[HarfBuzz] Ligatures and color changes

[Khaled Hosny]

On Tue, Feb 19, 2013 at 05:34:40PM -0500, Behdad Esfahbod wrote:
> > Is there maybe a way to tell HarfBuzz to ignore ligatures if they span that
> > color boundary? Or is there maybe a way to (quickly) assess if "liga" would be
> > applied to a range of characters?
> 
> We don't have a good answer for this right now.  The way I want to eventually
> fix this in Pango is different: it is to pain the ligature glyph half in each
> color.  I think you can do the same using <canvas>.  Just use a gradient with
> a sharp color switch for the ligature.  It's a royal pain, but I think that's
> the most desirable rendering.  I may be wrong.

Gecko is (was?) using clipping to partially draw each part of the
ligature:
http://robert.ocallahan.org/2006/10/partial-ligatures_24.html

> As for *where* to cut the ligature, here's what you need:
> 
>   * Count the number of cursor positions *inside* the ligature.  For the "fi"
> ligature it's one.  And we have one cursor position before the ligature, so in
> this case we need to cut it in two pieces,
> 
>   * The common heuristic then is to cut the advance width of the ligature
> (well, cluster really) into two equal pieces.  If you want to be fancy, you
> can call hb_ot_layout_get_ligature_carets(), and if the number of carets
> matches what you expect (1 in this case I believe?), you can use the returned
> caret positions instead of equally dividing the ligature.  I haven't seen
> anyone implementing this though, as it gives very marginal improvements over
> the heuristic.

It can make quite some different with some Arabic ligatures, but few
fonts implement it because few (no?) engines support it :)

Regards,
Khaled