Re: Control picture glyphs

From: Kenneth Whistler (
Date: Wed Aug 29 2007 - 16:57:18 CDT

  • Next message: Asmus Freytag: "Re: Control picture glyphs"

    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

    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.


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