From: Jonathan Rosenne (firstname.lastname@example.org)
Date: Wed May 02 2007 - 11:22:37 CST
The following comments are not specific to Arabic:
It isn't just a matter of what the Unicode standard requires, but rather of
what the various languages that use the script require. Unicode endeavored
to document these requirements.
The "practice" should not be taken as various computer implementations but
books and manuscripts. Computer implementations have progressed quite a lot
in recent years, but I don't believe they have reached the stage for most
scripts of being qualified as determining practice.
> -----Original Message-----
> From: email@example.com
> [mailto:firstname.lastname@example.org] On Behalf Of John Hudson
> Sent: Wednesday, May 02, 2007 7:19 PM
> To: Andreas Prilop
> Cc: email@example.com
> Subject: Re: Arabic ligatures and vowel signs
> Andreas Prilop wrote:
> > The Unicode standard requires that "transparent characters" such as
> > Arabic vowel signs do not affect the joining of Arabic
> letters and the
> > formation of ligatures.
> > http://www.unicode.org/versions/Unicode4.0.0/ch08.pdf
> > However, the practice seems to be quite different.
> > I notice that in Windows 2003/XP only the typefaces
> > Microsoft Sans Serif and Times New Roman show the
> > lam-alif ligature with transparent characters:
> > http://www.unics.uni-hannover.de/nhtcapri/temp/lam-alif.html6
> > The other typefaces show the joining of lam and alif
> > but no ligature.
> I hope that most of those fonts have been updated in Vista.
> Most of them are pretty old
> now, and predate the specific fetaures of OpenType that make
> it possible for ligation to
> occur in the presence of intervening combining marks.
> In addition to the ones shown in your graphic, the MS Arabic
> Typesetting font also allows
> ligation in the presence of combining marks, as do all the
> Arabic OT fonts that I've
> worked on for other clients in the past few years (e.g. Adobe Arabic).
> The mechanism for enabling such ligation is easy to
> implement: it is a flag in the glyph
> substitution lookup that determines whether ALL, NONE or some
> subset of marks is
> processed. The flag should normally be set to NONE for
> ligature lookups. However, the
> second stage of the implementation is less easy, because it
> requires providing dynamic
> mark positioning data for each underlying component in the
> ligature. In the case of the
> lam_alif ligature, the lam is the first component and the
> alif the second. Such mark
> positioning is fiddly and time consuming.
> The other approach to this issue, of course, is Tom Milo's:
> don't use any ligatures (I
> mean this in the technical sense of a single glyph
> representing more than one character).
> John Hudson
> Tiro Typeworks www.tiro.com
> Gulf Islands, BC firstname.lastname@example.org
> We say our understanding measures how things are,
> and likewise our perception, since that is how we
> find our way around, but in fact these do not measure.
> They are measured. -- Aristotle, Metaphysics
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