Re: Changing slur behavior

[Aaron Hill]

On 2020-03-26 2:24 am, Kevin Barry wrote:
> I think the right way to do what you are trying is to write an
> engraver that listens for slur events and alters the spacing of the
> music. (I would write one if I could, but I'm not at that level yet.)

Firstly, know that slurs are not the only way to define melismata.  You 
can instruct LilyPond to have a single lyric syllable span multiple 
notes using \melisma and \melismaEnd:

%%%%
\version "2.20.0"

\fixed c' { b4( g a2) | b4\melisma g a2\melismaEnd }
\addlyrics { one two }
%%%%

This does not solve the spacing aspect of the original post, but it 
shows how you can avoid messing about with slurs.

Assuming you really want to use slurs to adjust spacing, here might be 
one way to do it:

%%%%
\version "2.20.0"

Slur_spacing_engraver =
#(lambda (context)
  (let ((slur-event-dir #f))
    (define (change-spacing dir)
      (ly:broadcast
        (ly:context-event-source context)
        (ly:make-stream-event
          (ly:make-event-class 'spacing-section-event)
          '(())))
      (let ((score-context (ly:context-find context 'Score)))
        (for-each
          (lambda (args)
            (apply ly:context-pushpop-property
              score-context 'SpacingSpanner
              (if (< 0 dir) (drop-right args 1) args)))
          `((base-shortest-duration ,(ly:make-moment 8))
            (shortest-duration-space 1)
            (spacing-increment 1)))))
    (make-engraver
      ((start-translation-timestep engraver)
        (set! slur-event-dir #f))
      (listeners
        ((slur-event engraver event)
          (let ((dir (ly:event-property event 'span-direction #f)))
            (set! slur-event-dir dir))))
      ((process-music engraver)
        (if (ly:dir? slur-event-dir)
          (change-spacing slur-event-dir))))))

<< \context Voice = "melody" \with {
     \consists "Melody_engraver"
     \override Stem.neutral-direction = #'()
     \consists \Slur_spacing_engraver
     \omit Slur
   } \fixed c' {
     | b4 f8 g16 c' a2
     | b4( f8 g16 c' a2)
     | b4 f8 g16 c' a2
   }
   \context Lyrics \lyricsto "melody" {
     | "normal spacing" _ _ _ _
     | "tightly-spaced"
     | "normal spacing" _ _ _ _
   } >>
%%%%

The engraver listens for slur-events, injecting a spacing-section-event 
and adjusting some context properties as needed.  The logic is very 
simple, and there are no safety checks.  Be warned this likely could 
fail with more complicated music.


-- Aaron Hill

slur-spacing.cropped.png
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