Re: ct, fj and blackletter ligatures

Date: Wed Nov 06 2002 - 11:06:29 EST

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    On 11/06/2002 05:05:17 AM "William Overington" wrote:

    >I am thinking here of ordinary TrueType fonts on a Windows 95 platform
    >on a Windows 98 platform.

    So, by "ordinary" you mean a TTF with a cmap table but no GSUB or other
    tables that perform glyph transformations (though fonts containing such
    tables are just as much TrueType fonts as fonts that are not -- and some
    fonts with such tables were part of some versions of Win 98).

    > I was under the impression that the reason that
    >an ordinary TrueType font will not process a ZWJ sequence on those
    >was that both the operating system and the ordinary TrueType font do not
    >have the capabilities to process ZWJ sequences.

    Given your definition of "ordinary TrueType font", glyph transformations
    are not possible, by definition. But your definition isn't all that
    relevant: fonts that contain tables to perform glyph transformations can
    be used on *any* flavour of Win32 (or other platforms), given appropriate

    It is true that neither the western versions of Win95 or Win98 had
    OS-level capability of applying tables inside TrueType fonts to perform
    glyph transformations. The Mideast version of at least Win98 (not sure
    about Win95) did make use of some such tables (at that time, the
    technology was known as TrueType Open). But any application software on
    any platform could make use of such tables, provided the software is
    written to do so.

    You'll probably come back to say, "But I was talking about 'ordinary
    TrueType fonts'." If you insist on an invalid assumption, there's no way
    to argue against it. It's like saying, "software with a character-mode UI
    is not capable of displaying bitmap graphics" -- true, but irrelevant.

    > My understanding is that
    >even an OpenType font with ZWJ sequence facilities will not work on a
    >Windows 95 or Windows 98 platform.

    It can, given software that knows how to process such sequences to do
    glyph substitutions.

    >As far as I know, there is no requirement in Unicode that the rendering
    >system should notify, perhaps using an Alert dialogue box or similar, the
    >end user that the ZWJ request has been "made yet not fulfilled".
    >Can an advanced format font supply such a message to the rendering system
    >for onward notification of the end user?

    Yes, actually, there is a built-in feedback mechanism: the font provides
    to the rendering system outline data for the c glyph and the t glyph, and
    the rendering system rasterises those outlines in consecutive order, so
    the user sees a glyph sequence "ct" rather than the ct ligature. This
    feedback mechanism even works on older systems.

    A font implementer could make use of this built in capability to provide
    even more explicit information: a font feature might be used to cause
    invisible characters to be displayed in some way (similar to seeing a
    raised circle for the non-breaking space when you set Word to show
    non-printing characters).

    If you really want a dialog box to popup providing notification to the
    user, I'm wondering how many times as the file is opened and a page is
    rendered you'd like this popup to appear? 17 times if there are 17
    instances of < c, ZWJ, t > that are not rendered as a ct ligature? Not on
    my system, thank you.

    >Also, perhaps some method of asking a font to declare a
    >list of the code points for which it has a specific glyph would be

    Software simply needs to inspect the cmap table. No new mechanism is
    needed for this. You're enumerating solutions that need to be built for
    problems that don't exist.

    >There seems to be a gap between the Unicode Technical Committee encoding
    >characters into a file and the process of making sure that the desired
    >is rendered correctly on an end user's platform with good provenance.

    It is not the job of the Unicode Technical Committee to define guidelines
    or review implementations for rendering of text.

    >feel that that issue needs to be addressed. Hopefully the Unicode
    >Committee will wish to take that task upon itself.

    I assure you, they will not.

    - Peter

    Peter Constable

    Non-Roman Script Initiative, SIL International
    7500 W. Camp Wisdom Rd., Dallas, TX 75236, USA
    Tel: +1 972 708 7485
    E-mail: <>

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