Re: Extracting approximate outlines from FT ?

[Han-Wen Nienhuys]

On Fri, Apr 24, 2020 at 5:54 PM Han-Wen Nienhuys <address@hidden> wrote:
>
> On Fri, Apr 17, 2020 at 7:56 PM Werner LEMBERG <address@hidden> wrote:
> > > I've been looking into performance, and one big source of slowness
> > > is calculating skylines. I found that we calculate exact skylines
> > > based on glyph shapes for some symbols.
> > >
> > > For example, for the F clef, we compute an outline of 23 and 20
> > > segments, based on sampling the exact bezier curves in the glyph.
> > > This is silly (because the F clef is obscured by the staff symbol),
> > > but even if it weren't, it seems extremely expensive overkill.  Can
> > > you recommend a mechanism by which we can get a linear approximation
> > > of the curve directly out of freetype?
> >
> > FreeType doesn't offer this, sorry – at least not in a form that would
> > be usable for LilyPond IMHO.  You could use `FT_Outline_Decompose` and
> > register your own handler for simplifying Bezier arcs – given the
> > well-behaving of font outlines (at least in Feta), maybe the direct
> > use of control points are sufficient, possibly doing one or at most
> > two de Casteljau subdivisions?
>
> Good idea. I'll keep it in mind. Other question: is there a standard
> wrt to outline direction? If I can assume that external outlines go in
> clockwise order, I could skip left-pointing path elements when
> creating the upwards skyline, cutting the amount of work in half.
> Also, is there is way to detect internal curves (eg. the inner curve
> of the O glyph?).

In https://docs.microsoft.com/en-us/typography/opentype/spec/ttch01 it
is explained that outside curves go clockwise. Does that hold for
other font types too? (PFA, OTF?)

-- 
Han-Wen Nienhuys - address@hidden - http://www.xs4all.nl/~hanwen