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 Post subject: Automatic glyph substitution for Arabic text
PostPosted: Sun Apr 03, 2011 9:21 am 
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Joined: Tue Oct 12, 2010 3:35 pm
Posts: 7
Is there an algorithm spec that defines the rules of substitution for Arabic characters (and possibly from other scripts). You know, which character becomes which depending on position in words.

I've been reading the TTF spec and according to it this kind of info should be in the the font's metamorphosis table but after looking at the fonts in my windows\fonts dir I saw that only one font had such table (a CJK one or something) so I thought it must be in the unicode spec.

Thanks!


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 Post subject: Re: Automatic glyph substitution for Arabic text
PostPosted: Tue Apr 05, 2011 11:18 am 
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Joined: Mon Feb 01, 2010 6:18 pm
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Well, first of all, characters don't become other characters, there are only different glyph variants. The positional forms are fairly well documented in most beginning Arabic texts - even Webster's Dictionary - although I believe there are complexities involved with some Arabic styles, like Nastaliq. I believe the OpenType features are <INIT>, <MEDI>, and <FINA>, and you can map the glyphs you want for the positional substitutions.

Just to make it clear: characters do not change into other characters depending on position. When you create a typeface, you need to create positional forms for basically every Arabic character, and then use OpenType substitution to have the positional forms selected.

This brings us to the crux of your question, the answer to which is fairly mundane: Unicode does not specify the different forms of a character, as the Standard is not prescriptive for ANY character, let alone the Arabic blocks. The introduction to the Arabic script in Chapter 8 does contain a fairly comprehensive list of example positional forms, but again, this is a descriptive list, not prescriptive.

-Van


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 Post subject: Re: Automatic glyph substitution for Arabic text
PostPosted: Tue Apr 05, 2011 5:51 pm 
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Joined: Tue Dec 01, 2009 2:49 pm
Posts: 189
The full layout of Arabic will result in complicated mixture of glyph substitution, ligature formation and glyph positioning. The ultimate details of that definitely depend on the font (and/or the layout engine). The basic principles that Van alluded to are just that: basic.


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 Post subject: Re: Automatic glyph substitution for Arabic text
PostPosted: Wed Apr 06, 2011 1:04 am 
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Joined: Mon Feb 01, 2010 6:18 pm
Posts: 79
Thank you, Asmus. I had completely left out all the Lam-Alif ligatures, and these are essential to every Arabic style I have ever seen - admittedly not nearly as comprehensive to someone who can read and speak Arabic/Urdu/&c.

I think the greatest takeaway from this whole thing, Mr. Bumblebee, is that, as Asmus said, we can only give you a very cursory introduction, and the only information you will find in the Standard is a more detailed cursory introduction - with pictures! In order to truly design an Arabic typeface, you need to do some hefty research on a scribal traidition, such as Kufi, Thuluth, Diwani, Naskh, Ruqaa, Maghrebi, &c. By the time you can create a legible typeface, all the questions about positional forms and ligatures will be far behind you.

Best of luck,
Van


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 Post subject: Re: Automatic glyph substitution for Arabic text
PostPosted: Wed Apr 06, 2011 2:42 pm 
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Joined: Tue Oct 12, 2010 3:35 pm
Posts: 7
Thanks a lot, guys!

Well, well. I'm generally interested for two reasons:
  1. I've made an ebook reader for java mobile (a very restricted platform) and would like to include support for more scripts. at the moment it doesn't support bidi, but supports hyphenation for various languages
  2. I'm thinking of making a TTF engine for the next version of my reader so I really needed to know whose responsibility the glyph substitution would be.

Interestingly enough, arialuni.ttf does contain the mentioned table tags: init, medi, fina liga. Unfortunately, they seem to be a part of the OpenType spec, not the TrueType one. And I was wishing for going away with something simpler like TTF's metamorphosis table!

Again, thank you for you help. I highly appreciate your attitude!

Regards,
Svetlin


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 Post subject: Re: Automatic glyph substitution for Arabic text
PostPosted: Wed Apr 06, 2011 4:17 pm 
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Joined: Tue Dec 01, 2009 2:49 pm
Posts: 189
Arabic and other complex scripts require opentype. It's as simple as that.


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 Post subject: Re: Automatic glyph substitution for Arabic text
PostPosted: Thu Apr 07, 2011 2:48 pm 
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Joined: Sat Dec 04, 2010 10:25 pm
Posts: 4
asmus wrote:
Arabic and other complex scripts require opentype. It's as simple as that.

ahem... or SIL Graphite, or Apple AAT, or any of a number of other technologies that support complex script. But you knew that ;)

dumbledore wrote:
And I was wishing for going away with something simpler like TTF's metamorphosis table!

I suspect what you are referring to is Apple's MORT table which is part of the Apple Advanced Typography (AAT) system. Historically, then, the metamorphosis table is not actually part of TTF, but is part of Apple's extension to TTF for advanced typography, just like OpenType and SIL Graphite are both extensions to TTF for advanced typography.

And some might suggest that metamorphosis table isn't "simpler" the OpenType :)

Bob


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