Tips & Tricks on Editing and
Displaying Unicode Text
Murray Sargent III
Microsoft Corporation
Text Services Group, Word
What’s RichEdit?
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RichEdit 3.0 is set of plain/rich-text, single/multiline
Unicode/ANSI edit controls in single world-wide binary
Multilevel undo, message & com interfaces, Word
compatibility, pretty rich text
Outline view, zoom, font binding, latest in IME support,
and rich complex script support (BiDi, Indic, and Thai)
Next version: pagination, nested tables, tight wrap, 2D
math (maybe!)…
Clients: Office dialogs, WordPad, Outlook RTF editor,
Pocket Word,…
Introduction
Discuss some problems in manipulating multilingual Unicode text:
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Multiple fonts to display Unicode plain text
Neutral characters, deunifying characters that look different in
different scripts
Working with complex scripts, like Arabic
Using keyboards to enter Unicode characters conveniently
Maintaining backward compatibity with previous character sets
Navigating through text that includes “multicharacters”
Implementing glyph variants and surrogate pairs
Font Binding
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Most Unicode characters belong to scripts
Associate with each position in a document a “font bundle”
When inserting characters, assign each one to a script
For CJK, check surrounding characters for Kana and Hangul as clues
to use Japanese or Korean fonts instead of Chinese
Assign scripts to neutrals and digits
Keyboard language, especially IMEs, provide strong binding clues
Format inserted characters with fonts assigned to scripts. Check
current font to see if it supports required script
Font Binding Problems
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Character not in any script, e.g., mathematical, arrows,
dingbats: use current font or bind to font with font
signature covering appropriate Unicode range. Or invent
new script ID
Font signature may be zero, i.e., unsupported. Call
EnumFontFamiliesEx() to enumerate all charsets for
facename
Font signature may claim support for Unicode ranges, but
miss some characters. cmap reveals support on codewise
basis (slow to access)
Ironically, charset or codepage is a good script ID
Language Detection & Font Binding
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Korean and Japanese are often easy to spot because of Hangul and
Kana characters, respectively
For CJK can convert back to codepage and see if errors occur (Ken
Lunde’s suggestion)
For proofing purposes, accurate language identification is needed. For
font binding, script identification is usually sufficient
Typically more than one language corresponds to a script, e.g., Latin
script. Essentially only one uses the Korean script
Natural language processing techniques allow good language
identification if more than a few words are involved, e.g., a sentence
Big Fonts
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BitStream Cyberbit has most Unicode characters (“big
font”)
Some big fonts have CJK glyph variants for Japanese vs
Simplified Chinese vs Traditional Chinese vs Korean
Font-binding code needs to avoid unnecessary (and
unwanted) font binding with such fonts
Recognize such fonts by using font signature Unicode
ranges and script (codepage) information
Font Sizing
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In dialogs, 8-pt Latin characters are commonly used
8-pt Chinese characters are hard to read, so better to use 9
points in combination with 8-pt Latin characters
Latin characters have bigger descenders than Chinese
characters, since latter only need room for underline
Combining 8-pt Latin characters with 9-point Chinese
characters and keeping same baseline increases line height
to 9 pts plus extra height for Latin descender
Result is more like 10 points: shifts text too high in dialog
box originally designed to handle one language
Complex Scripts
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Unicode covers many complex scripts, e.g., Arabic, Thai
Complex-scripts require layout engine that translates
character codes to glyph indices (often referencing
ligatures)
General Unicode text engine has to have access to
complex-script layout engine
At the previous Unicode conference David Brown
discussed such an engine, Uniscribe, which runs on all
Windows platforms and is shipped with recent versions of
Internet Explorer
For performance: only use CS engine if needed
Neutrals
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Many characters are neutral or “multiscript” and can be rendered with
many different fonts
E.g., blank, ASCII punctuation, ASCII in general, other punctuation,
and decimal digits
Some scripts render neutrals very differently than others and Unicode’s
occasional “over-unification” has complicated what font to use
E.g., Western ellipsis consists of three dots on baseline, while a
Japanese ellipsis has three raised dots
Unicode Standard gives detailed rules for neutrals in BiDi text
Simple rule: neutrals are surrounded by nonneutral characters of same
kind should be rendered with font of nonneutrals
Compatibility characters, such as ASCII fullwidth characters, reveal
which script they belong to
Backward Compatibility
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Unicode text engine has to be able to import and export text in other
standards, which are defined by their codepages
Given nonUnicode plain text, which codepage should one use to
convert to/from Unicode?
