Merging simultaneous voices into shared note heads

2026-08-03T12:36:59Z

Mkouhia: Created page with "This snippet defines <code>\combineVoices</code>, a music function that takes any number of rhythmically identical voices written inside <code><< ... >></code> and merges them into a single voice under one stem. Wherever two or more of the voices share the same pitch at a given moment, only one note head is printed; wherever the pitches differ, a chord containing all of the distinct pitches is printed instead. This differs from the default functionality, where double no..."

This snippet defines <code>\combineVoices</code>, a music function that takes
any number of rhythmically identical voices written inside
<code><< ... >></code> and merges them into a single voice under one stem.
Wherever two or more of the voices share the same pitch at a given
moment, only one note head is printed; wherever the pitches differ, a
chord containing all of the distinct pitches is printed instead.

This differs from the default functionality, where double note heads are printed in case of shared pitches.

<lilypond version="2.24">
% Return a list 0, 1, ..., n - 1.
#(define (int-range n)
(let loop ((i 0) (acc '()))
(if (= i n)
(reverse acc)
(loop (+ i 1) (cons i acc)))))

% Drill through any single-child wrapper node (as introduced by, e.g.,
% \relative) until a music object with a real, non-empty 'elements
% list is reached, and return that list.
#(define (music-elements m)
(let ((elts (ly:music-property m 'elements)))
(if (and (list? elts) (pair? elts))
elts
(let ((child (ly:music-property m 'element)))
(if (ly:music? child)
(music-elements child)
'())))))

% Return the NoteEvents contained in a single moment of music. A
% plain note is a bare NoteEvent; an explicit chord <...> is an
% EventChord wrapping one NoteEvent per pitch. Rests and skips
% contain no pitch and contribute nothing.
#(define (note-events moment)
(cond
((eq? (ly:music-property moment 'name) 'NoteEvent)
(list moment))
((eq? (ly:music-property moment 'name) 'EventChord)
(filter (lambda (e) (eq? (ly:music-property e 'name) 'NoteEvent))
(ly:music-property moment 'elements)))
(else '())))

% A comparable key for a pitch, since pitch objects do not compare
% correctly with equal?.
#(define (pitch-key p)
(list (ly:pitch-notename p) (ly:pitch-alteration p) (ly:pitch-octave p)))

% Merge one moment's worth of notes, taken from several voices, into
% a single EventChord with duplicate pitches removed.
#(define (merge-chords moments)
(let ((seen '())
(notes '()))
(for-each
(lambda (moment)
(for-each
(lambda (note)
(let ((key (pitch-key (ly:music-property note 'pitch))))
(if (not (member key seen))
(begin
(set! seen (cons key seen))
(set! notes (cons note notes))))))
(note-events moment)))
moments)
(if (null? notes)
(car moments) ; every voice rests here: keep a rest
(make-music 'EventChord 'elements (reverse notes)))))

combineVoices =
#(define-music-function (music) (ly:music?)
(let* ((voices (ly:music-property music 'elements))
(seqs (map music-elements voices))
(lens (map length seqs)))
(make-music 'SequentialMusic 'elements
(map (lambda (i) (merge-chords (map (lambda (s) (list-ref s i)) seqs)))
(int-range (apply min lens))))))

\new Voice \combineVoices <<
\relative { c''4 f8 d e16 f g8 d4 }
\relative { c''4 d8 b c16 d e8 b4 }
>>
</lilypond>

[[Category:Simultaneous notes]]
[[Category:Chords]]
[[Category:Snippet]]

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Merging simultaneous voices into shared note heads