Center text below hairpin dynamics

This example provides a function to typeset a hairpin (de)crescendo with some additional text below it, such as “molto” or “poco”. The added text will change the direction according to the direction of the hairpin. The Hairpin is aligned to a DynamicText grob.

The example also illustrates how to modify the way an object is normally printed, using some Scheme code.

\version "2.24"

hairpinWithCenteredText =
#(define-music-function (text) (markup?)
  #{
    \once \override Voice.Hairpin.after-line-breaking = 
      #(lambda (grob)
        (let* ((stencil (ly:hairpin::print grob))
               (par-y (ly:grob-parent grob Y))
               (dir (ly:grob-property par-y 'direction))
               (staff-line-thickness
                 (ly:output-def-lookup (ly:grob-layout grob) 'line-thickness))
               (new-stencil (ly:stencil-aligned-to
                 (ly:stencil-combine-at-edge
                   (ly:stencil-aligned-to stencil X CENTER)
                   Y dir
                   (ly:stencil-aligned-to
                     (grob-interpret-markup
                       grob
                       (make-fontsize-markup
                         (magnification->font-size
                           (+ (ly:staff-symbol-staff-space grob)
                              (/ staff-line-thickness 2)))
                           text)) X CENTER))
                 X LEFT))
               (staff-space (ly:output-def-lookup
                 (ly:grob-layout grob) 'staff-space))
               (par-x (ly:grob-parent grob X))
               (dyn-text (grob::has-interface par-x 'dynamic-text-interface))
               (dyn-text-stencil-x-length
                 (if dyn-text
                   (interval-length 
                     (ly:stencil-extent (ly:grob-property par-x 'stencil) X))
                   0))
               (x-shift 
                 (if dyn-text 
                   (- 
                     (+ staff-space dyn-text-stencil-x-length)
                     (* 0.5 staff-line-thickness)) 0)))
  
        (ly:grob-set-property! grob 'Y-offset 0)
        (ly:grob-set-property! grob 'stencil 
           (ly:stencil-translate-axis
            new-stencil
            x-shift X))))
  #})

hairpinMolto = 
\hairpinWithCenteredText \markup { \italic molto }

hairpinMore = 
\hairpinWithCenteredText \markup { \larger moltissimo }

\relative c' {
  \hairpinMolto
  c2\< c\f
  \hairpinMore
  c2\ppppp\< c\f
  \break
  \hairpinMolto
  c2^\< c\f
  \hairpinMore
  c2\ppppp\< c\f
}