From: John Hudson (john@tiro.ca)
Date: Tue Nov 04 2008 - 11:21:17 CST
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
level.
> 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
http://www.tiro.com/John/mirroring.gif
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 www.tiro.com Gulf Islands, BC tiro@tiro.com You can't build a healthy democracy with people who believe in little green men from Venus. -- Arthur C. Clark
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