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change clef inside a grob-callback?

From: Mark Polesky
Subject: change clef inside a grob-callback?
Date: Thu, 16 Jul 2009 19:10:15 -0700 (PDT)

Is there a way to change the clef from within a grob callback?

I'm trying to create an auto-clef-change function that will find the
average staff-position within a chord and change the clef if needed. But
I'm a little stuck. I found a way to calculate each chord's average
staff-position, but it's within a grob-callback. And I can't figure out
how to change the clef from there, or if it's even possible. So for the
moment, I'm having the function display the would-be clef decisions in
the console.

Maybe an entirely different approach is needed -- I don't know.
Can anyone look this over and point me in a helpful direction?

Thanks so much!
- Mark

\version "2.13.2"

#(define (first-note? note-grob)
   (let* ((note-column     (ly:grob-parent note-grob X))
          (note-grob-array (ly:grob-object note-column 'note-heads))
          (first-note      (ly:grob-array-ref note-grob-array 0)))
    (eq? note-grob first-note)))

#(define (get-avg-staff-pos note-grob)
   (let* ((note-column     (ly:grob-parent note-grob X))
          (note-grob-array (ly:grob-object note-column 'note-heads))
          (note-count      (ly:grob-array-length note-grob-array)))

     ;; maybe there should be a ly:grob-array-map procedure...
     (let loop ((staff-pos-sum 0) (i 0))
       (if (>= i note-count)
           (/ staff-pos-sum note-count)
           (loop (+ staff-pos-sum
                      (ly:grob-array-ref note-grob-array i)))
                 (1+ i))))))

autoClefOn =
  \override NoteHead #'before-line-breaking =
    #(lambda (grob)
       (if (first-note? grob)
           (if (< (get-avg-staff-pos grob) -6)
               (display "bass\n")
               (display "treble\n"))))

autoClefOff =
  \revert NoteHead #'before-line-breaking

\relative {
  b <g' b d> c, <g b d>


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