How to pass an ApplyOutputEvent to a music function? (Vertical bracket indicating which hand should play)

[Valentin Villenave]

Greetings everybody,

(Neil: please forgive me for directly Ccing you, but your input is
always of such great help to me :)

I'm trying to address a question that's sometimes asked on the list:
http://lists.gnu.org/archive/html/lilypond-user/2008-08/msg00903.html

Basically, here's what I've been able to come up with: it's a dirty
hack around the Fingering object.

lhBracket =
#(define-music-function (parser location music) (ly:music?)
  (let ((current-staff-position 0))
   ; this shouldn't be needed!!!
   (set! current-staff-position 1)
   (make-music 'ApplyOutputEvent
    'origin location
    'context-type 'Voice
    'procedure
    (lambda (grob grob-origin context)
     (let ((staff-pos (ly:grob-property grob 'staff-position)))
      (if (number? staff-pos)
       (set! current-staff-position staff-pos)))))
   #{ \once \set fingeringOrientations = #'(left)
   $(add-bracket current-staff-position music "l.h.")
   $music #}))

#(define (add-bracket pos music text)
  (if
   (equal? (ly:music-property music 'name) 'EventChord)
   (let ((note (car (ly:music-property music 'elements))))
    (set! (ly:music-property note 'articulations)
     (append (ly:music-property note 'articulations)
      (list
       (let ((obj (make-music 'FingeringEvent))
             (bracket-length (- (* pos -0.5) 7))
             (v-space (/ (+ 7 pos) 7.0)))
        (set! (ly:music-property obj 'tweaks)
         (acons 'self-alignment-Y 1
          (acons 'text (markup #:normal-text #:fontsize 3
                        #:center-column (
                          #:postscript (format #f "
                         .5 0 -.5 0 lineto
                         -.5 0 -.5 ~a lineto
                         stroke" bracket-length)
                          #:vspace v-space
                          #:with-dimensions '(0 . 0) '(0 . 0)
                          #:center-align text
                        ))
           (ly:music-property obj 'tweaks))))
        obj))))
  )))

{
  \lhBracket
  c''1
  d'
  % This bracket shouldn't be too long!
  \lhBracket
  c'
}

It works well, but the bracket's length isn't computed automatically.
This would depend on the notehead's staff-position, which I can
retrieve using an ApplyOutputEvent, but I suspect by then the
music-function has already been evaluated, and therefore the
current-staff-position property isn't updated.

(Of course, the bracket length algorithm should also be improved and
the vspace hack is dirty as hell, but you get the picture.)

I suspect a solution would be to code a whole engraver in Scheme, but
I still am kinda new to this :-(

I hope someone can help!

Cheers,
Valentin

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