-base glyphs for attachment to base
characters. In Myanmar, this includes "Kinzi" and "Asat" in addition
to tone markers, diacritical marks, above-base dependent vowels
(matras), and Vedic signs.
The blwm feature positions below-base glyphs for attachment to base
characters. In Myanmar, this includes subjoined consonants as well as
below-base dependent vowels (matras), medial consonants, tone markers,
diacritical marks, and Vedic signs.
The mark feature positions marks with respect to base glyphs.
The mkmk feature positions marks with respect to preceding marks,
providing proper positioning for sequences of marks that attach to the
same base glyph.
The older Myanmar script tag, <mymr>, has been deprecated. However,
shaping engines may still encounter fonts that were built to work with
<mymr> and some users may still have documents that were written to
take advantage of <mymr> shaping.
Sparse information is available about how the Microsoft Uniscribe
shaping engine treated <mymr> text runs. Documentation from the
HarfBuzz shaping engine suggests that the Uniscribe <mymr> shaper
did not perform a significant amount of reordering or application of
Indic-like GSUB features.