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

[Lucas Werkmeister]

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, [1]), 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 :)

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"
  }
>>

Screenshots of the results are at [2]. 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. 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 [3]. But still, it’s better than nothing IMHO.

Hope this helps,
Lucas

[1]: https://lists.gnu.org/archive/html/lilypond-user/2015-05/msg00817.html
[2]: https://imgur.com/a/Pd4dm
[3]: http://lilypond.org/doc/v2.18/Documentation/notation/modifying-shapes.en.html#Known-issues-and-warnings-76