- From: fantasai <fantasai.lists@inkedblade.net>
- Date: Wed, 20 Oct 2004 14:18:24 -0400
- To: www-style@w3.org
- CC: w3c-css-wg@w3.org, w3c-i18n-wg@w3.org, www-i18n-comments@w3.org
I've been working on a revision of CSS3 Text to clean up its handling of
vertical text layout. An explanation of the system, targetted primarily at
members of the Unicode mailing list, is up at
http://fantasai.inkedblade.net/style/discuss/vertical-text/
I will be creating a separate writeup that discusses implementation concerns
and compatibility with W3C technologies.
The full text follows for archival and discussion purposes. If you want to
/read/ the document, you're best off with the HTML version; it has real
links and embedded diagrams.
Comments are welcome. CSS/W3C-specific comments should go to www-style@w3.org;
others should go to the unicode mailing list[1]. You can also send directly to
me (or the CSS WG mailing list if you're a W3C Member), but please post them
publicly so everyone can read. :)
~fantasai
[1] Subscription: http://www.unicode.org/consortium/distlist.html
Archives: http://www.unicode.org/mail-arch/
###########################################################################
Robust Vertical Text Layout
by fantasai <http://fantasai.inkedblade.net/contact>
Few formatting systems today can handle vertical text layout, and most
of those only lay out text in right-to-left columns. This document
outlines a system that can not only handle common scripts in vertical
right-to-left columns, but that can _gracefully_ accept uncommon
script combinations and left-to-right text columns. The model is
described here as a CSS system, but the concepts can apply to non-CSS
systems as well.
The CSS model and Unicode provide support for logical text layout, but
only in horizontal flow. Although CSS3 Text attempts to use horizontal
BIDI controls to handle vertical BIDI, the system it sets up is
ill-defined and inflexible, relies on assumptions that may not hold
true, and requires a styled document's content and its markup to be
adapted to the CSS rather than the other way 'round. A better design
would use the intrinsic properties of the characters and an expansion
of Unicode's logic to lay out the text. A layout model thus based on
the logic and knowledge of writing systems can scale to gracefully
handle any combination of scripts, can correctly (if not optimally)
lay out text with any combination of styling properties, and can
integrate well with the layered Unicode + Markup + Styling model of
semantically-tagged documents.
The examples in this text require support for Unicode BIDI and Arabic
shaping, and fonts for Simplified Chinese and Arabic/Farsi. Most
diagrams are available in SVG, but inline versions are in PNG with
fallbacks in GIF.
Recommended browsers (recent versions):
* Opera <http://www.opera.com/>
* Gecko-based (such as Mozilla, Firefox, or Camino (Mac OS X))
<http://www.mozilla.org/>
* Microsoft Internet Explorer or Safari may suffice.
More about Unicode fonts and other software
<http://www.alanwood.net/unicode/>
1. Background
1. The ‘Cascade’ in Cascading Style Sheets
2. CSS and Unicode Bidi
2. Abusing Directionality and Its Consequences: A Case Study of
CSS3 Text
3. Describing Text Flow
1. Physical vs. Logical Description
2. Intrinsic Directionality and Orientation
1. Script Classification by Directionality
3. Logical Text Flow
1. Implying Direction
2. Orienting by Block Progression
3. The Three Switches of Logial Text Layout
4. Implementing A Logical Text Layout System
1. Composing Lines of Text
1. Character Ordering
2. Glyph Orientation
1. Vertical Scripts
2. Horizontal Scripts
3. Punctuation
3. Character Shaping
2. Understanding Character Properties
5. Why and How the Unicode Consortium Should Be Involved
1. What happens if Unicode chooses not to standardize the
additional character data?
6. About the Author and the Status of CSS3 Text
7. Acknowledgements
8. Appendix: Vertical Scripts in Horizontal Flow
Background
The "Cascade" in Cascading Style Sheets
Unlike many formatting systems, in which styling properties are
definitively applied to a page element at one point, CSS collects and
applies to the element multiple style rules from the author, reader,
and user agent. In case of a conflict, the origin of the rule and the
specificity of the rule's element selector determine which of the
conflicting property values takes effect on the element. This process
of sorting and applying style rules is called cascading[1], and it
allows style rules from multiple sources and with separate formatting
purposes to interact in a rigorous way.
