[HarfBuzz] Setting initial cluster value

[Kelvin Ma]

It’s only a character in a conceptual sense, the text is really a list of
chars and objects like

['t', 'h', 'i', 's', ' ', {FONT_POSITIVE: 'bold'}, 'i', 's', ' ', 'b', 'o',
'l', 'd', 'e', 'd', {FONT_NEGATIVE: 'bold'}, ' ', 't', 'e', 'x', 't']

These types of characters are all over a Knockout document; they can create
fractions, formulas, radicals, page numbers, etc. (They can also be nested
but let’s not get into that). They can take up horizontal space and they
get set into the line just like any Harfbuzz glyph output, though the
fontstyle chars have a width of zero by default (but they can be set to
have width to do useful stuff like italic correction)

[image: Inline image 1]

^ See the small pink triangles? Those are the font characters; they can be
selected, typed, and deleted just like any other character. (The square
root is also another object-character, but one that takes up horizontal
space.)

I may be able to replace all the object-chars with spaces to make a big
paragraph string that could be passed into the shaper on an index basis,
though the mere presence of a space character would probably ruin the
cross-run arabic shaping. Alternatively I could just strip the
object-chars, but that would destroy the cluster values which would make
editing impossible. 🙃

On Sat, Jun 25, 2016 at 4:27 PM, Behdad Esfahbod <behdad.esfahbod at gmail.com>
wrote:

> On Jun 25, 2016 12:33 PM, <kelvinsthirteen at gmail.com> wrote:
> >
> >
> >
> > > On Jun 25, 2016, at 1:39 PM, Khaled Hosny <khaledhosny at eglug.org>
> wrote:
> > >
> > > On Sat, Jun 25, 2016 at 01:06:27PM -0400, Kelvin Ma wrote:
> > >>>>>> Don’t you
> > >>>>> need
> > >>>>>> context to be ignored if the boundaries of the text you want to
> shape
> > >>>>> fall
> > >>>>>> inside a cluster? Like in the string 'af[fluency s]tate' where
> only
> > >>> the
> > >>>>>> 'fluency s' is supposed to be shaped?
> > >>>>>
> > >>>>> Depends on why you are shaping “fluency s” alone, if it is because
> of,
> > >>>>> say, font change, then you need HarfBuzz to know the context
> otherwise
> > >>>>> you get broken Arabic shaping.
> > >>>>
> > >>>> Well font change would produce a separate run that wouldn’t know
> about
> > >>> the
> > >>>> other runs so context can only be within a same-direction,
> same-font run.
> > >>>
> > >>> This is wrong, font change shouldn’t break Arabic shaping, so you
> have
> > >>> to pass the context even in this case.
> > >>>
> > >>
> > >> If the text consists of text strings separated by formating objects,
> each
> > >> text string doesn’t know about what’s around it. Because that’s at a
> much
> > >> higher level in the code and harfbuzz can only handle a single font
> in a
> > >> single run at a time. To artificially jam in the neighboring runs for
> each
> > >> shaping attempt would involve an inordinate amount of string
> concatenation
> > >> and searching on the fly.
> > >
> > > One can always fix his code to not do wrong assumptions. When doing
> text
> > > layout you always need the full paragraph, and you should have it
> around
> > > after itemisation. Itemisation does not have to be done by splitting
> > > text, you can just store run start indices and lengths.
> >
> > No, meaning font styling is created by inline styling objects. They’re
> like inline images except they have zero width. So a font change is really
> stored as a special character in between the two sections. This character
> is not understood by harfbuzz, which is why it does not make sense to pass
> anything containing it into the shaper.
>
> That's your design's limitation.  You still can fix it by using custom
> Unicode funcs with HarfBuzz, that returns a "default-ignorable" Unicode
> property for your placeholder codepoints.  I just checked and it wouldn't
> work right now; I'll fix that.  What placeholder character do you use?  Can
> you change that?
> >
> > >
> > > Regards,
> > > Khaled
> > _______________________________________________
> > HarfBuzz mailing list
> > HarfBuzz at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/harfbuzz
>
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