Re: Question about the directionality of "Old Hungarian" (document N3531)

From: John Hudson (
Date: Tue Nov 04 2008 - 11:21:17 CST

  • Next message: Kent Karlsson: "RE: Question about the directionality of "Old Hungarian" (document N3531)"

    Kent Karlsson wrote:

    > I'm not sure what you mean by "character level mirroring" vs. "glyph
    > mirroring" here.

    By 'character level mirroring' I mean application of BidiMirroring.txt.
    This, as you and Asmus note, is intended as a fallback mechanism in the
    absence of glyph mirroring, but I don't believe it is reliably
    anticipated as such. There are already applications that perform this
    kind of 'character level mirroring', and they have done so in the
    absence of any standardised mechanism for glyph level -- i.e.
    font-controlled -- mirroring. This means that what was intended as a
    fallback mechanism may be active regardless of the presence or absence
    of glyph mirroring in newer fonts, so the latter must be aware of this
    and not reverse mirroring that has already been applied at the character

    > When the bidi algorithm says not to mirror, the font is *not* to do
    > mirroring (based on any bidi results). Otherwise there is something
    > wrong. That is why I was talking about what the bidi algorithm says
    > to mirror, and not to mirror, as well as the (ugly) loophole for
    > LTR overridden to RTL (but, currently, no such loophole for RTL
    > overridden as LTR).

    This presumes that Unicode is the last word on glyph directionality, but
    I don't think it is. As Chris Fynn notes, at the glyph level the
    decision to flip or not flip the form of a letter may be a stylistic
    variation, even a user preference as catered for by the individual font.

    > This would be something handled by the font handling system (mirror
    > the given glyph),

    That presumes that simply flipping the outline produces visually
    acceptable results. But type designers mean by mirroring involves design
    of directionally reversed glyphs, which is seldom achieved by simply
    flipping an outline. Consider this image

    contrasting a flipped letter e with one designed as a reversed form (for
    phonetic transcription in this case, but the same holds true for
    direction mirroring). The flipped form looks distorted because the
    ductus is reversed.

    Such designed mirrored forms need to be accessed, which implies a GSUB
    feature in OT or similar mechanism in AAT or Graphite.

    John Hudson

    Tiro Typeworks
    Gulf Islands, BC
    You can't build a healthy democracy with people
    who believe in little green men from Venus.
                        -- Arthur C. Clark

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