[1] http://www.w3.org/TR/CSS21/cascade.html#cascade
*Cascading means that style properties specified together are not
guaranteed to take effect together.* This raises the design standards
for creating CSS properties and pushes them towards a more logical,
rather than physical, description of the intended design.
CSS and Unicode BIDI
CSS2 introduced the direction and unicode-bidi properties to
incorporate markup directives such as HTML's dir attribute into the
CSS rendering model, and to allow the use of markup semantics in
assigning BIDI embeddings. The direction property can take the values
ltr and rtl, and this value inherits to descendant elements. The
unicode-bidi property assigns embeddings and overrides in the
direction given by the direction property. Its behavior is defined in
terms of the Unicode embedding and override codes.
/* map 'dir' attribute to 'direction' + embedding*/
*[dir="ltr"] {direction: ltr; unicode-bidi: embed; }
*[dir="rtl"] {direction: rtl; unicode-bidi: embed; }
/* embed quotations so they always stay as a single unit */
q {unicode-bidi: embed;}
When applied to a block of text, the direction property specifies the
block's embedding direction; CSS documents do not use heuristics to
guess the block's embedding direction.
These properties are meant to reflect BIDI distinctions necessary for
the proper ordering of text. Authors in general are discouraged[2]
from using the properties in favor of the direct markup that would
trigger the appropriate values.
[2] http://www.w3.org/TR/i18n-html-tech-bidi/#ri20030728.092130948
Abusing Directionality and Its Consequences: A Case Study of CSS3 Text CR
CSS3 Text was intended to update and expand the text layout
capabilities of CSS2 by adding support for more international
typesetting features and introducing controls for laying out vertical
text. It defines a 'block-progression' property, which switches the
line stacking direction, and hijacks 'rtl' and 'ltr' values of the
'direction' property to use as an inline-progression control in
vertical text.
Cite: http://www.w3.org/TR/2003/CR-css3-text-2003051/#writing-mode
| writing-mode: direction: block-progression: Common Usage:
| lr-tb ltr tb Latin-based, Greek, Cyrillic
| (and many others)
| rl-tb rtl tb Arabic, Hebrew
| tb-rl ltr rl Chinese, Japanese, Korean
| tb-lr rtl lr Traditional Mongolian
It is a good example of how _not_ to set up a vertical text system.
In order to interface with the Unicode BIDI Algorithm[3], CSS3 Text
maps vertical scripts' character directionality based on the
paragraph's block progression.
[3] http://www.unicode.org/reports/tr9/
Because all vertical scripts in Unicode are assigned a canonical
directionality of left-to-right, BIDI reordering proceeds as normal
when the text columns are stacked right-to-left.
However, if the columns of text are stacking the other way--from
left to right--then the same characters (which so far are all given
left-to-right directionality in Unicode) are treated as right-to-left
characters (R). This was done because left-to-right scripts such as
Latin read bottom to top when the lines of text were ordered left to
right. You can see this often on image and table captions when the
text runs along the left side. The first line of text runs from bottom
to top, and lines stack with each one to the right of the previous
(left to right). In this case, top-to-bottom scripts _must_ go in the
direction _opposite_ the left-to-right text, and the opposite of ltr
is rtl.
Messing with the directionality of vertical scripts messes with other
bits of text layout as well, and much of this interaction was left
undefined. Character shaping, for example, depends on the
BIDI-reordered string being in normal order. Not only character
ordering, but the character shaping algorithm and the font rendering
code all need to compensate for the altered input to the BIDI
algorithm, and CSS3 Text failed to explain the necessary changes.
