This document details the shaping procedure needed to display text runs in the Khmer script.
Table of Contents
The Khmer or Cambodian script is a descendant of the Brahmi script, and follows many of the same general patterns found in Indic scripts. However, Khmer incorporates enough distinctions of its own that it is generally not advisable to attempt supporting it in a general-purpose Indic shaping engine.
The Khmer script is used to write multiple languages, most commonly Khmer, Tampuan, Krung, Cham, and Pali. In addition, Sanskrit may be written in Khmer, but the Khmer script is not used for Vedic texts, therefore Khmer text runs are not expected to include any glyphs from the Vedic Extensions block of Unicode.
The Khmer script tag defined in OpenType is <khmr>.
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 Khmer script can include sequences of multiple vowels and, therefore, multiple matras.
Halant and Virama are both standard terms for the vowel-killer" mark. Unicode documents use the term "virama" most frequently, while OpenType documents use the term "halant" most frequently.
The Khmer block does include a version of "halant" mark, "Viriam"
(U+17D1). Its usage in Khmer text, however, differs significantly
from the usage of "halant" in other Brahmi-derived and Indic
orthographies.
Chandrabindu (or simply Bindu) is the standard term for the diacritical mark indicating that the preceding vowel should be nasalized. In Khmer, the chandrabindu mark is known as the "nikahit".
The term base consonant is also critical to Khmer 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).
Each consonant in Khmer bears one of two inherent vowels. The two sets of letters that correspond to these inherent vowels are referred to as registers. In Khmer text, a register shifter mark can be used to replace the letter's inherent vowel with the inherent vowel of the other register.
Some consonants in one register have a corresponding consonant in the other register; for these consonant pairs, a register shifter is not employed.
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. The below-base, subscript forms of letters are the most frequently seen secondary forms. However, some secondary forms are post-base, and the secondary form of "Ro" is a pre-base-reordering form.
These secondary letter forms are known in Khmer as coeng forms. Most coengs are consonants, although, in certain cases, independent vowels may take on coeng forms.
The Khmer block of Unicode does not encode the coeng forms of
letters as separate codepoints. Instead, the "Sign Coeng" (U+17D2)
codepoint is a control character used to indicate that the following
letter should be rendered in its coeng form. The "Sign Coeng" has no
visual representation of its own.
Note: Despite the potentially confusing name in the Unicode standard, "Sign Coeng" (
U+17D2) itself should not be referred to as a coeng. The term "coeng" refers to the form of the letter that follows the "Sign Coeng" control character.
:::{figure-md}
Coeng form of Kha :::
Although coengs are typically attached to the base consonant of a syllable, in certain circumstances coengs may also be attached to an independent vowel.
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 Khmer 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,
Khmer 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 Khmer 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
NUKTA 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.
Note: In Khmer, the
NUKTAclass is used for the chandrabindu mark, "Nikahit" (U+17C6). This more correctly reflects the shaping behavior of the nikahit mark than does theBINDUclass used in other scripts.
"Viriam" (U+17D1), Khmer's "halant"-like codepoint, is classified as
PURE_KILLER rather than the more common VIRAMA. This is to
indicate that the "Viriam" suppresses the inherent vowel of a
consonant but is not used between consonants to trigger the formation
of a subjoined form.
"Sign Coeng", the coeng-form generator, is classified as
INVISIBLE_STACKER. This is to indicate that the "Sign Coeng"
codepoint itself is never rendered as a visible glyph.
"Toandakhiat" (U+17CD) is classified as CONSONANT_KILLER. This
mark indicates that the previous consonant is not pronounced. Note
that "Toandakhiat" is a diacritic mark, and that its class,
CONSONANT_KILLER is not a subclass of consonant.
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]).
Khmer uses one subclass of consonant, CONSONANT_POST_REPHA. This
subclass is used only for "Robat", the above-base form of "Ro". The
"Robat" is similar to the "Reph" found in many Indic scripts but,
unlike "Reph", "Robat" is encoded as a separate codepoint; therefore, it is
not formed by a special sequence of control characters.
