From: John Hudson (firstname.lastname@example.org)
Date: Wed May 02 2007 - 10:18:43 CST
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.
> 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:
> 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).
-- Tiro Typeworks www.tiro.com Gulf Islands, BC email@example.com 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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