For example, Mongolian is a cursive vertical script. Like Arabic (to
which it is related), it is also a shaping script: a letter at the
beginning of a word is shaped differently from one in the middle or at
the end. Unicode defines Mongolian to be a left-to-right script, so
shaping makes the leftmost character of a word into an initial and the
rightmost character into a final. If, however, the Mongolian word is
ordered right-to-left, then the initial letter of the word will be on
the right, and therefore shaped as a final and not an initial. This is
because shaping happens _after_ reordering. Vertical Mongolian text
treated like this will look upside down and read like a bunch of
gibberish, and no amount of glyph rotation can fix the problem. To
make the letters connect properly under the CSS right-to-left
override, the Mongolian parts of the text would need to be shaped in
reverse and then have their glyphs rotated 180°--but this is not even
mentioned in CSS3 Text.
To accomodate CSS3 Text's ill-defined tweaks to BIDI reordering
(and character shaping and font rendering), a layout system can't
simply pass the string to standard Unicode processing functions.
Assume, however, that the layout system manages to hold up internally
the pretense that "top-to-bottom" is "right-to-left". It still needs
to interact with BIDI instructions from the outside world, which
doesn't share the delusion. In an effort to make the outside world
_seem_ like it's adapted to these changes, CSS3 Text instructs the
designer to use 'direction: rtl' when assigning 'block-progression: lr'
to a block of top-to-bottom text (such as Mongolian or Chinese, both
actually ltr scripts), in effect asking him to lie about the text's
properties. Like most lies, it seems to work in the general case, but
as the situation gets complicated, the system breaks down...
* Foremost, if the expected block progression fails to take effect--
whether through the cascade or through lack of UA support--the
text direction and the assigned embedding direction no longer match
and the subtleties of Unicode BIDI can wreak havoc on the order of
the text.
<example>
* CSS embeddings set on elements _within_ the formatted block are no
longer necessarily going the right way.
<example>
* HTML dir attributes that were added with the assumption of
regular, horizontal text might or might not need to have their
effects be reversed.
<example>
* There is no mention of how character shaping should happen.
In conclusion, abusing directionality controls to make a limited
system lay out text correctly doesn't scale. It's a hack, not a
solution.
Describing Text Flow
To describe how a text flows into lines, one needs to know three
things:
Image: Three Vectors
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/text-flow-vectors-tb.svg>
* which way the text flows within a line (inline progression)
* which way the lines stack (block progression)
* which way the glyphs are facing (glyph orientation)
However, not all combinations of text direction and glyph orientation
are valid. Therefore if certain of the character's inherent
characteristics are known, it is often possible to derive one from the
other. Unicode systems take advantage of this model in horizontal
text: you don't have to manually tell every run of Hebrew to order
itself right-to-left, and you don't need to specify that Mongolian
characters turn themselves sideways when the text is running
horizontally left-to-right.
Logical vs. Physical Description
In a purely physical layout scheme, each of these text layout
properties would be given as an absolute: The inline progression of
this run of English is top to bottom, its glyph orientation is 90
degrees clockwise, its block progression is from right to left.
Image: Diagram of vectors for rotated English
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/text-flow-vectors-rl.svg>
However, because the
interrelationships among these properties are realized in the author's
mind and not in the system,
* The author must manually intervene any time there is a script
change.
* If one of the three properties fails to take effect (because of
the Cascade or lack of UA support), then the layout breaks and the
text becomes unreadable.
A better system would embed knowledge of different scripts' intrinsic
characteristics and define style properties in terms of the
relationships among them.
Intrinsic Directionality and Orientation
Each script has a characteristic writing direction, and each character
in Unicode is assigned a directionality value based this
characteristic. Unfortunately, Unicode currently only defines
horizontal directionality even though vertical and bi-orientational
scripts have a vertical directionality as well. For example, while
English can go either top to bottom or bottom to top (since it doesn't
have a vertical directionality), Japanese must only go from top to
bottom, even in a left-to-right block progression. Mongolian also has
top-to-bottom vertical directionality. Unlike Japanese however, it has
no definite horiziontal directionality (just a canonical one for BIDI
purposes).
Script Classification by Directionality
Scripts can be classified into three orientational categories:
horizontal
Scripts that have horizontal, but not vertical, directionality.
Includes: Latin, Arabic, Hebrew, Devanagari
vertical
Scripts that have vertical, but not horizontal, directionality.
Includes: Mongolian, Manchu
bi-orientational
Scripts that have both vertical and horizontal directionality.
