[Fontconfig] Monospace detection

[jerrycasiano at gmail.com]

On my system out of 7099 fonts only 975 contain Panose information.

I don't think processing this info would have much of an impact anyway
but just something to keep in mind.


On Thu, 2019-05-30 at 18:25 +0900, Akira TAGOH wrote:
> Hi,
> 
> We have two values in spacing property to consider if a font is
> monospaced or not. FC_MONO and FC_DUAL.  Right now fontconfig decides
> it when:
> 
> - single advance value available in a font -> FC_MONO
> - two advance values available in a font and larger value is twice of
> smaller value -> FC_DUAL
> 
> This code worked for fonts which has smaller language coverage and
> satisfy older Unicode specifications that non-Latin characters
> assumes
> double-widths or so.
> 
> Nowadays, various glyphs are included in even monospaced fonts which
> doesn't meet the above criteria (i.e. Noto Mono). it may be a time to
> update and improve this.
> 
> What do you think?
> 
> I'm not sure what approach is better though, the idea that comes to
> my
> mind is to refer sFamilyClass and Panose values in OS2 table. if
> there
> are any better idea or opposed to this, please let me know.
> 
> BTW given that we decide it like that, it isn't DUAL anymore. need a
> good name for that too perhaps, but anyway.
> 
> Regards,