[MPEG-OTSPEC] Some research on TT instructions and cubics

Behdad Esfahbod behdad at behdad.org
Fri Sep 22 17:41:49 CEST 2023

On Fri, Sep 22, 2023 at 6:45 AM Jonathan Kew <jfkthame at gmail.com> wrote:

> On 22/09/2023 13:01, Skef Iterum wrote:
> > How one cubic control point should move in a given case can
> > greatly depend on the position of the other control point. In general
> > the "behavior" of cubics is more complex than that of quadratics --
> > they're stranger creatures.
> The question of how cubic control points should behave may be trickier,
> yes -- I haven't tried to look into this. I think the existing TrueType
> algorithm would indeed handle them (as described above), but whether
> this would be a useful result is unclear.

It's not that unclear.  If the control points fall within the geometric
range of the two consecutive touched points, then the whole operation would
just be a geometric scale, with similar results for cubics and quadratics.

The other cases result in possibly one cubic control point moving *with*
one of the touched points. While this is more general than the quadratic
case, type-design tools show us that this is a valid and useful operation
and does *not* make the cubic go crazy or anything.

In general, because all Beziers are contained within the convex hull of
their control box, I don't see this as a problem. Of course, to be
experimented with.

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