Robust Vertical Text Layout and CSS3 Text:
Using the Unicode BIDI Algorithm to Handle Complexities
in Typesetting Multi-Script Vertical Text
Elika Etemad - W3C CSS Working Group / Princeton University
Intended Audience: Software Engineers, International Typography Experts
Session Level: Intermediate, Advanced
As cross-cultural written communication increases, the technology underlying that communication needs to handle the intersection of their layout conventions. Vertical text is the traditional mode of text layout for many East Asian writing systems. It is also used for effects such as vertical headers in horizontal layout. However, few formatting systems today can do true vertical text layout, and most of those only can only handle common scripts in right-to-left columns. Methods for typesetting left-to-right columns or uncommon script combinations such as Mongolian and Arabic thus often involve unwieldy BIDI overrides and delicate glyph rendering tweaks. These workarounds are awkward and can break the portability of the underlying text. The model outlined in this paper uses the intrinsic properties of the characters and an expansion of Unicode's logic to lay out the text without these hacks. Such a system can scale to gracefully handle any combination of scripts, can correctly lay out text with any combination of styling properties, and can integrate well with the layered Unicode + Markup + Styling design of semantically-tagged documents on the Web. This model, developed for the next revision of the CSS3 Text Module, is described here as a CSS system, but the concepts can apply to non-CSS layout systems as well.
This presentation focuses on methods for automatically handling character ordering, shaping, and glyph orientation switches when typesetting lesser-known scripts and unusual script combinations in vertical layout, without reworking existent horizontal layout algorithms or adding script-specific new modules. Assumes some familiarity with Unicode BIDI.