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Creating and Supporting OpenType Fonts for Myanmar Script

Introduction

Microsoft Typography
Last updated: December 2013

This document presents information that will help font developers in creating OpenType fonts for Myanmar script as covered by the Unicode Standard 6.0. The Myanmar script is used to write the Myanmar language. It is also used to write other languages including Pali and Sanskrit.

Contents

Introduction

This document targets developers implementing shaping behavior compatible with the Microsoft OpenType specification for the Myanmar script. It contains information about terminology, font features and behavior of the Myanmar shaping engine. While it does not contain instructions for creating Myanmar fonts, it will help font developers understand how the Myanmar shaping engine processes Myanmar text.

Terms

The following terms are useful for understanding the layout features and script rules discussed in this document.

Base glyph – Any glyph that can have a diacritic mark attached to it. Layout operations are defined in terms of a base glyph, not a base character, as a ligature may act as a base

Character – Each character represents a Unicode character code point. A character may have multiple glyph forms

Cluster – A group of characters that form an integral unit in Brahmi-derived scripts, oftentimes this corresponds to a syllable

Consonant – Myanmar consonants have an inherent vowel (the short vowel /a/). For example, “Ka” and “Ta”, rather than just “K” or “T”

Consonant conjunct (aka ‘conjunct’) – A ligature of two or more consonants

Format controls – special formatting characters used in the shaping process of Myanmar scripts (U+200c and U+200D). These characters have no visual appearance, except when an application chooses to display zero width glyphs

Glyph – A glyph represents a form of one or more characters

Kinzi – A reduced form of certain consonant signs that is written as an above base mark on a following base. This corresponds to a reph in other Brahmi-derived scripts.

Ligature – A combination of glyphs that join to form a single glyph

Matra (dependent vowel) – Used to represent a vowel sound that is not inherent to the consonant. Dependent vowels are referred to as “matras” in Sanskrit. They are always depicted in combination with a single consonant, or with a consonant cluster

OpenType layout engine – The library responsible for executing OpenType layout features in a font. In the Microsoft text formatting stack, it is named OTLS (OpenType layout services)

OTLS – OpenType Layout Services

OpenType tag – A 4-byte identifier for script, language system or feature in the font

Shaping engine - Code responsible for shaping input, classified to a particular script



Shaping Engine

The Uniscribe Myanmar shaping engine processes text in stages. The stages are:

  1. Analyzing the characters
  2. Reordering characters
  3. Applying OpenType GSUB features to get the correct glyph shape
  4. Applying OpenType GPOS features to position glyphs or marks

The descriptions which follow will help font developers understand the rationale for the Myanmar feature encoding model, and help application developers better understand how layout clients can divide responsibilities with operating system functions.

Analyzing the characters

The run of text that the shaping engine receives for the purpose of shaping is a sequence of Unicode characters. The shaping engine divides the text into syllable clusters and identifies character properties. Character properties are used in parsing syllables and identifying their parts as well as determining whether any contextual reordering is required.

Additionally, the engine verifies that the run consists of valid clusters and inserts a placeholder glyph (U+25CC) wherever combining marks occur without a valid base.

In the Myanmar engine, OpenType features are applied in two stages, first GSUB features are applied to the logical cluster. Then, GSUB and GPOS features are applied to the entire run.

In the diagrams below, the rules for forming clusters are given in terms of the classes of characters in the character stream. The meanings of the symbols are:

A Anusvara class (1032, 1036)
As Asat (103A)
C Consonants and Independent vowels (1000-1020, 103F, 104E, 1050, 1051, 105A-105D, 1061, 1065, 1066, 106E-1070, 1075-1081, 108E, AA60-AA6F, AA71-AA76, AA7A)
D Myanmar digits except zero (1041-1049, 1090-1099)
D0 Myanmar digit zero (1040)
DB Dot below (1037)
GB Generic base characters (00A0, 00D7, 2012–2015, 2022, 25CC, 25FB–25FE)
H Halant/virama (1039)
IV Independent vowel (1021-102A, 1052-1055)
J Joiners (200C, 200D)
K A special sequence of three characters (<1004 | 101B | 105A>, 103A, 1039)
MH Medial consonants Ha, Mon La (103E, 1060)
MR Medial consonants Ra (103C)
MW Medial consonants Wa, Shan Wa (103D, 1082)
MY Medial consonants Ya, Mon Na, Mon Ma (103B, 105E, 105F)
O SCRIPT_COMMON characters in a Myanmar run
P Punctuation (104A, 104B)
PT Pwo and other tones (1063, 1064, 1069-106D, AA7B)
R Reserved characters from the Myanmar Extended-A block (AA7C-AA7F)
S Symbols (104C, 104D, 104F, 109E, 109F, AA70, AA77-AA79)
V Visarga and Shan tones (1038, 1087-108D, 108F, 109A-109C)
VAbv Above base dependent vowel (102D, 102E, 1033-1035, 1071-1074, 1085, 1086, 109D)
VBlw Below base dependent vowel (102F, 1030, 1058, 1059)
VPre Pre base dependent vowel (1031, 1084)
VPst Post base dependent vowel (102B, 102C, 1056, 1057, 1062, 1067, 1068, 1083)
VS Variation selectors (FE00–FE0F)
WJ Word joiner (2060)
WS White space (any white space character including ZWSP)
X* sequence of zero or more occurrences of X. Since this could extend a cluster indefinitely an arbitrary limit of 31 characters in a sequence has been used
X+ sequence of one or more occurrences of X
<X | Y> disjunction of elements: X or Y
[X] optional (zero or one) occurrence of X
# occurrence of a boundary
× no boundary allowed at indicated position
÷ boundary allowed at indicated position
^ Except