Includes: Han, Hangul, Yi, Ogham
Bi-orientational scripts may be further classified by how their glyphs
transform when switching orientations. CJK characters translate; they
are always upright. Other scripts, such as Ogham and some variants of
classical Yi, must be rotated.
Logical Text Flow
Implying Direction
Scripts in their native orientation need no additional stylistic hints
for proper layout: their inline progression and glyph orientation are
both intrinsically mandated, so the style system can know by itself
how to lay them out. *Directionality and glyph orientation overrides
are not necessary and should not be used._*(In fact, using them
degrades the system by creating a tangle of dependencies, as
demonstrated in the section on the current version of CSS3 Text.)
Scripts in a foreign orientation don't need directionality or glyph
overrides either. They just need a few hints: whether to translate
upright, or, if they're rotated sideways, which side is "up". Given
that, the rules for laying out the text in its native orientation are
enough to determine the inline progression and exact glyph
orientation.
Orienting by Block Progression
For scripts in a non-native orientation, the natural inline text flow
depends on the direction of line stacking: the text is most
comfortably laid out as if the whole text block were merely rotated
from the horizontal. For example, English text in vertical lines that
stack from left to right will face with the glyphs' tops towards the
left and the text direction running from bottom to top. The same text,
by the same logic, would in a right-to-left line stacking context face
right and flow within each line from top to bottom.
Image: Diagram of text block rotation
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/text-flow-natural.svg>
Note: Merely rotating the rendered text from a horizontal layout is
not sufficient because while the primary script is horizontal, it may
include some vertical text (such as Chinese) that would need to be
laid out appropriately for vertical lines.
Putting this logic into the style system is straightforward: define
"up" for non-native glyphs to point to the beginning of the line
stack, and the inline progression follows from that orientation. The
glyph orientation and inline progression will thus adapt to whichever
block progression happens to take effect.
This layout scheme is most appropriate for dealing with text that has
been turned on its side for layout purposes--as for page headers
or captions or table headings. However, a major use case for laying
out text in a non-native orientation is mixing horizontal and vertical
scripts, which introduces the requirement of making the secondary
scripts flow well in the context of the primary script.
For example, a primarily Mongolian document, which has vertical lines
stacking left to right, usually lays its Latin text with the glyphs
facing the right. This makes the text run in the same inline
progression as Mongolian and face the same direction it does in other
East Asian layouts (which have vertical lines stacking right to left),
but the glyphs are facing the _bottom_ of the line stack rather than
the top, something they wouldn't do in a primarily-English paragraph.
Image: Text Flow Vectors in Mongolian Text
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/mongolian-vectors.jpg>
Yet another common layout is to keep the horizontal script's glyphs
upright and order them from top to bottom; this is frequently done
with Latin-script acronyms in vertical East Asian text.
Image: <http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/vertical-acronym.gif>
To handle these layouts, the style system needs to offer controls for
choosing among these different layout schemes. Note, however, that
scripts in their native orientations do not need these hints; only the
non-native ones do. Also, this is only one simple scheme switch here:
there's no need for the designer to set separate absolute inline
progression and glyph orientation controls or to set styling
properties on each text run of a different script.
The Three Switches of Logical Text Layout
In summary, to lay out a block of arbitrary, mixed-script text, the
layout system needs to offer only three controls:
* primary script's directionality (BIDI property)
* block progression direction (stylistic property)
* glyph orientation scheme (stylistic property)
Formalized into CSS syntax, this becomes:
direction
Primary directionality. Can take the following values
ltr
Left-to-right directionality in horizontal text; No
inherent directionality in vertical text. (Horizontal
script) Examples: Latin, Tibetan
rtl
Right-to-left directionality in horizontal text; No
inherent directionality in vertical text. (Horizontal
script) Examples: Arabic, Hebrew
ttb
Top to bottom directionality in vertical text; No
inherent directionality in horizontal text. (Vertical
script) Example: traditional Mongolian
ltr-ttb
Left to right directionality in horizontal text; Top to
bottom directionality in vertical text. (Bi-orientational
script) Examples: Han, modern Yi
ltr-btt
Left to right directionality in horizontal text; Bottom
to top directionality in vertical text. (Bi-orientational
script) Example: Ogham
block-progression
Block progression (line stacking) direction. Can take the
following values
tb
Top-to-bottom line stacking (horizontal text). Typically
used for most non-East-Asian layout.
