Re: Ligatures (was: Unify the Platforms: Cairo+FreeType+Harfbuzz Everywhere (except TTY))

[Clément Pit-Claudel]

On 18/05/2020 13.39, Eli Zaretskii wrote:
>> From: Clément Pit-Claudel <address@hidden>
>> Date: Mon, 18 May 2020 13:31:30 -0400
>>
>> I've attached a screenshot with a few examples, though I couldn't find a 
>> font that displays ae as æ.
> 
> Thanks.  Once again, I wonder how they decide where each parts starts
> and ends.  The examples show very simple cases, so it's hard to know
> where this ends.

Hi Eli,

I asked on Firefox' Matrix server.  Here is a lightly edited transcript:

cpitclaudel> Hi all.  I noticed that Firefox has this nifty feature that makes 
it possible to move the cursor within a ligature (for example, with the right 
font config, "ffi" can be rendered as "ffi" while allowing the cursor to move 
between the individual glyphs that make up that composition).   Is the 
extraction of ligature information and the rendering done by Firefox itself, or 
by a lower-level library?  Most font shaping libraries I've seen don't seem to 
return glyph-decomposition information for ligatures, so I'm curious to 
understand how Firefox does it ^^

jfkthame> Firefox uses harfbuzz to handle the font shaping (ligature rules, 
etc). I'd expect what you describe to work pretty much the same in other 
browsers too, fwiw.
krosylight

cpitclaudel> Thanks! But Harfbuzz doesn't give sub-glyph information for 
ligatures, does it? So how does Firefox know where to put the caret when it 
moves through a ligature?

jfkthame> it doesn't, really - it just knows how many underlying characters are 
represented by the ligature glyph, and divides the advance width up into that 
many slices (usually that works pretty reasonably, but it's possible to come up 
with fonts where the inaccuracy becomes obvious)

jfkthame> In principle, OpenType fonts can provide specific positions for the 
caret within a ligature (see the LigatureCaretList subtable within the GDEF 
table), but in practice that's rarely supported or used (harfbuzz can provide 
this information if it's present, see the hb_ot_layout_get_ligature_carets 
function, but currently firefox doesn't use it anyhow)

cpitclaudel> Thanks, that's very useful! How does that work for glyphs like 
"fs" displayed as ẞ or "fz" displayed as ß? Does Firefox move in that single 
glyph? (I couldn't find a font that does that, otherwise I'd have tested it ^^) 
Thanks a lot for your help :)

jfkthame> Yes, it'd be the same - doesn't matter what the specific characters 
are, if there's a ligature of two characters Firefox would put the caret 
half-way through the ligature glyph when it is between the component characters 
in the underlying text

jfkthame> btw, if you're on a mac (or have access to one), you can see an 
extreme case if you try the word "Zapfino" in the font Zapfino .... the entire 
word is a single 7-character ligature, and the seven equal slices that Firefox 
treats it as for selection/editing purposes don't match up to the visual shapes 
of the sub-glyphs at all well

HTH,
Clément.