Re: Control picture glyphs

From: James Kass (
Date: Tue Aug 28 2007 - 19:06:42 CDT

  • Next message: Mark Davis: "Re: Control picture glyphs"

    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.

    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.

    > The font should always have invisible CJKVS characters if it has CJK
    > characters: otherwise it screws up normal rendering. The NORMAL rendering of
    > a CJK character plus CJKVSn, if the pair is not supported by the font, is
    > the CJK character alone. Period. No gummed up box after it.

    There are no valid sequences involving CJK characters plus VS characters.

    Any user getting an undesired display is free to choose a font
    which matches the user's expectations.

    Just as any user getting unexpected results, such as pictures
    being swapped for a font's control pictures by an application,
    is free to discard that application and get something which
    matches *that* user's expectations.

    Because of the size of CJK fonts, it's unrealistic to expect two
    versions of each large font; one for editing/input and the other
    for display.

    VS control pictures are useful during editing/input and are already
    supported by plain-text applications which have no "show hidden"
    mode, like Notepad, when the font includes its own control pictures
    and the author/editor inserts a space between the base character
    and the combining mark.

    If somebody sends me data which includes a CJK base character plus
    a VS character, and that VS character is not officially registered in
    an official sequence, I expect instant visibility of the fact that there's
    a problem with the data. Other users may have different expectations,
    that's OK. Likewise, if somebody sends a CJK base character plus a
    valid VS character (for a valid sequence), and a control picture displays;
    it is an immediate visual indication of a shortcoming in the font.

    Once again, the NORMAL display doesn't get screwed with. The control
    picture in a font which supports VS sequences would never be part of
    the normal display, as long as the data includes only valid sequences.

    Can you show me any problem with my fonts where my VS control
    pictures screw-up a normal display? (I'm only talking about valid
    sequences here, but not limited to CJK.)

    CJK fonts are especially interesting, as they are often fixed-width
    fonts. Unless I'm mistaken, fixed-width fonts can't include arbitrary
    zero-width glyphs interspersed with positive advance width glyphs.
    So, with any valid sequence of CJK base character plus VS character,
    the user will either see:

    1) The single variant glyph. (If the font supports VS characters and
    has a look-up and glyph for that sequence.)

    2) The original base glyph plus a control picture for the VS character.
    (If the font includes control pictures for VS characters but does not
    have glyph substitution enabled for that sequence.)

    3) The original base glyph plus the font's missing glyph. (If the font
    does not support VS characters at all.)

    4) The original base glyph plus a full-width whitespace. (If the font
    maps VS characters to no-contour glyphs.)

    Of the above four, number one is optimal. In my opinion, number two
    is better than either three or four.

    If Adobe's proposal for a very large number of what are essentially
    font-variant CJK glyphs is approved, then, depending on the font
    selected for display, there may well be VS control pictures showing
    up in the display. Unless an application has a user-controlled option
    to strip VS characters from the display, the user is free to choose
    another font if the user doesn't like those VS control pictures.

    Besides, normal CJK display could not be affected at all as long
    as normal CJK *text* doesn't use VS characters. In those rare
    cases where a need to preserve glyph variant distinctions in
    plain text for two variants which are unifiable under the rules
    of Annex S *has been demonstrated*, then any CJK font supporting
    VS characters should include appropriate glyph substitution data
    for that sequence. If there is no need to distinguish a pair of
    variants in plain text, then VS characters should not be used in
    the first place. (And such sequences should not be approved, either.)

    Best regards,

    James Kass
    with apologies for length.

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