:::{figure-md}
Robat :::
"Robat" is a consonant, but it is classified as a combining mark in Unicode. For shaping purposes, "Robat" behaves like the "Nukta" mark found in many Indic scripts.
The Khmer glottal-stop consonant "Qa" (U+17A2) carries an inherent
vowel and is also capable of accepting dependent vowels (matras). It
is sometimes used in place of an independent vowel. For shaping
purposes, however, this usage of "Qa" does not demand any special
treatment.
Other characters, such as symbols and punctuation, 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.
Multi-part dependent vowels (matras) may be designated with compound
mark-placement subclasses (such as TOP_AND_LEFT_POSITION) that
denote all of the mark-placement positions occupied.
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.
The Khmer block in Unicode includes all of the codepoints necessary to write Khmer language text. The Khmer Symbols block contains miscellaneous symbols used for lunar-date calendars. The Khmer Symbols codepoints do not evoke any special behavior from the shaping engine.
Separate character tables are provided for the Khmer and Khmer Symbols
blocks as well as for other miscellaneous characters that
are used in <khmr> text runs:
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+1780 |
Letter | CONSONANT | null | ក Ka |
U+17C6 |
Mark [Mn] | NUKTA | TOP_POSITION | ំ Nikahit |
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 Khmer 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 (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,Sign_Coeng,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 hgher-level typesetting environment.
Several codepoints in the Khmer block are deprecated and their usage in new documents is officially discouraged. The deprecated codepoints are:
- "Qaq" (
U+17A3) - "Qaa" (
U+17A4)
Usage of three other codepoints is also discouraged, although the codepoints have not been deprecated. These codepoints are:
- "Inherent Aq" (
U+17B4) - "Inherent Aaa" (
U+17B5) - "Sign Beyyal" (
U+17D8)
Although usage of these codepoints in text is discouraged, shaping engines encountering them in a text run should handle the situation gracefully.
Processing a run of <khmr> 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 involves 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.
Khmer 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, Khmer's specific shaping characteristics include:
- The first consonant of a syllable is always the base consonant.
Note: For comparison with the General Indic shaping model, this characteristic would correspond to
BASE_POS_FIRST.
- The "Robat" form, which is analogous to "Reph" or "Repha" in Indic scripts, in separately encoded as a non-spacing mark codepoint. The "Robat" form does not require reordering.
Note: For comparison with the General Indic shaping model, the Robat -encoding characteristic would correspond to
REPH_MODE_VISUAL_REPHA, and the reordering characteristic would be null or some other designation indicating that the "Robat" is not reordered.
- The below-base forms feature is applied to consonants after the base consonant.
Note: For comparison with the General Indic shaping model, this characteristic would correspond to
BLWF_MODE_POST_ONLY.
-
The ordering position for left-side matras, as with Indic scripts, is
POS_PREBASE_MATRA. -
The ordering positions for right-side, below-base, and above-base matras is the same. All are reordered to immediately after the last post-base consonant.
Note: For comparison with the General Indic shaping model, this characteristic would correspond to
MATRA_POS_TOP,MATRA_POS_BOTTOM, andMATRA_POS_RIGHTtaking the ordering positionPOS_AFTER_POST.
A syllable in Khmer consists of a valid orthographic sequence that may be followed by a "tail" of modifier signs.
Note: The Khmer Unicode block enumerates eight modifier signs, "Nikahit" (
U+17C6), "Reahmuk" (U+17C7), "Bantoc" (U+17CB), "Kakabat" (U+17CE), "Ahsda" (U+17CF), "Samyok Sannya" (U+17D0), "Bathamasat" (U+17D3), and "Atthacan" (U+17DD).
Because texts written in Khmer 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.
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 consonant's inherent vowel sound. This vowel sound can be changed by a dependent vowel (matra) sign or by a register shifter following the consonant.
