[XeTeX] On combining diacritics again

[Jonathan Kew]

On 19 Jan 2006, at 3:39 pm, Malte Rosenau wrote:

>
> Unicode-based TeX for Mac OS X <xetex at tug.org> schrieb am 19.01.06  
> 11:17:59:
>> Hello,I have read in another thread (subject: "misplaced combining  
>> diacritical marks") about the way XeTeX and Cocoa (differently)  
>> handle diacritics stacking. The suggestion there is to use a font  
>> like Doulos SIL, which includes features for stacking arbitrary  
>> diacritics signs that work in XeTeX.
>> The same problem (XeTeX overlaps, instead of stacking, diacritics  
>> in pdf output) occurs when combining, say, U+0117 (Latin Small  
>> Letter e with dot above) with U+0301 (Combining Acute Accent). The  
>> output is fine with Doulos SIL, but not with other fonts (e.g.  
>> Gentium). I was wondering whether  the reason for this behaviour  
>> still depends on the text rendering engine, since in this case I  
>> am not stacking two "combining accents", but rather a "letter"  
>> with an accent (I guess U+0117 should be considered as a unique  
>> symbol, not a combination of 'e' with an accent).
>
> No, I think it's the font. Gentium has no OpenType specific  
> features, while Doulos has loads. I'm still trying to figure out  
> how this actually works, as I'd love to reencode one of my coptic  
> fonts to the latest unicode standard, where diacritics like U+0301  
> or U+304 are used as accents.

It's true, to some extent, that it depends on the text rendering  
engine, in the sense that the Cocoa TextView (as used in TextEdit,  
TeXShop's editor window, etc) will stack diacritics automatically,  
without any specific help from the font. The positioning of the  
diacritics may not be ideal, however; it's often more widely spaced  
than a type designer would prefer.

XeTeX doesn't do this; it *only* performs diacritic positioning (etc)  
in accordance with font tables. Gentium doesn't currently include  
OpenType tables, and so diacritics won't stack; Charis SIL and Doulos  
SIL do have OpenType positioning tables, and so diacritics will stack  
properly.

JK