Well-formed Clusters

Well-formed Myanmar character clusters can have combinations of groups as defined below. There are three options:

Simple non-compounding cluster

<P | S | R | WJ| WS | O | D0 >

Punctuation (P), symbols (S), reserved characters from the Myanmar block (R), word joiner (WJ), white space (WS), and other SCRIPT_COMMON charcters (O) contain one character per cluster.

Cluster terminating in Halant

[K] <C | IV | D | GB>[VS] (H <C | IV> [VS])* H

Optional Kinzi with a required base sign and optional variation selector. Zero or more stacked consonants or full vowel forms and optional variation selector. A halant character terminates the cluster.

Complex cluster

[K] <C | IV | D | GB>[VS] (H <C | IV> [VS]) (As)* [MY] [MR] [<MW [As] | [MW] MH [As]>] (VPre)* (VAbv)* (VBlw)* (A)* [DB [As]] (VPst [MH] (As)* (VAbv)* (A)* [DB [As]])* (PT (A)* [DB [As]])* (V)* [J]

Optional Kinzi with a required base sign and optional variation selector. Zero or more stacked consonants or full vowel forms and optional variation selector followed by Zero or more Asat signs. Zero or one of each medial consonant, a single Asat may follow MW and MH, but may not precede MH. Zero or more dependent pre-base and above-base vowels. Zero or more below-base vowels. Zero or more Anusvara and zero or one dots below, if there is a dot below, a single Asat is permitted. Zero or more sequences of a post-base vowel, followed by zero or one medial H, zero or more Asat, zero or more vowels above, and zero or one dots below, if there is a dot below, a single Asat is permitted. Zero or more sequences of a Pwo tone mark each of which may have zero or one dots below, if there is a dot below, a single Asat is permitted. Zero or more visargas. Zero-width joiner and zero-width non joiner will be the last item in any cluster in which they occur.

img

The cluster rules need to permit this level of complexity in order to be able to handle the full range of possible encoded sequences. However, only test cases will try to exercise more than a few of these positions in a single cluster:

င်္က္ကျြွှေို့်ာှီ့ၤဲံ့းႍ

img

Reordering characters

Once the Myanmar shaping engine has analyzed the run as described above, it creates a buffer of appropriately reordered elements (glyphs) representing the cluster according to the rules given below.

Pathological Reordering Example

A run containing many of the possible items in a single syllable would reorder as follows:

Key

Kinzi Base Medial Ra Prebase vowel Anusvara

INPUT

1004 103A 1039 1000 1039 1000 103B 103C 103D 1031 1031 102D 102F 1036 102C 1036

REORDERED

1031 1031 103C 1000 1004 103A 1039 1039 1000 103B 103D 102D 1036 102F 102C 1036

The OpenType lookups in a Myanmar font must be written to match glyph sequences after re-ordering has occurred. OpenType fonts should not have substitutions that attempt to perform the re-ordering. If a font developer attempted to encode such reordering information in an OpenType font, they would need to add a huge number of many-to-many glyph mappings to cover the general algorithms that a shaping engine will use.

Apply OpenType GSUB features

Uniscribe calls OTLS to apply the features. All OTL processing is divided into a set of predefined features (described and illustrated in the Feature section of this document). Each feature is applied to the entire run and OTLS processes them. Uniscribe makes as many calls to the OTL Services as there are features. This ensures that the features are executed in the desired order.