rl
Right-to-left line stacking (vertical text). Typically
used for traditional CJK layout.
lr
Left-to-right line stacking (vertical text). Typically
used for traditional Mongolian layout.
text-orientation-vertical
Glyph orientation scheme to use in vertical text. Can take the
following values
natural
Non-vertical script runs are laid out as if "up" was
towards the top of the line stack (left or right,
depending on the block progression in effect). (Vertical
scripts are laid out as vertical scripts.)
left
Non-vertical script runs are laid out as if "up" was
towards the left side of the line stack. (Vertical
scripts are laid out as vertical scripts.)
right
Non-vertical script runs are laid out as if "up" was
towards the right of the line stack. (Vertical scripts
are laid out as vertical scripts.)
upright
Non-vertical scripts' characters read top to bottom, with
each grapheme cluster oriented upright. (Vertical scripts
are laid out as vertical scripts.)
For handling vertical-only scripts in horizontal layout, a
text-orientation-horizontal property is also necessary; it
takes effect only when the block progression is top-to-bottom.
To keep the discussion less verbose, I am relegating
consideration of horizontal layout to an appendix.
As long as the directionality is set correctly for the text (and it
should be set automatically from the content/markup as long as the
designer doesn't touch it later), any combination of the
block-progression and text-orientation stylistic values will result in
a correct (though perhaps not optimally-designed) text layout.
The style system can thus handle most of the intricacies of laying out
both usual and unusual combinations of text by itself. What it needs
to do this, however, is to know the intrinsic properties of the
characters and the logic of laying them out.
Implementing A Logical Text Layout System
Composing Lines of Text
Handling block-progression is very straightforward: just stack the
composed lines in the stacking direction. Composing the lines of text
is more complicated. The text needs to go through three processing
steps.
Character Ordering
Character ordering is where the BIDI algorithm gets applied. *The
algorithm remains essentially unchanged when dealing with vertical
text: what changes is the data.* Specifically, the directionality
values of certain characters are mapped into the algorithm differently
depending on the styling context.
The Unicode Bidirectional (BIDI) Algorithm deals with two
directions: left-to-right (towards right) and right-to-left (towards
left), defined to be the same as the script directionalities involved.
Although this multi-directional model has several more directionality
values, the BIDI algorithm here still deals with only two directions:
it just abstracts them so that they could just as easily be
bottom-to-top (towards top) and top-to-bottom (towards bottom). To
avoid the apparent absurdity of mapping right to left and such things,
I will call the two BIDI directions "high" (H) and "low" (W).
(Implementations, no doubt, will prefer to call them "left" and
"right" to map directly into the Unicode BIDI algorithm.)
It is important to keep in mind that these directions are abstract. We
will map "left", "right", "top", and "bottom" to "high" or "low" based
on the values of text-orientation and block-progression. *The mapping
applies to everything: the individual character's directionality,
embedding and override codes, the CSS direction values, HTML dir
attributes, etc.* Once the line is composed, we then lock "high" and
"low" to the appropriate sides of the block as we stack the lines
according to block-progression.
Directionality Mapping: Vertical Case
In vertical context, bi-orientational scripts use their vertical
directionality and behave as vertical, not horizontal, scripts. Han,
for example, as a ltr-ttb script, is treated as ttb (top to bottom),
_not_ ltr (left to right). The ltr-ttb value for direction is
correspondingly treated the same way as the value ttb.
For text-orientation: right (and text-orientation: natural in a
right-to-left block progression):
* Map ttb and ltr to htl (high to low)
* Map btt and rtl to lth (low to high)
Image: Diagram of 'right' Mapping
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/bidi-right.svg>
Run the Unicode BIDI Algorithm with its "left" being our "high" and
its "right" being our "low".
