[XeTeX] On combining diacritics again

Jonathan Kew jonathan_kew at sil.org
Wed Jan 25 12:52:24 CET 2006

On 25 Jan 2006, at 7:57 am, Adam Twardoch wrote:
> The nice property of the OpenType format is that you can have both  
> types of layout tables in one font (OTL and AAT). Such fonts are  
> rather tricky to build but it can be done. With such a font, you  
> can get e.g. Arabic shaping in Cocoa applications using AAT and in  
> Mellel or InDesign ME using OTL, all from the same font.

We've done this in the Doulos SIL and Charis SIL fonts (supporting  
extended Latin and Cyrillic character sets), for example. (The  
features available in these are primarily related to minority  
language support--e.g., language-specific variations in glyph shapes  
or rendering behavior--rather than typographic sophistication, however.)

> Jonathan was on the SIL team that built the free SIL Arabic fonts  
> (http://scripts.sil.org/ArabicFonts ). They have chosen to build  
> two separate fonts for each design, one in OTL and one in AAT. This  
> had surely some practical reasons, one of them being that the AAT  
> font needed more precomposed glyphs due to the lack of mark  
> attachment support in AAT. But I guess one could theoretically take  
> the glyphset and the features of the AAT font and basically just  
> add the OTL features of the other one, and it would principally  
> work (the redundant precomposed mark-to-base glyphs would not be  
> used by the OTL code that would use dynamic positioning instead).

We were intending to ship a single multi-technology font for each  
design, as with the Latin fonts, but made separate OT and AAT  
versions at the last moment. This was purely because Apple shipped OS  
X 10.4 with the ATSUI bug that preferred OT tables over AAT, even  
though it didn't really know how to use the Arabic OT features. Under  
10.4 - 10.4.2, such a combined font would not work in AAT-based apps  
because the presence of the OT tables masks the AAT behavior required  
for proper display.

Now that 10.4.3 and later corrects this issue, I expect we'll go back  
to building a single font with both layout technologies. This  
promotes easier document portability, in that the "same" font (known  
by a single name) will work in multiple apps and platforms... e.g, MS  
Word on Windows, OpenOffice.org (anywhere), TextEdit on Mac OS X,  
etc.; then a document formatted using "Scheherazade" will display  
properly in any of these environments.


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