The steps of the shaping process are outlined below. Not all of the features listed must be used by all languages using the Myanmar script.

Shaping features:

  1. Localized forms
    1. Apply feature ‘locl’ to preprocess any localized forms for the current language
  2. Basic shaping forms
    1. Apply feature ‘rphf’ to get kinzi glyph forms
    2. Apply feature ‘pref’ to get pre-base glyph forms
    3. Apply feature ‘blwf’ to get below-base glyph forms
    4. Apply feature ‘pstf’ to get post-base forms
  3. Presentation forms
    1. Apply feature ‘pres’ to substitute pre-base glyph forms
    2. Apply feature ‘abvs’ to substitute above-base glyph forms
    3. Apply feature ‘blws’ to substitute below-base glyph forms
    4. Apply feature ‘psts’ to substitute post-base glyph forms

Note: since the presentation form features are applied simultaneously over the entire cluster, several features are operationally equivalent to a single feature. Multiple features are provided as an aid for font developers to organize the lookups they implement.

Apply OpenType GPOS features

The shaping engine next processes the GPOS (glyph positioning) table, applying features concerned with positioning. All features are applied simultaneously to the entire cluster.

The font developer must consider the effects of re-ordering when creating the GPOS feature and lookup tables.

  1. Kerning
    1. Apply feature ‘kern’ to provide pair kerning between glyphs for better typographic quality. Note this feature may be disabled by some applications
    2. Apply feature ‘dist’ to make any required distance adjustments
  2. Mark placement
    1. Apply feature ‘mark’ to position diacritic glyphs relative to the base glyph
    2. Apply feature ‘mkmk’ to position diacritic glyphs relative to each other


Features

The features listed below have been defined to create the basic forms for languages that use the Myanmar script. Regardless of the model an application chooses for supporting layout of complex scripts, Uniscribe requires a fixed order for executing features within a run of text to consistently obtain the proper basic form. This is achieved by calling features one-by-one in the standard order listed below.

The order of the lookups within each feature is also very important. For more information on lookups and defining features in OpenType fonts, see Encoding feature information in the OpenType font development section.

The standard order for applying Myanmar features encoded in OpenType fonts:

Feature Feature function Layout operation Required
Localized forms

locl

 

GSUB

 
Basic shaping forms

rphf

Kinzi substitution

GSUB

X

pref

Pre-base substitution

GSUB

X

blwf

Below-base substitution

GSUB

X

pstf

Post-base form substitution

GSUB

X

Presentation forms

pres

Pre-base substitution

GSUB

 

abvs

Above-base substitution

GSUB

 

blws

Below-base substitution

GSUB

 

psts

Post-base substitution

GSUB

 
Kerning:

kern

Pair kerning

GPOS

 

dist

Distance adjustments

GPOS

X

Mark placement

mark

Mark positioning

GPOS

 

mkmk

Mark to mark positioning

GPOS

 

[GSUB = glyph substitution, GPOS = glyph positioning]

Feature examples

The registered features described and illustrated in this document are based on the Microsoft OpenType font Myanmar Text (mmrtext.ttf). Myanmar Text contains layout information and glyphs to support all of the required features of the Myanmar script.

The illustrations in the following examples show the result of that particular feature being applied. Features must be written to match glyph sequences after re-ordering has occurred. Note that the input context for a feature may be the result of a previous feature having already been applied.

Localized forms

Feature Tag: “locl”

This feature is used in association with OpenType language system tags to trigger lookups that will select alternate glyphs needed for language-specific typographic conventions. The ‘locl’ should not be used in association with the default language system, but only used with other language system tags. See the Appendix of this document for language system tags associated with the Myanmar script.

img

Basic shaping forms

Feature Tag: “rphf”

This feature is used to substitute the kinzi forms. Kinzi sequences are reordered after valid base, see above. If there is no valid base, a dotted circle glyph is inserted by the shaping engine to serve as the base. Kinzi sequences should be substituted with a mark glyph so that it can be positioned above the preceding base character after reordering. The kinzi substitution must be done with the rphf feature.

img

Feature Tag: “pref”

This feature is used to substitute the pre-base forms. For example, the Myanmar Text font uses this feature to substitute different width forms of the medial consonant Ra (MR).

img

Feature Tag: “blwf”

This feature is used to substitute the below-base forms. For example, the Myanmar Text font uses this feature to form stacked consonant glyphs.

img

Feature Tag: “pstf”

This feature is used to substitute the post-base forms. For example, the Myanmar Text font uses this feature to substitute the combination of the vowel sign tall Aa and a following Asat.

img

Presentation forms

Feature Tag: “pres”

