[HarfBuzz] Different results when shaping sub-sections of text

[Jamie Dale]

Thanks for the info. I actually started working with the extracted cluster
information right before you sent this message, although that was to
minimise the shaping requests I need to make by extracting the info (where
possible) from the shaped data for the entire run of text.

I'll admit that colour only was a bad example, but aside from also being
able to change the font or font size, our rich-text can also contain
completely user-defined widgets. This can make extracting out the style
information... tricky, since I don't really know how it's being used (and
may actually be part of a nested control, such as a button or hyperlink).

Rich-text itself is actually a secondary concern right now, my primary
concern is selection highlighting (which uses a similar mechanism, as text
is broken into runs where it is selected, since selection can change the
text colour). That said, selection isn't allowed to change the font used so
I can more easily combine the selected and non-selected text into a single
shape, however I'm still unsure how ligatures would be handled in that case.

I'll use English for simplicity since I can actually read it. Imagine I
have the text "Magnificent", where the "fi" has been combined into a
ligature. If I were to select "Magnif", then in order to change the colour
of that portion of the text, the ligature would have to be split. This
doesn't present a readability issue for English, but would it present
issues for other languages?

-Jamie.

On 6 October 2015 at 22:45, Khaled Hosny <khaledhosny at eglug.org> wrote:

> On Tue, Oct 06, 2015 at 08:08:00PM +0100, Jamie Dale wrote:
> > I suspect that the first shape has used some ligatures, and the second
> > shape was unable to do that due to being unable to combine the glyphs (I
> > have previously seen this with the "fi" ligature in English).
> >
> > If both of these forms are considered acceptable, then I'm happy enough,
>
> Shaping parts of text separately is generally a bad idea as you lose any
> OpenType interaction between these parts, so you only do it when it is
> absolutely necessary (e.g. due to font change). Though your second image
> is still barely legible, it loses all the contextual substitutions
> specified in the font and gives a very suboptimal result, but it can
> make the text illegible in many other cases, for example when shaping
> "لا". I expect Indic scripts to suffer more legibility-wise.
>
> The proper way it to identify rich-text attributes that shouldn’t break
> shaping (color, underline, overline, etc.) and apply them after shaping,
> using cluster values to do the reverse glyph to character index mapping
> (while at it, use HB_BUFFER_CLUSTER_LEVEL_MONOTONE_CHARACTERS so that
> you get more finer cluster mapping).
>
> Regards,
> Khaled
>
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