Re: How Firefox shows characters missing in font

[David De La Harpe Golden]

Lennart Borgman wrote:
> We discussed how to show characters missing in a font a while ago.
> Here is an example of how Firefox handles it. It shows four hex digits
> in a square.


Note that the SIL Unicode BMP fallback font (consisting of a bunch of 
squares with numbers in) is AFAIK free. The apple one with an icon per 
script is gratis, but I dunno about its license.

http://scripts.sil.org/CMS/scripts/page.php?site_id=nrsi&item_id=UnicodeBMPFallbackFont
http://www.unicode.org/policies/lastresortfont_eula.html
http://en.wikipedia.org/wiki/Fallback_fon

Trouble is, if you install the SIL fallback font as an ordinary font,
it seems to sometimes "win" in emacs (and possibly elsewhere) when
it shouldn't. And of course emacs doesn't _know_ it's a fallback.
If fallback fonts are supported, they'd need to be special cased in the 
emacs code or OS code or both, and emacs IMO would best expose that some 
glyphs are supplied are known fallbacks, not real glyphs. i.e.  C-u C-x 
= 's "display:" should  say "no font available, using fallback font 
(glyph code) ...", not "by this font (glyph code) ..." without 
indicating it was a fallback choice.

I guess on MacOSX specifically the LastResort font is already 
special-cased if you use native apis, but I have no idea what emacs on 
ns makes of it.

Fallback fonts are of course only one approach to achieve the effect 
(with the advantage users who prefer a per-script style could use drop 
in a font with the per-script style of fallback glyph instead of the 
numbers-in-boxes style as they saw fit) - alternatively, the hex values 
could be drawn into the boxes at the emacs or toolkit level like the 
boxes currently are, I think firefox uses that approach rather than  a 
fallback font.

Actually, emacs could of course allow options where it (a) draws empty 
boxes as current, (b) draws in hex digits itself, or (c) allows use of a 
user-defined/system-supplied special fallback font, of course. Someone 
"just" needs to write the code to do all that...