Re: Create custom arpeggio-like engraver

[Jean Abou Samra]

Le 17/11/2021 à 20:50, Dimitris Marinakis a écrit :
> I need a generic engraver that works similar to the default arpeggio 
> one but I don't want to override the arpeggio engraver or any other 
> default ones in case I need to have multiple events simultaneously.
>
> I saw a similar thread with a custom arpeggio stencil that had both up 
> and down arrows but I need to keep this engraver its own thing in this 
> case.
>
> I thought about searching inside the lilypond internal files to see 
> how the default arpeggio spanner works but if someone else can help me 
> through this I'd really appreciate it.
>
> Essentially, I'm interested in the ability of the arpeggio spanner to 
> calculate its length based on the distance of the outer notes. I will 
> replace the line stencil with something that will scale up/stretch 
> depending on the vertical length of that spanner.
>
> Sorry if my terminology is wrong (engraver, spanner etc.)


How about this?

\version "2.22.1"

#(define-event-class 'custom-arpeggio-event 'music-event)

#(define (define-event! type properties)
   (set-object-property! type
                         'music-description
                         (cdr (assq 'description properties)))
   (set! properties (assoc-set! properties 'name type))
   (set! properties (assq-remove! properties 'description))
   (hashq-set! music-name-to-property-table type properties)
   (set! music-descriptions
         (sort (cons (cons type properties)
                     music-descriptions)
               alist<?)))

#(define-event!
  'CustomArpeggioEvent
  '((types . (custom-arpeggio-event post-event event))
    (description . "")))

#(set-object-property! 'CustomArpeggio 'is-grob? #t)
#(set-object-property! 'CustomArpeggio 'translation-type? 
ly:grob-properties?)

\layout {
  \context {
    \Global
    \grobdescriptions #(acons 'CustomArpeggio
                              (assq-ref all-grob-descriptions 'Arpeggio)
                              all-grob-descriptions)
  }
}

#(ly:register-translator
  (lambda (context)
    (let ((arpeggio-event #f)
          (arpeggio #f))
      (make-engraver
        (listeners
          ((custom-arpeggio-event engraver event)
             (set! arpeggio-event event)))
        ((process-music engraver)
           (if arpeggio-event
               (set! arpeggio
                     (ly:engraver-make-grob engraver 'CustomArpeggio 
arpeggio-event))))
        (acknowledgers
          ((rhythmic-head-interface engraver grob source-engraver)
             (if arpeggio
                 (ly:pointer-group-interface::add-grob arpeggio 
'side-support-elements grob)))
          ((stem-interface engraver grob source-engraver)
             (if arpeggio
                 (begin
                   (ly:grob-set-parent! arpeggio Y grob)
                   (ly:pointer-group-interface::add-grob arpeggio 
'stems grob))))
          ((note-column-interface engraver grob source-engraver)
             (if arpeggio
                 (ly:pointer-group-interface::add-grob grob 
'conditional-elements arpeggio)))
          ((arpeggio-interface engraver grob source-engraver)
             ; NB ==============================================
             ; Place custom arpeggios on the left of normal ones.
             ; If this is not what you want, flip 'arpeggio' and
             ; 'grob' below.
             ; =================================================
             (ly:pointer-group-interface::add-grob arpeggio 
'side-support-elements grob)))
        ((stop-translation-timestep engraver)
           (set! arpeggio-event #f)
           (set! arpeggio #f)))))
  'Custom_arpeggio_engraver
  '())

\layout {
  \context {
    \Voice
    \consists Custom_arpeggio_engraver
  }
}

customArpeggio = #(make-music 'CustomArpeggioEvent)

{
  <c' c''>\customArpeggio
  \override CustomArpeggio.color = red
  <c' c''>\arpeggio\customArpeggio
}


The engraver is adapted from the default Arpeggio_engraver,
of which the C++ code can be found at
https://gitlab.com/lilypond/lilypond/-/blob/master/lily/arpeggio-engraver.cc
For information on Scheme engravers, see
https://extending-lilypond.readthedocs.io/en/latest/translation.html

To get the vertical extent of the CustomArpeggio in
a stencil callback, read the 'positions property. For
example:

\score {
  \layout {
    #(layout-set-staff-size 30)
  }
  {
    \override CustomArpeggio.stencil =
      #(lambda (grob)
         (let ((positions (interval-scale (ly:grob-property grob 
'positions)
(ly:staff-symbol-staff-space grob))))
           (ly:stencil-translate-axis
            (grob-interpret-markup
             grob
             (make-draw-squiggle-line-markup
              0.5
              (cons 0 (interval-length positions))
              #t))
            (car positions)
            Y)))
    <c c''>\arpeggio\customArpeggio
  }
}

Best,
Jean