From: Kenneth Whistler (kenw@sybase.com)
Date: Wed Aug 29 2007 - 16:57:18 CDT
James Kass responded to Mark Davis:
> Mark Davis wrote,
>
> > No. This is the core of the problem. If an application wants to Show Hidden,
> > and reveal VS characters, it can have a special rendering mode to do so,
> > where it replaces DI and whitespace by pictures. And it will have to manage
> > the pictures because it can't depend on any fonts to do it.
>
> Any font which supports control picture characters should be
> able to do it; that's what those control picture characters are for.
Not really. That is one way that they *can* be used, but the
set was never generalized as intending to cover all default
ignorables, nor even the important subset of default
ignorables.
The 2400..243F collection is really a bunch of compatibility
characters for character sets that encoded a collection of
C0 graphic display symbols (in particular CNS 11643-1992, Row
34, 01..33), plus a few random other forms
tossed in to cover graphic display symbols for space and
another couple from control code standards. Note that
there is a complete set of graphic display symbols for C0
codes, but none for C1 control codes, nor *any* of the
Unicode format control characters.
> If an application "shows hidden" and substitutes its own control
> picture for U+0020 when the font already has a perfectly good
> control picture mapped at U+2420, then the application is broken,
> far as I'm concerned.
And I think that statement is just wrong. There is nothing
in the standard that requires or even suggests that
applications wanting to have a "show hidden" mode for
text containing control codes and/or Unicode format control
characters must first assume that it check mappings into
24XX codes and let the font display those if it has them.
And the defectiveness of the 24XX characters for this purpose,
even *if* the font supports display of those characters --
because they are limited essentially to C0 control codes
and SPACE, and don't cover very important Unicode
format controls like the bidi controls -- dictate that it
would be unwise for most applications to assume that they
could depend on the fonts to do anything rational or consistent
for this.
I'll defer to others for the discussion of the CJK VS issues.
--Ken
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