Re: Extracting approximate outlines from FT ?

[Werner LEMBERG]

Hello Han-Wen,


> 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?

Looking up the internet, I found these documents that might be of help
to implement the handler otherwise:

  https://pomax.github.io/bezierinfo/
  https://raphlinus.github.io/graphics/curves/2019/12/23/flatten-quadbez.html

BTW, chances are good that Raph Levien's algorithm gets extended to
third-order Beziers during a FreeType GSoC project this year :-) [*]


    Werner


[*] I think he has already done the math but not coded it.