For text-orientation: left (and text-orientation: natural in a left-to-right
block progression):
* Map ttb and rtl to lth (low to high)
* Map btt and ltr to htl (high to low)
Image: Diagram of 'left' mapping
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/bidi-left.svg>
Run the Unicode BIDI Algorithm with its "left" being our "high" and
its "right" being our "low".
For text-orientation: upright
* Map ttb, ltr, and rtl to htl (high to low)
* Map btt to lth (low to high)
Image: Diagram of 'upright' mapping
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/bidi-upright.svg>
Run the Unicode BIDI Algorithm with its "left" being our "high" and
its "right" being our "low".
Glyph Orientation
Before the system can paint the text (or even do alignment), it needs
to know how to rotate the glyphs. For vertical and bi-orientational
scripts, this is simply "rotate me to my intrinsic position". This
doesn't mean "don't rotate me, I'm supposed to be upright", however,
because *the standard representation of a character in a font is the
one used in horizontal text*.
Vertical Scripts
Han and Kana and Hangul and Yi do need to be kept upright (0°
rotation) because they use the same orientation in both horizontal and
vertical text. Mongolian (and Ogham), however, rotate from one context
to the other and so their glyphs must be rotated 90° from their
horizontal orientation when used in vertical context. Part of the
system's knowledge, therefore, needs to be which scripts need to be
rotated and which merely translated into place. Given that and the
script's directionality, the exact rotation can be derived as follows:
System's Knowledge of Vertical Scripts' Properties -
Han/Hangul/Kana/Yi Mongolian/Manchu Ogham
(cannonical) horizontal
directionality......... LTR (LTR) LTR
vertical directionality TTB TTB BTT
transformation translation rotation rotation
System's Derivation of Vertical Scripts' Orientation
Han/Hangul/Kana/Yi Mongolian/Manchu Ogham
horizontal orientation
(vector direction)....
inline progression 90° 90° 90°
glyph orientation 0° 0° 0°
transformation........
inline progression rot 90° rot 90° rot -90°
glyph orientation static rot (90°) rot (-90°)
vertical orientation
(vector direction)....
inline progression 180° 180° 0°
glyph orientation 0° 90° 270°
Horizontal Scripts
For horizontal scripts, the method is "rotate me according to the
relevant text-orientation style".
For text-orientation: right or text-orientation: natural in a right-to-left
block progression:
Rotate horizontal scripts' grapheme clusters 90° to the right.
Image: Glyphs rotated right
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/glyph-right.svg>
For text-orientation: left or text-orientation: natural in a left-to-right
block progression:
Rotate horizontal scripts' grapheme clusters 90° to the left.
Image: Glyphs rotated left
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/glyph-left.svg>
For text-orientation: upright
Keep glyphs for horizontal scripts upright and stack grapheme clusters
vertically.
Image: Glyphs translated upright
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/glyph-upright.svg>
Punctuation
Transformations for punctuation, being somewhat arbitrary and
stylistic, should be handled by using vertical glyph variants given in
the font, but only when the direction of the text is a vertical or
bi-orientational directionality or text-orientation-vertical is
upright. (If the text is primarily horizontal text rotated sideways,
then the punctuation should likewise be horizontal punctuation rotated
sideways.)
Character Shaping
Character shaping is the process of selecting, based on context, which
of several allographs of a letter should be used. This is typical of
cursive scripts like Arabic and Mongolian, where the shape of a letter
depends on whether it comes at the start of a word, in the middle of a
word, or at the end of a word.
Image: Diagram of Arabic shaping
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/shaping.svg>
According to UAX 9, character shaping occurs _after_ BIDI reordering:
the Arabic character shaped as an "initial" will always be on the
right, even if the text is given a left-to-right override. This
ensures that the letters always connect. (An initial form on the left
side of the word would be trying to connect to nothing.)
To deal with the multiple orientations of vertical layout, the shaping
logic needs to know not just the reordered string of characters, but
which side of the line is "up". If we turn the glyphs all upside-down,
for instance, the shaping needs to be done in reverse.
Image: Diagram of reverse (upside-down) shaping
<http://fantasai.inkedblade.net/style/discuss/vertical-text/diagrams/reverse-shaping.svg>
Because in vertical text Arabic and Mongolian can go in the same direction
or in opposite directions, merely inverting the entire character string
before passing it to standard Unicode shaping functions doesn't work.
