[HarfBuzz] HarfBuzz 1.0 API; the message you were hoping would never come

[Jonathan Kew]

On 3/1/14 11:28, James Clark wrote:
> On Fri, Jan 3, 2014 at 5:07 PM, Jonathan Kew <jfkthame at googlemail.com
> <mailto:jfkthame at googlemail.com>> wrote:
>
>
>           In order to do fallback,
>         you need to do character to glyph mapping.
>
>
>     Not necessarily. You need to know the character repertoire supported
>     by the font, but you may not need to actually map to glyphs. In
>     Firefox, for instance, font fallback is done based on a per-font
>     *bit* map of supported Unicode codepoints. So at the font fallback
>     stage, we know whether the character is present, but do not map it
>     to a glyph.
>
>
> Isn't this introducing an additional step that's not strictly necessary?
> Presumably the bitmap is constructed from the font's cmap (unless you
> use the OS/2 ulUnicodeCodeRange*, which wouldn't be 100% accurate).

Yes, it's constructed from the cmap; but it allows us to maintain 
internally-cached character coverage data for all the available fonts 
(often hundreds or even thousands of them) in a -much- more compact form 
than keeping whole cmap tables around.

(Somewhat OT: In the context of a web browser, there's also the CSS 
@font-face unicode-range descriptor to consider [though it's not yet 
supported in FF]: we may not actually want to use a given font resource 
for all the characters that its cmap covers. Treating character coverage 
for font-matching purposes as something distinct from actual cmap tables 
may be helpful here.)

But obviously, this isn't the only possible approach.

JK