Re: A slur across a line break with change of clef in another staff generates bad output

[Thomas Morley]

Hi Lucas,

2018-03-04 0:03 GMT+01:00 Lucas Werkmeister <address@hidden>:
> Hi everyone,
>
> I stumbled across a bug where Lilypond generates a bad slur if the slur
> crosses a line break and there is a simultaneous clef change in a different
> staff. I found a previous report for this bug in the mailing list archives
> (2015-05-30, same subject), but with Google Code being archived and
> SourceForge currently in Disaster Recovery mode, I’m not sure if the bug is
> currently tracked anywhere (or even fixed in the development version), so I
> thought I could still post my tiny(ish) example along with a workaround, and
> perhaps it would be helpful to someone :)

It's
https://sourceforge.net/p/testlilyissues/issues/3287/
not fixed yet

>
> Example (you can replace the 7 with other numbers), with workaround
> commented out:
>
> \version "2.18.2"
> <<
>   \new Staff {
>     s1*7 |
>     % \temporary \alterBroken control-points #'(((0.3 . 1.5) (1.3 . 2.3)
> (2.5 . 2.3) (3.5 . 1.5)) ((3.3 . 1.5) (4.3 . 2.3) (5.5 . 2.3) (6.5 . 1.5)))
> Slur
>     r2.. d''8( | \break
>     % \revert Slur control-points
>     c'')
>   }
>   \new Staff {
>     \clef "violin"
>     s1*7 s1 \clef "bass"
>   }
>>>
>
> The control points aren’t perfect,
> since I couldn’t figure out how to get the automatically chosen control
> points (for the second half of the broken slur, we could just reuse these,
> since only the first half is bad, and the first half could be based on them
> as well), so I had to try to match them by eyeballing.

For this purpose the shape-command was developed, thanks David Nalesnik.
Though, you may try to set minimum-length instead, directly or via \alterBroken.
In my eyes it looks already nice then.

All three possibilities demonstrated below (comment in/out to test):

\version "2.18.2"

<<
  \new Staff {
    s1*7 |
    r2..

    %% (1) the \shape command
    \shape
      #'(
         ;; change cps of first part
         ((1.8 . 0)(1.8 . 0.8)(1 . 0.8)(1 . 0))
         ;; don't change cps of second part, thus provide an empty list
         ()
         )
      Slur

    %% (2) override 'minimum-length
    %\once \override Slur.minimum-length = 6

    %% (3) \alterBroken with 'minimum-length
    %\alterBroken minimum-length #'(6 0) Slur
    d''8( | \break
    c'')
  }
  \new Staff {
    \clef "violin"
    s1*7 s1 \clef "bass"
  }
>>

> Also, this
> unfortunately doesn’t work if you want double slurs, since AFAIK it’s not
> possible to set the control points in that case. But still, it’s better
> than nothing IMHO.


For shaping double-slurs the following may help (I used some extreme
values for the show-effect):

\version "2.18.2"

%% c/p from shape-function in music-functions-init.ly
#(define (shape-curve offsets grob coords)
  (let* ((orig (ly:grob-original grob))
         (siblings (if (ly:spanner? grob)
                       (ly:spanner-broken-into orig) '()))
         (total-found (length siblings)))
    (define (offset-control-points offsets)
      (if (null? offsets)
          coords
          (map coord-translate coords offsets)))

    (define (helper sibs offs)
      (if (pair? offs)
          (if (eq? (car sibs) grob)
              (offset-control-points (car offs))
              (helper (cdr sibs) (cdr offs)))
          coords))

    ;; we work with lists of lists
    (if (or (null? offsets)
            (not (list? (car offsets))))
        (set! offsets (list offsets)))

    (if (>= total-found 2)
        (helper siblings offsets)
        (offset-control-points (car offsets)))))

#(define (shape-double-slurs offsets)
  (lambda (grob)
    (let* ((dir (ly:grob-property grob 'direction)))
      (ly:grob-set-property! grob 'control-points '())
      (ly:grob-set-property! grob 'after-line-breaking
        (lambda (grob)
          (let ((cps (ly:slur::calc-control-points grob)))
            (ly:grob-set-property! grob 'control-points
              (if (> dir 0)
                  (shape-curve (car offsets) grob cps)
                  (shape-curve (cdr offsets) grob cps)))))))))

shapeDoubleSlurs =
#(define-music-function (parser location offsets)(pair?)
#{
  \once \override Slur.before-line-breaking = #(shape-double-slurs offsets)
#})


\score {
  \new Staff {
    \set doubleSlurs = ##t
    \shapeDoubleSlurs
      #'(
         ;; upside Slur
         (
           ((0 . 0) (14 . 10)(-14 . 10)(0 . 0))
         )

         . ;; n.b it's a pair

         ;; downside Slur
         (
           ((0 . 0) (0 . -6)(-20 . -6)(0 . 0))
         )
         )
    c'1( c''2 c'')
    \shapeDoubleSlurs
      #'(
         ;; upside Slur
         (
           ;; first part
           ((0 . 0) (-8 . 2)(8 . 2)(-2 . 0))
           ;; second part
           ((6 . 0) (0 . 0)(0 . 0)(-6 . 0))
         )

         . ;; n.b it's a pair

         ;; downside Slur
         (
           ;; first part
           ((0 . 0) (20 . -4)(-20 . -4)(0 . 0))
           ;; second part
           ((0 . 0) (13 . -3)(-10 . -4)(0 . 0))
         )
         )
    c'1( c''2 c''
    \break
    c''2 c'')
  }
  \layout { line-width = 60 }
}


HTH,
  Harm