Modified slur stencil with added markup

[Urs Liska]

Hi,

I modified the snippet 
http://lilypond.org/doc/v2.19/Documentation/snippets/text#text-center-text-below-hairpin-dynamics 
to create a slur with added centered text. It took me little effort to 
adapt the snippet to work with a slur instead of a hairpin but I have 
two problems I didn't manage to solve on my own. Here's the code:

\version "2.19.82"

annotatedSlur =
#(define-music-function (padding text) ((number? 1) markup?)
   #{
     \once \override Slur.after-line-breaking =
     #(lambda (grob)
        (let*
         ((stencil (ly:slur::print grob))
          (dir (ly:grob-property grob 'direction))
          (markup-stencil (grob-interpret-markup grob text))
          (new-stencil
           (ly:stencil-aligned-to
            (ly:stencil-combine-at-edge
             (ly:stencil-aligned-to stencil X CENTER)
             Y dir
             (ly:stencil-aligned-to markup-stencil X CENTER)
             padding)
            X LEFT)))
         (ly:grob-set-property! grob 'stencil new-stencil)))
   #})

{
  \annotatedSlur
  %\markup \score { c''' }
  \markup "hin."
  c''4 ( g' g' c'' )
}

The first problem is that the new stencil is incorrectly aligned 
horizontally to the note heads. The *whole* new object is now aligned to 
the left edge of the parent note column (rather than the center of the 
note head). I might get back to some old code to figure out how to 
retrieve the necessary data to calculate an offset of half the notehead, 
but before going down that road I'd like to know if there's not an 
easier way to combine two stencils while keeping one of them (the slur) 
exactly where it was before.

The alignment problem is especially problematic when the "markup" is 
wider than the slur.


The second problem is that this function doesn't increase the new 
stencil's notion of its Y-extent. Concretely: this doesn't push the 
whole thing downward. You can see this from the attached image showing 
how the added \markup \score is cut off at the upper paper edge.

I tried to calculate the height of the markup and add it to the 
stencil's Y-extent, but this didn't work. I can see why this won't have 
any effect in after-line-breaking, but is there any way to make the 
modified slur aware of its modified height? Is there a way to calculate 
the markup's dimensions already in the music-function stage?

I had no luck with that. I succeeded to retrieve the markup's extent by 
inserting another callback before-line-breaking, but I failed to 
essentially *adding that height to the original slur's Y-extent. (Which 
somewhat makes sense since that information may not be readily available 
yet at that stage ...)

Any suggestions?

Thanks
Urs

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