Re: Font selection algorithm?

[Robert Siemer <Robert Siemer-i18n backsla sh>]

Allow me to pick up this thread again:

On 01/21/2010 03:17 AM, Behdad Esfahbod wrote:
> On 01/19/2010 07:52 PM, Robert Siemer wrote:
> > On 01/19/2010 04:18 AM, Behdad Esfahbod wrote:
> > > On 01/18/2010 12:50 AM, Robert Siemer wrote:
> >
> > > > It is really hard to improve the configuration (through fontconfig),
> > > > without knowing which font request pattern to intercept. -- Even in a
> > > > specific app with FC_DEBUG set it is hard fish the right request chain.
> > >
> > > Something like:
> > >
> > > $ fc-match --sort "sans:lang=fa"
> > >
> > > is all you need to check.
> >
> > So Pango does at most one fontconfig request for each given text?
>
> One per language.

Assume that the application gives no hints: What is the algorithm to 
derive “languages” from a text? (Just the current locale language, or 
something more?)

What happens when out-of-language characters pop up?

Is it like the following:  ?

1) Pango gets a (untagged, no language) text to layout
2) Pango launches a sans:lang=locale request and gets a list from fontconfig
3) every character covered in the first font from the result list is 
laid out
4a) if a character comes up that is i) not covered but ii) is considered 
the same language is tried to layout with the next font from the list 
(recursive)
4b) if a char is i) not covered and ii) is not considered the same 
language: that results in a new lookup like “sans:lang=new_language” and 
a covering font is searched in the new result list  (*)
5) Pango falls back to the very first font from the original result list 
as soon a a character from the initial (locale derived) language is hit
...and so on...


(*) Does Pango still use the infamous lang=“xx” requests in case it 
can’t derive a language?
Fontconfig has to be adapted to always return a “unicode-complete” font 
at the bottom of the list to avoid boxes to be drawn. (Pango does never 
ask for specific glyphs, but languages, right?)


Regards,
Robert


> > So to
> > avoid missing characters it requires a corresponding fontconfig
> > configuration, and does not launch adapted requests for e.g.
> > "sans:lang=someotherlang" or "sans:charset=..."?
>
> It does for lang, not charset.
>
>
> > In the example above: is the "sans" the default in Pango if not
> > overwritten in the app? Or is the default the empty pattern?
>
> I think sans is the default, yes.
>
> > In a standard gtk/gnome app, the ":lang=..." part comes from the current
> > locale or is it text driven?
>
> Defaults to locale, but the app can override (eg, firefox passes the web page
> language).
>
> behdad
>
> > Thanks again,
> > Robert
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