[XeTeX] Problem with Deja Vu Sans fonts

[Kenneth Reid Beesley]

Date: Tue, 9 Mar 2010 10:39:47 +0100
From: Peter Dyballa <Peter_Dyballa at Web.DE>

<snip>
Does Vietnamese have your multi-accented characters?

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Vietnamese orthography is a notable challenge for rendering multiple  
diacritics on the same letter.

As a simpler example, in the de facto standard orthography for Hopi,  
spoken in Arizona, USA, there are glyphs involving

1.  o  (or uppercase O)
2.  dieresis
3.  and an acute, grave or circumflex accent

The dieresis should be rendered above the o (or O), and the acute or  
grave or circumflex should be rendered above the dieresis.

E.g. in Unicode you could have the following sequence of characters

U+006F		LATIN SMALL LETTER O
U+0308		COMBINING DIAERESIS
U_0301		COMBINING ACUTE ACCENT

and it should be rendered correctly (o, with dieresis above, with an  
acute accent stacked above the dieresis).    Unicode does include a  
single character with the o+dieresis, and the following sequence of  
characters is equivalent and should be rendered the same by a good  
rendering engine.

U+00F6		LATIN SMALL LETTER O WITH DIAERESIS
U+0301		COMBINING ACUTE ACCENT

Unicode does not provide a single letter for o, with dieresis, with an  
acute, grave or circumflex accent.

When typesetting Hopi in XeTeX, I get good results with these  
sequences using the Doulos SIL font.  In gvim (MacVim) I get good  
results using the DejaVu Sans Mono font.

I'm definitely not an expert in fonts, but I assume that the font  
itself should include information for each glyph about where combining  
diacritics, including stacks of combining diacritics, should be  
rendered.  And the rendering engine should be able to access and use  
that information.   In my experience, the handling of combining  
diacritics is a weak point in fonts and rendering engines, let alone  
the handling of _stacked_ combining diacritics.

Any correction or clarification would be appreciated.

Ken


******************************
Kenneth R. Beesley, D.Phil.
P.O. Box 540475
North Salt Lake, UT
84054  USA