For a syllable beginning with a consonant, the base consonant is the first consonant of the syllable.
Unlike Indic scripts, where the vowel sound designates the end of the syllable, Khmer syllables can end with final consonants that occur after a dependent vowel (matra).
All post-base consonants in a valid syllable will be preceded by "Sign Coeng" marks. This includes final consonants.
BaseC Sign-Coeng Post-baseC Matra Sign-Coeng FinalC
In some Khmer words, an independent vowel can occur in a subjoined position like a post-base consonant. In such instances, the independent vowel will be preceded by "Sign Coeng".
BaseC Sign-Coeng IndependentVowel
The algorithms for identifying syllables and for correctly identifying the base consonant include test to recognize these sequences.
As with other Brahmi-derived and Indic scripts, the consonant "Ro" receives special treatment.
- A post-base "Ro" must be reordered to a visually pre-base position. This move is performed during the initial reordering stage.
- "Robat", the above-base variant of "Ro", is encoded as a combining mark rather than as a full consonant. "Robat" does not, however, require reordering by the shaping engine.
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 Khmer 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 Khmer-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_ = The consonant "Ro"
_consonant_ = `CONSONANT` - _ra_
_vowel_ = `VOWEL_INDEPENDENT`
_nukta_ = `NUKTA` | `CONSONANT_POST_REPHA`
_zwj_ = `JOINER`
_zwnj_ = `NON_JOINER`
_matra_ = `VOWEL_DEPENDENT` | `PURE_KILLER` | `CONSONANT_KILLER`
_syllablemodifier_ = `SYLLABLE_MODIFIER` | `BINDU` | `VISARGA`
_placeholder_ = `PLACEHOLDER` | `CONSONANT_PLACEHOLDER`
_dottedcircle_ = `DOTTED_CIRCLE`
_registershifter_ = `REGISTER_SHIFTER`
_coeng_ = `INVISIBLE_STACKER`
_symbol_ = `SYMBOL` | `AVAGRAHA`Note: The
CONSONANT_POST_REPHAshaping class is merged with theNUKTAshaping class to reflect the correct orthographic behavior of "Robat".
These identification classes form the bases of the following regular expression elements:
C = _consonant_ | _ra_ | _vowel_
N = (_zwnj_? _registershifter_)? (_nukta_ _nukta_?)?
Z = _zwj_ | _zwnj_
CN = C N?
MATRA_GROUP = Z? _matra_ N?
SYLLABLE_TAIL = (_syllablemodifier_ _syllablemodifier_?)?
PARTIAL_CLUSTER = N? (_coeng_ CN)* MATRA_GROUP* (_coeng_ CN)? SYLLABLE_TAILUsing the above elements, the following regular expressions define the possible syllable types:
A valid syllable will match the expression:
(C | _placeholder_ | _dottedcircle_) PARTIAL_CLUSTERThe 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 and pre-base-reordering "Ro".
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_BASE_CONSONANT
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 in Khmer 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. It includes some categories not utilized in Khmer.
The basic positions (left to right) are "Reph" (POS_RA_TO_BECOME_REPH), dependent
vowels (matras) and consonants positioned before the base
consonant (POS_PREBASE_MATRA and POS_PREBASE_CONSONANT), the base
consonant (POS_BASE_CONSONANT), above-base consonants
(POS_ABOVEBASE_CONSONANT), below-base consonants
(POS_BELOWBASE_CONSONANT), consonants positioned after the base consonant
(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 base consonant. 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 stage 2, step 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. - The consonant stopped at will be the base consonant.
Second, any multi-part dependent vowels (matras) must be decomposed
into their left-side and right-side components. Khmer has five
two-part dependent vowels, "Oe" (U+17BE), "Ya" (U+17BF), "Ie"
(U+17C0), "Oo" (U+17C4), and "Au" (U+17C5).
Each of these dependent vowels decomposes into a left-side component
identical to the single-part dependent vowel "E" (U+17C1) plus a
right-side component.
