On 3/1/14 11:57, Khaled Hosny wrote:
On Fri, Jan 03, 2014 at 10:07:50AM +0000, Jonathan Kew wrote:
On 3/1/14 06:12, James Clark wrote:
On Mon, Dec 30, 2013 at 7:26 PM, Jonathan Kew <[email protected]
<mailto:[email protected]>> wrote:
There's overlap here with the process of font-matching (choosing the
font(s) to be used for a given text sequence), which is clearly out
of scope for harfbuzz. If a given Unicode character is not supported
(exactly, or via a *canonical* [de]composition) by a given font,
there are several possible outcomes: just render the font's .notdef
glyph; render some synthetic representation of the codepoint
(hexbox); render a compatibility-equivalent character/sequence, if
such exists; choose a different font.
I agree with this model. There's a general problem of what you might
call "fallback": what to do if the requested font does not have a glyph
for the requested character.
This makes me realize that I don't understand the big picture of how
this fallback process interacts with harfbuzz. 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.
One issue with that approach is you lose HarfBuzz’s ability to do
Unicode composition/decomposition for unsupported characters, which
should generally produce better resulting than font fallback. I wounder
HarfBuzz can provide an API to help clients decide whether HarfBuzz can
shape a given character with a given font or not? Or would that be too
slow for practical use?
This is an interesting point, and one that we're still thinking about.
Behdad has talked at some point about possibly having such an API, but
we've never finalized anything.
In LibreOffice we just pass the characters to HarfBuzz, and if returns a
0 glyph id for any character, font fallback is used for that one (some
times with some of the surrounding characters to make sure clusters are
rendered with the same font).
Yes, that's another possibility, though it wouldn't fit very readily
into the current Gecko text architecture. It may well be a more natural
approach for some other clients, though.
JK
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