Re: adjusting vertical positions of start and end of tie

[Thomas Morley]

Am Fr., 7. Aug. 2020 um 00:34 Uhr schrieb Werner LEMBERG <wl@gnu.org>:
>
>
> > Look at this example
> >
> >
> >   \relative c' {
> >     \clef "alto" d'2 \tweak positions #'(8 . 4) ~
> >     \clef "treble" d2
> >   }
> >
> > The `\tweak positions` doesn't work – which is kind-of expected.
> > However, for this very situation it would be nice if I could change
> > the start and end position of the slur manually.
>
> Of course, it would be even better if LilyPond could handle this
> situation automatically :-)
>
>
>     Werner

Hi Werner,

you could translate the control-points _iff_ left/right-bound's
staff-positions are not equal.

Below a proof-of-concept. Though it transforms linear, this is the
wrong math here, thus the Tie looks not very well, too late to dive
into Bezier-things...:

Anyway, here's the code:

 \relative c' {
      \override Tie.after-line-breaking =
      #(lambda (grob)
         (let* ((left-bound (ly:spanner-bound grob LEFT))
                (left-x-pos (ly:grob-property left-bound 'staff-position))
                (right-bound (ly:spanner-bound grob RIGHT))
                (right-x-pos (ly:grob-property right-bound 'staff-position))
                (cps (ly:grob-property grob 'control-points))
                (first-cp (car cps))
                (last-cp (last cps))
                (x1 (car first-cp))
                (x2 (car last-cp))
                (y1 (cdr first-cp))
                (y2 right-x-pos)
                (gradient (/ (- y2 y1) (- x2 x1)))
                (y-off (/ (- (* x1 y2) (* x2 y1)) (- x1 x2)))
                (func (lambda (x) (- y1 (+ (* gradient x) y-off)))))
           (if (not (eqv? left-x-pos right-x-pos))
              (ly:grob-set-property! grob 'control-points
                 (map
                   (lambda (cp)
                     (cons (car cp) (- (cdr cp) (func (car cp)))))
                   cps)))))

    \clef "alto" d'2~
    \clef "treble" d2
    d~ d
  }

Maybe a starting point for an automagic solution.

Cheers,
  Harm