[XeTeX] weird kerning with GFS Didot's italic face

[Jonathan Kew]

On 6 Jul 2006, at 10:35 pm, Ralf Stubner wrote:

> Jonathan Kew <jonathan_kew at sil.org> writes:
>
>> Fontspec doesn't seem to have a predefined option for this (how about
>> it, Will?!), but you can declare one; I think the form would be
>> something like this (untested):
>>
>> 	\newfontfeature{NoKerning}{-kern}
>> 	\setromanfont[NoKerning]{GFS Didot}
>
> Actually fontspec supports 'Kerning=Off'.

Sorry, I missed that. Oh, I see, I was looking at an old version of  
the doc.... oops!

> Since this would be
> counterproductive for all but the italic shape in this case, it is
> proably best to use 'ItalicFeatures={Kerning=Off}'. Of course, a  
> *little
> bit* of kerning for [VW][eo] would be nice, hence switching the  
> kerning
> off completly is no real solution.
>
> Related question: The 'kern' feature defaults to 'on'. Are there other
> features that are on by default? Where is that defined?

The current default list is ccmp, liga, clig, kern, palt, mark, mkmk.  
(For Latin and other "default-behavior" scripts, that is; the complex  
Indic and Middle-eastern scripts have their own special shaping  
engines that apply script-specific features as needed for proper  
rendering, and don't currently support user overrides.)

The defaults are set in the ICU source code; see  
OpenTypeLayoutEngine.cpp. The standard ICU layout engine just  
supports a couple of options (kerning, ligatures, both, or neither);  
xetex extends this to allow any individual feature tag to be  
activated or deactivated.

JK