Re: Two arpeggio lines (up and down) next to each other

[David Kastrup]

Lukas-Fabian Moser <lfm@gmx.de> writes:

>> Jean Abou Samra knew how to fix that behaviour of arpeggios, so
>> here's a (I hope) working solution:
>
> It seems we can continue this game of "LilyPond experts explain how to
> do things and I only have to post the resulting, ever simpler
> examples" for some time. :-)
>
> Harm pointed out in
> https://gitlab.com/lilypond/lilypond/-/issues/6193#note_701181244 that
> it suffices to do:
>
> \version "2.22"
>
> arpeggioUpDown = {
>   \override Arpeggio.stencil =
>   #(lambda (grob)
>      (let ((one #f) (two #f) (combined-stencil #f))
>        (ly:grob-set-property! grob 'arpeggio-direction UP)
>        (set! one (ly:arpeggio::print grob))
>        (ly:grob-set-property! grob 'arpeggio-direction DOWN)
>        (set! two (ly:arpeggio::print grob))
>        (set! combined-stencil
>              (ly:stencil-add
>               one
>               (ly:stencil-translate-axis two 1.25 X)))
>        (ly:grob-set-property!
>         grob 'X-extent
>         (ly:stencil-extent combined-stencil X))
>        (ly:pointer-group-interface::add-grob
>         (ly:grob-parent grob X) 'elements grob)
>        combined-stencil))
>   \override Arpeggio.direction = #RIGHT
> }

set! is very unidiomatic Scheme.  You could use
(let* ((one (begin (ly:grob-set-property! ...) (ly:arpeggio::print grob)))
      ((two ...

but I don't even know whether there is a guarantee that
ly:arpeggio::print is happy with the grob being modified after the call
and before the stencil is getting used.  Looking at the C++ code, that
appears to be the case.  Maybe giving ly:arpeggio::print an optional
overriding direction argument could be cleaner?  It doesn't help with
creating user-level code for current LilyPond versions, of course.

-- 
David Kastrup