Re: Joining Arabic Letters

I was not speaking about ligatures like lan+alef. But really about the
contextual forms chosen from base letters (and independantly of the
diacritics applied to them, except for a few of them that use
different shapes in some combinations for these contextual joining
forms and that are encoded distinctly in the UCS to allow exactly a
difference of these contextual shapes in some joining contexts).

I have never said that the glyphs was mandatory. But the joining
behavior of each letter (independantly of whever ligatures are applied
on top of them) must be kept. So in a combination like <LAM,
diacritic, ALEF>, the joining behavior of each letter must be kept,
even if there's a mapping to a single glyph for <LAM, diacritic>, that
has itself no ligature bound with the following ALEF. In that case it
is perfectly acceptable to use a font for <LAM+diacritic> and another
for ALEF. The absence of the ligature in the first font will have no
impact on the readability of the text because the ligature is only
recommended but not mandatory for the script.

I just want to say that the encoding of a separate diacritic between
base letters that would otherwise join cleanly if using only one font
should not prevent each font to use the correct contextual form when
two fonts are used for each letter, even if these "joins" may not look
very cleanly connected. Using the non-joining letter forms at font
boundaries is not acceptable for Arabic.

Le 31 mars 2012 07:52, Asmus Freytag <asmusf_at_ix.netcom.com> a écrit :
> On 3/30/2012 5:36 PM, Philippe Verdy wrote:
>>
>> Le 30 mars 2012 20:08, Julian Bradfield<jcb+unicode_at_inf.ed.ac.uk>  a écrit
>> :
>>>
>>> On 2012-03-30, Andreas Prilop<prilop4321_at_trashmail.net>  wrote:
>>>>
>>>> I think a better idea is to have joining glyphs always even for
>>>> different typefaces. At least the Unicode Standard should say
>>>> what should happen when Arabic characters of different typefaces
>>>> follow each other.
>>>
>>> How can it? Unicode is about plain text. As soon as you start talking
>>> about different typefaces, you're out of scope.
>>
>> Not really. Even if there is only one typeface involved, the joining
>>
>> behavior of Arabic letters is normative and in scope.
>>
>>
>
> The discussion was about joining about typeface boundaries, which is
> nonsense, of course.
>
> In order to make characters "join", the glyphs for each have to be designed
> to allow
> such "joining". In cases where the join results in a ligature, it's patently
> obvious that you
> can't have a typeface boundary in the middle of a ligature....
>
> Now there's always something that renderers could do to provide fall-back
> solutions.
> For example, they could see whether one or the other typeface has the full
> ligature
> and arbitrarily move the boundaries of the typeface runs. For a "mandatory"
> ligature
> like "lam-alif" that might almost be reasonable. (Just as fallback rendering
> of diacritics
> is somewhat reasonable).
>
> However, I rather have layout engines that work really well in sensible
> cases, then tryiing
> to cover weird situations ("ransom notes"). that don't (or shouldn't) occur
> in practice.
>
> That said, some aspects of script rendering are of course in scope for the
> Unicode Standard.
>
> The natural scope for Unicode derives from character identity.
>
> Characters are encoded to represent certain entities in text. For characters
> that are
> members of scripts this means that there is an understood relation between
> character
> sequences and words (or fragments of words) in a given writing system that
> is supported
> by that script.
>
> If the lam alif ligature is "mandatory," that tells the user that the
> character sequence for
> this is expected to be <lam, alif> with no joiner character between the two
> characters,
> nor the use of any dedicated character code for the ligature.
>
> The same goes for general joining behavior - for Arabic the default is
> described in
> the Standard, so that users know when to add ZWJ or ZWNJ for override.
>
> And so on...
>
> However, it's out of scope for Unicode to mandate anything about how to
> treat "defective"
> font bindings - Julian got that right.
>
> A./
Received on Sat Mar 31 2012 - 01:36:11 CDT