From: Peter Constable (petercon@microsoft.com)
Date: Tue Dec 09 2003 - 03:30:23 EST
From: unicode-bounce@unicode.org on behalf of Kenneth Whistler
>> Unicode doesn't prevent styling, of course. But having 'logical' order
>> instead of 'visual' makes it a hard task for the application and the
>> renderer.
>> This is witnessed by the thin-spread support for this.
>
>Yes...
Ken conceded the claim too readily. Glyph re-ordering due to a logical encoding order that is different from visual order may mean that certain types of styling (of the re-ordered character) may not be supported in some implementations, but it does *not* mean that this is, in general, a hard task. Style information is applied to characters, and as long as there is a 1:m association between characters and glyphs and there is a path to transform the styling information to match the character/glyph transformations, styling is in principle possible. (There's a constraint that styling might not be possible if the styling differences require different fonts but the glyph transformations that occur require rule contexts to span such a style boundary.)
(Expecting one component from a precomposed character to be styled differently from the rest, however, would be somewhat hard.)
In particular, for reordering this is easy to demonstrate by considering a hypothetical complex-script rendering implementation in which processing is divided into two stages: character re-ordering, and glyph transformation. In the first stage, all that happens is that a string is mapped to a temporary string used internally only, in which characters are reordered into visual order. (Surroundrant characters with no decomposition would be mapped into multiple internal-use-only virtual characters.) Thus, a styled string such as <string>k<span color="red">e</span></string> would transform in the first stage to <string><span color="red">e</span>k</string>. There is nothing hard in such processing.
(Of course, whether it is harder to get people to implement support for one thing rather than another is an entirely different question.)
Peter Constable
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