Unlike many other scripts, the decompositions of multi-part dependent vowels in Khmer are not defined as canonical in Unicode. Some of the right-side components that would result from these decompositions do not correspond to assigned Unicode codepoints.
Instead, fonts often substitute the default glyph with a right-side-component glyph using GSUB substitutions. The decomposition step performed here allows the left-side component to be correctly reordered by the shaping engine.
"Oe" (
U+17BE) decomposes to "U+17C1,U+17BE""Ya" (
U+17BF) decomposes to "U+17C1,U+17BF""Ie" (
U+17C0) decomposes to "U+17C1,U+17C0""Oo" (
U+17C4) decomposes to "U+17C1,U+17C4""Au" (
U+17C5) decomposes to "U+17C1,U+17C5"
Two of the multi-part dependent vowels, "Oe" (U+17BE) and "Oo"
(U+17C4), can be decomposed into existing Unicode codepoints. If
desired, the corresponding decompositions are:
"Oe" (
U+17BE) decomposes to "U+17C1,U+17B8""Oo" (
U+17C4) decomposes to "U+17C1,U+17B6"
However, shaping engines should take note of the fact that these decompositions are non-canonical and therefore, if the active font's design employs non-standard stylistic choices, the results may not appear as expected.
:::{figure-md}
Multi-part matra decomposition :::
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.
Third, 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, above-base, and below-base dependent-vowel (matra)
signs are tagged POS_AFTER_POST.
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.
Fourth, all pre-base-reordering consonants must be tagged with
POS_PREBASE_CONSONANT.
Khmer has one pre-base-reordering consonant: "Ro".
:::{figure-md}
Pre-base-reordering Ro :::
Fifth, the remaining consonants and independent vowels should be tagged with the appropriate positions.
- All
VOWEL_INDEPENDENTs and allCONSONANTs other than "Ro" occurring after the base consonant (found in step one) and must be tagged asPOS_BELOWBASE_CONSONANT. - A
CONSONANTs orVOWEL_INDEPENDENTs in the syllable occurring after a dependent vowel (matra) must be tagged asPOS_FINAL_CONSONANT.
In a valid syllable, such post-base consonants (of class CONSONANT)
and independent vowels (of class VOWEL_INDEPENDENT) will be preceded by a
"Sign_Coeng" glyph.
Note: The consonant "Robat", of class
CONSONANT_POST_REPHA, is not included in the classes checked here and must not be tagged in this step. "Robat" should not appear in a post-base position in a valid syllable.
Sixth, all marks must be tagged.
Several Khmer marks that are categorized in Unicode as syllable
modifiers or that modify consonants are allowed to occur mid-syllable
in Khmer words. Therefore, they are not tagged for the POS_SMVD
position that is typically reserved for syllable modifiers and Vedic
signs.
| Codepoint | Sorting Position | Glyph |
|---|---|---|
U+17CB |
POS_ABOVEBASE_CONSONANT |
់ Bantoc |
U+17CD |
POS_ABOVEBASE_CONSONANT |
៍ Toandakhiat |
U+17CE |
POS_ABOVEBASE_CONSONANT |
៎ Kakabat |
U+17CF |
POS_ABOVEBASE_CONSONANT |
៏ Ahsda |
U+17D0 |
POS_ABOVEBASE_CONSONANT |
័ Samyok Sannya |
U+17D1 |
POS_ABOVEBASE_CONSONANT |
៑ Viriam |
U+17D3 |
POS_ABOVEBASE_CONSONANT |
៓ Bathamasat |
U+17DD |
POS_ABOVEBASE_CONSONANT |
៝ Atthacan |
All remaining marks, including "Sign Coeng", 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 base consonant and those marks after the base consonant.
-
Initially, all remaining marks should be tagged with the same positioning tag as the closest preceding consonant.
-
For each consonant after the base consonant (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 base consonant "own" the marks that follow them, while all consonants after the base consonant "own" the marks that come before them. When a syllable does not have any consonants after the base consonant, the base consonant should "own" all the marks that follow it.