On localized systems, system code page is a good bet
In multilingual text, you can enter text using keyboards in a variety of
languages that need either Unicode or multiple code pages
For searching text, best choice seems to be to use the current keyboard
code page
If text begins with a UTF-8 BOM, use UTF-8 conversion
If text begins with a rich-text header, e.g., “{\rtf” or “<html>” or
“<!doctype html”, use appropriate conversion routine
Backward Compatibility (cont)
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Need a little rich-text functionality (minimal language tagging) to
display Unicode plain text unambiguously in some CJK scenarios
This functionality handles font choices and language-dependent glyph
variants
There can be a disparity between typed text and set text
When a user types in text using a keyboard charset, edit engine knows
charset and therefore can insert accurate Unicode text including which
CJK glyph variant to use
Client gets text as pure ANSI (or Unicode) text without script clues
Would be handy to have script tags. Language tags also work, but are
a case of overkill unless proofing tools are to be supported
Unicode on Win95/98
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Win95/98 supports a limited subset of Unicode text functions
ExtTextOutW() works in most cases. Not on Win95J or with metafiles,
so convert back to ANSI whenever possible
Device drivers may not handle Unicode text
With TrueType it’s possible to force downloading of fonts and use
Unicode more reliably
A number of GDI text APIs aren’t implemented, e.g.,
GetGlyphOutlineW().
GetStringTypeExW is stubbed out, so all references to character
property tables have to go through a codepage translation
(WideCharToMultiByte()).
Text boxes, list boxes, comboboxes are all ANSI; use RichEdit for
Unicode
Unicode Keyboard Input
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National keyboards provide ways to input many Unicode characters.
E.g., Greek, Russian, and all ordinary European text.
IMEs (input method editors) let you type phonetic characters to get a
partially composed character sequence. Then type blank to request
composition. If the composition is reasonably unique, you get a fully
composed character; else you get menu of possible resolutions.
To enter Unicode Hex input type a Unicode hexadecimal code into the
text. type a special hot key, e.g., Alt+x, to convert the hex to a Unicode
character
Type Alt+X to replace a character by its hexadecimal number.
Input Sequence Checking. Vietnamese, Thai, and Indic languages
don’t allow all Unicode sequences to be valid and utilize special input
sequence checking code to disallow illegal sequences. For example,
Vietnamese only allows tone marks on vowels.
Unicode Surrogates
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Discuss 3 display models that could enable Win9x/WinNTx based
applications to display higher-plane characters (those in the 16 planes
above the BMP). Ideas are still under development...
