Re: font-backend mechanism on Windows and Mac?

[Jason Rumney]

Adrian Robert wrote:
> Another possibility is the metrics implementation.  In text_extents()
> the W32 and Xft impls query the font itself for the metrics every
> time.  With Xft this may be fast if the font is client-side (and it
> does it for the whole string, not each char), but possibly under
> Windows it is not.  In Cocoa nsfont.m metrics are cached in blocks of
> 256 chars to deal with this.
This might make a difference, the old Windows code used to cache the
metrics of ASCII characters for each font.

> font.c.  (But drivers _should_ do a full-string determination like
> xftfont.c does if they can, so they wouldn't need it.)
>   
I think the lack of comments for the font backend has led us to
implement this incorrectly.
Actually, when text_extents is called with a metrics argument, nglyphs
is always 1, so there is no need to loop over all the characters, and
there is only one metric to fill in, not an array of them. In the case
where metrics is NULL, the Windows version does do a full string
determination, but it has to convert the unsigned array of characters to
a short array for the Windows API functions.