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.
Note: No marks will precede the base consonant in a valid syllable.
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
pref
blwf
abvf
pstf
cfar
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 Khmer fonts. Nevertheless, shaping engines must apply anyccmpsubstitutions if they are present in the active font.
The pref feature replaces pre-base-consonant glyphs with
any special forms. In Khmer, this typically includes the
pre-base-reordering form of "Ro".
:::{figure-md}
Pre-base form substitution :::
The blwf feature replaces below-base-consonant glyphs with any
special forms. In Khmer, this usually means replacing the default
forms of letters with coeng (or subscript) forms.
The below-base forms feature is applied to glyphs occurring after the base consonant.
:::{figure-md}
Below-base form substitution :::
The abvf feature replaces above-base-consonant glyphs with any
special forms. In Khmer, this may include variant forms of above-base
dependent vowels and marks.
:::{figure-md}
Above-base form substitution :::
The pstf feature replaces post-base-consonant glyphs with any
special forms. In Khmer, this can include coeng forms of certain
consonants that include an ascending "arm" on the right-hand side as
well as variant forms for right-side matras and marks.
:::{figure-md}
Post-base form substitution :::
The cfar feature replaces any below-base-consonant or
post-base-consonant glyphs that occur immediately after a "Sign
Coeng,Ro" sequence with special presentation forms. This can include
contextual variants of post-base and below-base glyphs designed to
better interact, visually, with the final position of
pre-base-reordering "Ro".
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
blws
abvs
psts
calt
clig
liga
The pres feature replaces pre-base-consonant glyphs with special
presentations forms. In Khmer, this can include stylistic variants
of left-side dependent vowels (matras) or of pre-base-reordering "Ro".
:::{figure-md}
Pre-base presentation form substitution :::
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.
:::{figure-md}
Above-base presentation form substitution :::
The blws feature replaces below-base-consonant glyphs with special
presentation forms. In Khmer, this can include contextual ligatures
involving below-base dependent vowel marks (matras) or subjoined letters.
:::{figure-md}
Below-base presentation form substitution :::
The psts feature replaces post-base-consonant glyphs with special
presentation forms. This usually includes stylistic variants of
right-side dependent vowels (matras) or of subjoined letters featuring
right-side ascenders.
:::{figure-md}
Post-base presentation form substitution :::
The clig feature substitutes optional ligatures that are on by
default, but which are activated only in certain contexts.
Note: In some other scripts, substitutions made by
cligmay be disabled by application-level user interfaces. For Khmer, however, application ofcligsubstitutions in mandatory because these substitutions are important for typographic correctness, not merely for user preference.
:::{figure-md}
Contextual ligature substitution :::
The liga feature substitutes standard, optional ligatures that are on
by default. Substitutions made by liga may be disabled by
application-level user interfaces.
:::{figure-md}
Standard ligature substitution :::
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
kern
blwm
abvm
mkmk
Note: The
kernfeature is usually applied at this stage, if it is present in the font. However,kernis not mandatory for shaping Khmer text and may be disabled by user preference.Notably, the Microsoft Uniscribe shaping engine does not apply
kernlookups even if they are present in the font. For more information on Uniscribe compatibility, see the Uniscribe-bug-compatibility note.
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.
:::{figure-md}
Application of the dist feature :::
The abvm feature positions above-base glyphs for attachment to base
characters. In Khmer, this includes register shifters and syllable
modifiers, in addition to diacritical marks and above-base dependent
vowels (matras).
:::{figure-md}
Above-base mark positioning :::
The blwm feature positions below-base glyphs for attachment to base
characters. In Khmer, this can include coeng forms of letters as well as
below-base dependent vowels (matras).
:::{figure-md}
Below-base mark positioning :::
The mkmk feature positions marks with respect to preceding marks,
providing proper positioning for sequences of marks that attach to the
same base glyph.