Shaping occurs only within each directional level run. Shaping is also
constrained to runs of text in the same script; Mongolian characters,
from Arabic's point of view, form as concrete a boundary as Latin ones
do. It is therefore possible to break up the text into pieces that
have characters from no more than one shaping-affected script without
compromising the accuracy of the shaping. Then, for each run of text,
one can use the shaping script characters' glyph orientation (derived
above) to determine which way is "up" (0°) and hence which are the
"left" (-90°) and "right" (+90°) sides of the text run. Once that's
known the text run can be shaped, in reverse if necessary.
Understanding Character Characteristics
In addition to knowing the text, its primary directionality, and its
styling properties, the implementation needs to know something about
the characters themselves to be able to take advantage of the logical
model. For each character, the following information must be available
to the text layout algorithm:
* horizontal directionality: ltr or rtl
For vertical scripts this means the canonical directionality that
is used for fonts and for plaintext horizontal layout.
* vertical directionality: ttb, btt, or none
For horizontal scripts, this is none.
* glyph transformation between horizontal and vertical orientations:
translation, rotation, or not applicable
Only applies to scripts with vertical directionality.
Unicode only specifies the horizontal directionality. For some scripts
(not all), vertical directionality can be gleaned from the prose
chapters describing the different writing systems. Glyph
transformations are not given at all, only implied.
Why and How the Unicode Consortium Should Be Involved
The text layout model outlined in this document adds to the scope of
the Unicode BIDI Algorithm and requires additional knowledge of
character properties. This expansion should be part of Unicode rather
than an alteration defined in CSS3. Standardizing it at the Unicode
level rather than at the CSS level is more appropriate because
* The Unicode Consortium has the expertise necessary to specify the
character data correctly even for obscure scripts.
* The extended data and algorithms operate at the same level as the
existing Unicode specifications.
* Non-CSS systems wanting to use this model will have a solid base
to work off of rather than having to adapt bits of a high-level
protocol (CSS3) to fit their application.
* Standard Unicode APIs can be designed to handle the extended BIDI
and shaping manipulations so that each application will not need
to implement all of that itself.
* Intermediary systems such as HTML can accomodate the model by
building up from Unicode rather than down from CSS. (HTML would
need the new direction values for its dir attribute.)
* Unicode can add character-level support for
vertical-directionality by defining directionality control codes
to correspond with the vertical directionality requirements. This
will allow the same plain text to be properly flowed into vertical
layout contexts as well as horizontal ones.
What happens if Unicode chooses not to standardize the additional character
data
I will be including the results of my personal research as a normative
appendix to the next revision of CSS3 Text. Should the Unicode
Consortium provides the necessary character data, I will publish a new
version that removes the appendix and instead references the relevant
sections of the Unicode Standard.
About the Author and the Status of CSS3 Text
I am an invited expert for the CSS Working Group at the World Wide Web
Consortium (W3C) and the new editor of the CSS3 Text module. I intend
to completely rewrite the Text Layout chapter for the next version of
CSS3 Text based on the principles outlined herein.
Acknowledgements
Thanks go out to:
* Martin Heijdra at the Gest Library, for his guidance, expertise,
and enthusiasm. I had never imagined that the research staffer
helping me find books would turn out to be an expert on
international typography and Mongolian in particular.
* Ian Hickson, the members of the www-style mailing list, the
members of the CSS Working Group, and the contributors to the
Mozilla Project for tempering my technical skills and CSS
knowledge over the years
* The CSS Working Group for giving me a chance to fix everything I
found wrong in the CSS3 Text drafts.
* Last, but not least, Håkon Wium Lie and Opera Software <http://www.opera.com/>
for supporting me in creating this work, to the point of even _paying_
me to finish it. I only wish it hadn't taken so long so that I could
spend more time on QA. ;)
Appendix: Vertical Scripts in Horizontal Flow
<http://fantasai.inkedblade.net/style/discuss/vertical-text/appendix>
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Received on Wednesday, 20 October 2004 18:19:06 UTC