This feature is used to substitute presentation forms involving pre-base elements. For example, the Myanmar Text font substitutes the clipped forms of the medial consonant Ra using the pres feature.

img

Feature Tag: “abvs”

This feature is used to substitute presentation forms relating to above-base elements. For example, the Myanmar Text font uses this feature to form ligatures of above-base marks.

img

Feature Tag: “blws”

This feature is used to substitute presentation forms relating to below-base elements. For example, the Myanmar Text font uses this feature to substitute spacing forms of below-base marks vowels.

img

Feature Tag: “psts”

This feature is used to substitute presentation forms relating to post-base elements. For example, the Myanmar Text font uses this feature to substitute a presentation form for medial consonant Ya.

img

Kerning

Feature Tag: “kern”

This feature may be used to adjust the positioning of glyph pairs. The Myanmar Text font does not use this feature.

Feature Tag: “dist”

This feature may be used to adjust distances. The Myanmar Text font uses this feature add back width to spacing marks.

Note that mark glyphs have their width set to zero by OTLS. If a mark glyph must have width, it is necessary to add back lost width for correct display. The dist feature is a required feature and should be used for this purpose as well as other required distance adjustments.

img

Mark placement

Feature Tag: “mark”

This feature is used to position marks relative to a base glyph. The Myanmar Text font uses this feature to position above and below marks relative to the base glyph.

Note that the mark feature is a required feature and will always be triggered by the shaping engine. Fonts may use other mark positioning features such as abvm or blwm, but these are not required features.

img

Feature Tag: “mkmk”

This feature is used to position marks relative to each other. The Myanmar Text font uses this feature to position a sequence of marks.

Note that the mkmk feature is a required feature and will always be triggered by the shaping engine.

img

Handling invalid combining marks

Combining marks and signs that do not occur in conjunction with a valid base are considered invalid. Shaping engine implementations may adopt different strategies for how invalid marks are handled. For example, a shaping engine implementation might treat an invalid mark as a separate cluster and display the stand-alone mark positioned on some default base glyph, such as a dotted circle (U+25CC). (See Fallback Rendering in section 5.13 of the Unicode Standard 4.0.) Shaping engine implementations may vary somewhat with regard to what sequences are or are not considered valid. For instance, some implementations may impose a limit of at most one above-base vowel mark while others may not.

To allow for shaping engine implementations that expect to position an invalid mark on a dotted circle, it is recommended that a Myanmar OT font contain a glyph for the dotted circle character, U+25CC. If this character is not supported in the font, such implementations will display invalid signs on the missing glyph shape (white box).

Recommended Glyphs

Unicode code points that are strongly recommended for inclusion in any Myanmar font are:

Code point Description

U+200B

Zero Width Space

U+200C

Zero Width Non-Joiner

U+200D

Zero Width Joiner

U+25CC

Dotted Circle

U+002D

Hyphen-minus

U+00A0

No-break space

U+00D7

Multiplication sign

U+2012

Figure dash

U+2013

En dash

U+2014

Em dash

U+2015

Horizontal bar

U+2022

Bullet

U+25FB

White medium square

U+25FC

Black medium square

U+25FD

White medium small square

U+25FE

Black medium small square



Appendix

Appendix: Writing system and language tags

Features are encoded according to both a designated script and language system. The language system tag specifies a typographic convention associated with a language or linguistic subgroup. For example, there are different language systems defined for the Myanmar script; Burmese, Pali, and Sanskrit.

NOTE: The script tag for Myanmar script for use with the Myanmar shaping engine is mym2 and not mymr. The script tag mymr has limited support and should not be used.

Not all software applications support specific language tags for use when rendering text runs.

* NOTE: It is strongly recommended to include the “dflt” language tag in all OpenType fonts because it defines the basic script handling for a font. The “dflt” language system is used as the default if no other language specific features are defined or if the application does not support that particular language. If the “dflt” tag is not present for the script being used, the font may not work in some applications.

The following tables list the registered tag names for scripts and language systems.

Registered tags for the Myanmar script Registered tags for Myanmar language systems
Script tag Script Language system tag Language

“mym2”

Myanmar

“dflt”

*default script handling

   

ARK

BRM

Arakanese

Burmese

   

MON

Mon

   

KRN

Karen

   

QIN

Asho Chin

   

SHN

Shan

   

PLG

Rumai Palaung

   

SAN

Sanskrit

   

PAL

PLG

Pali

Palaung

Note: both the script and language tags are case sensitive (script tags should be lowercase, language tags are all caps) and must contain four characters (ie. you must add a space to the three character language tags).