First uses a plane index and a 16-bit offset
Second uses a flat 32-bit index
Third uses surrogate-pair ligatures
Models aren’t mutually exclusive, since they involve different cmaps
(compressed tables used to convert codepoints to glyphs)
All assume higher-plane characters are stored as standard Unicode
surrogate pairs
Alternative representations include straight 32-bit characters and UTF8, but aren’t as practical
Unicode Surrogates (cont)
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Using 2 16-bit surrogates to represent a single character complicates
more than measurement and display of characters:
Arrow-key handlers and other methods that change character position
must avoid ending up in between lead and trail surrogates
Input methods need to map to surrogate pair
Case changes, line-breaking rules, sorting, file formats, and backingstore manipulations in general have to recognize and deal with pairs
Surrogate code ranges make them easy to work with relative to
multibyte encoding systems
All three display models assume that GDI remains unchanged (need to
be able to run on OSs already in field
Also assume that 16-bit glyph indices are sufficient so that TrueType
rasterizer doesn’t need to be revised
Surrogate Planar Model
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Characters in font all belong to a particular plane
No changes required to OS. Applications extend font binding logic to
handle font switches to appropriate planes
Character indices remain 16-bit: allows ExtTextOutW family to be
used directly
Model easy for apps to use today in platform-independent way if no
complex scripts are involved
Complex scripts need layout engine. Then applications can ignore
model issue, since layout engine handles OS/font interactions
Truncated 16-bit code indices may map codes in higher planes to
common control or neutral codes
For surrogate-unaware text-processing code, some ranges would have
to be reserved in upper planes
Surrogate Flat and Ligature Models
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Flat 32-bit model uses 32-bit code to index into a new 32-bit cmap in
font file to translate the codes to 16-bit glyph indices
Glyph indices are used to access TextOut family
Method is too tricky for most applications to handle directly: need
surrogate-aware version of Uniscribe
Font binding is done using font signature
Alternatively, application could use 32-bit character strings with a 32bit TextOut family housed in platform-independent component
Ligature model requires use of complex-script engine to access ligature
tables
Comparison of Surrogate Models
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Ease of implementation: for simple scripts, planar model is easiest. In
worldwide-binary environment, need Uniscribe, which can handle
OS/font interactions
Performance: Code to glyph mapping has to be done at some point.
Uniscribe is slower and more RAM intensive than planar model or 32bit TextOut component
Flexibility: flat and ligature models can access chars in all 17 planes
even in same font; planar model one plane per font
Backward compatibility: planar model only needs appropriate fonts
and surrogate-aware apps to work on all Windows platforms
Flat and ligature models require a complex-script engine or a 32-bit
TextOut component to run on all Win9x/WinNTx platforms
Nonspacing Combining Marks
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Multicode characters (surrogate pairs, CRLFs, combining-mark and
variant-tag sequences) require special display/navigation handling
Render combining-mark sequences by standard systems calls and fonts
that support combining marks. Better display needs layout engine that
talks to OpenType
Simple caret movement across combining-mark sequences prevents
stopping inside a sequence. Backspace key deletes one mark at a time
Mouse-cursor hit testing leaves selection at beginning/end of
combining-mark sequence (more elegant model allows selection and
editing of individual marks)
Cool thing: if you can navigate past CRLF combinations, you can
modify corresponding code to handle surrogate pairs and combiningmark sequences quite easily
Glyph Variants
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Character variant: 1) Different character open to future
coding, 2) Prescribed variant (Mongollian), 3) Systematic
semantic variation (different forms like italic, bold, script,
Fraktur in math expressions)
Glyph variant: 1) Artistic variant: free variation (57 &s in
Poetica font), 2) Context preferred style (CJK languagebased variants), 3) Overloaded code points (U+005C: \ ¥
₩), 4) Historical variant: glyph changed over time
Identity variant: 2 external characters map to same
Unicode character
Handling Glyph Variants
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Character variant is open to separate encoding. But if already used,
complicates search algorithms (Ş vs Romanian S comma)
Two approaches: inline variant marks and out-of-plane annotations
Inline variant marks need to be ignored in some searches
Out-of-plane annotation is invisible in plain text and requires more
memory than inline variant mark
Semantically different characters, e.g., math italic b and math script b,
need to be distinguishable in searches, so separate encoding or use of
inline variant marks are desirable
Current proposal for inline variant marks defines 256 standard variant
codes in plane 14 as well as 256 codes for user-defined variant codes
Conclusions
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Have addressed issues encountered in creating Unicode
editors. Issues include:
Automatic choice of fonts for Unicode plain text
Handling nonUnicode documents in Unicode text engines
Ways to input Unicode text
Combining-mark sequences, surrogate pairs, navigation in
multicode text, and glyph variants
Some ideas have been implemented in RichEdit 3.0 control
and other text engines
Unicode surrogate pairs and glyph variants need
decisions...