Re: How do I do a transition between a grouped and a non-grouped part (example attached)

[Jean Abou Samra]

Le 30/07/2021 à 14:46, Daniel Benjamin Miller a écrit :
> Hi all,
>
> I want to replicate this feature, often found in manually-engraved 
> vocal scores. Although the feature I am talking about should be 
> obvious from the images, I will attempt to describe it. Basically, the 
> staff group starts partway through a system, as we switch from a solo 
> to a choral part. If necessary, another staff appears (filled with 
> rests as appropriate).
>
> I know it is possible to insert a [ bar manually (although I cannot 
> quite figure out how to do this in a way that looks like a staff group 
> across staffs). Since the staff grouping needs to continue in the 
> following systems, and since I don't want to manually break my score, 
> I don't think that's a reasonable option either.
>
> The solution I have been trying to work out essentially involves 
> trying to having the systems all be part of the staff group, but to 
> suppress the engraving of the staff group bracket where appropriate 
> (or something like that?). Still, it's proven to be a bit too 
> intricate for me.
>
> Has anyone else done this? I know there are some of you who have 
> greater expertise, and I'm probably not the first person to at least 
> try to include this element in a score.
>
> All the best,
> Daniel


Hi Daniel,

Too bad that I can't find the snippet again -- anyway, here is a
remixed version of a hacky piece of code I once wrote for the
French list:

\version "2.22.1"

#(define add-bracket
   (grob-transformer 'stencil
     (lambda (grob original)
       (let* ((vag (ly:grob-object grob 'axis-group-parent-Y))
              (elts (ly:grob-array->list
                      (ly:grob-object vag 'elements)))
              (system-start-delim
                (filter
                  (lambda (g)
                    (and (grob::has-interface g
'system-start-delimiter-interface)
                         (not (eq? 'SystemStartBar (grob::name g)))))
                  elts)))
         (if (eqv? 1 (length system-start-delim))
             (ly:stencil-combine-at-edge
               original
               X
               LEFT
               (ly:grob-property (first system-start-delim)
                                 'stencil)
               (ly:grob-property grob 'padding))
             (ly:warning "should have 1 delimiter, found ~a"
                         (length system-start-delim)))))))

forceBar =
\context Timing \applyContext
  #(lambda (context)
     (if (null? (ly:context-property context 'whichBar))
         (set! (ly:context-property context 'whichBar)
               "")))

midStaffBracket = {
  \forceBar
  \once \override StaffGroup.SpanBar.stencil = #add-bracket
}

\layout {
  \context {
    \StaffGroup
    % Padding added between the bracket and the bar in case they
    % are both present.
    \override SpanBar.padding = 0.8
  }
}



\new StaffGroup \with {
  %% Change the number of systems with no bracket at the beginning.
  \alterBroken transparent #(make-list 2 #t) SystemStartBracket
}
<<
  \new Staff {
    \repeat unfold 20 c'1
    c2 \midStaffBracket c2 \midStaffBracket
    \repeat unfold 79 c'1
  }
  \new Staff \repeat unfold 100 c'1
>>



(From a development perspective...)

To enable doing this more properly, one would have to start out
by making system start delimiters items rather than spanners. The
use of \alterBroken is not super natural here, it's tripped up
other before... Last time I looked, the problem was that in
theory you can always take the X-extent and X-offset of an
item before line breaking, and system start braces (as opposed
to brackets) have a horizontal extent that depends on the amount
of space between staves. So I wasn't very sure about the approach,
but probably this could be picked up.


Cheers,
Jean