Murray Sargent III

blabbingunequaledAI and Robotics

Oct 24, 2013 (3 years and 9 months ago)

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Murray Sargent III

Microsoft Corporation

Text Services Group, Word

Tips & Tricks on Editing and
Displaying Unicode Text

What’s RichEdit?


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:



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


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


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


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


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


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


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


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


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)


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


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


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


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 UTF
-
8, but aren’t as practical

Unicode Surrogates (cont)


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 backing
-
store 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


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


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 32
-
bit TextOut family housed in platform
-
independent component


Ligature model requires use of complex
-
script engine to access ligature
tables

Comparison of Surrogate Models


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 32
-
bit 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


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 combining
-
mark sequences quite easily

Glyph Variants


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 language
-
based 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


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


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...