Altering the shape of a default slur with a list of offsets (revised)

If you want to adjust the shape of a slur, one way is to specify a new list of control-points. This function offers a different approach by allowing you to adjust existing control-points. This is done by entering a list of offsets (in staff spaces) to the default coordinates:

\shapeSlur #'(x1 y1 x2 y2 x3 y3 x4 y4)

Setting a value to 0 leaves the default coordinate unchanged, so it is easy to maintain a slur's attachment-points while changing its interior.

This snippet is a revised version, by David Nalesnik, of the original snippet.

\version "2.24"

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% http://lists.gnu.org/archive/html/lilypond-user/2007-08/msg00539.html
% http://lists.gnu.org/archive/html/lilypond-user/2009-09/msg00495.html
% By Dmytro O. Redchuk, incorporating a function written by David Nalesnik
% Thanks, Neil, for your help!!

#(define ((alter-curve offsets) grob)
   ;; get default control-points
   (let ((coords (ly:slur::calc-control-points grob)))
     ;; add offsets to default coordinates
     (define (add-offsets coords offsets)
       (if (null? coords)
       '()
       (cons
         (cons (+ (caar coords) (car offsets))
               (+ (cdar coords) (cadr offsets)))
         (add-offsets (cdr coords) (cddr offsets)))))

     (if (null? offsets)
         coords
         (add-offsets coords offsets))))


#(define ((shape-slur offsets) grob)
   (let* (
          ;; have we been split?
          (orig (ly:grob-original grob))
          ;; if yes, get the split pieces (our siblings)
          (siblings (if (ly:grob? orig)
                        (ly:spanner-broken-into orig) '() ))
          (total-found (length siblings)))
     (if (>= total-found 2)
         ;; shape BROKEN
         ;; walk through siblings, find index in list
         ;; and apply offsets from list of offsets:
         (let loop ((n 0))
                   (if (eq? (list-ref siblings n) grob)
                       ;; return altered:
                       ((alter-curve (list-ref offsets n)) grob)
                       (if (< n total-found)
                           (loop (1+ n))
                           ;; end of list -- none found?!
                           ;; return defaults:
                           ((alter-curve '()) grob))))
         ;;
         ;; shape UNBROKEN
         ((alter-curve offsets) grob))))

shapeSlur =
#(define-music-function (offsets)
                        (list?)
  #{
    \once \override Slur.control-points = #(shape-slur offsets)
  #})
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% for version "2.17.97"

\markup {
  \underline "Default Slurs"
}

\relative c'' {
  d4( d' b g g,8 f' e d c2)
  \bar "||"
  d4( d' b g
  \break
  g,8 f' e d c2)
}

\markup {
  \underline "Modified"
}

\relative c'' {
  \shapeSlur #'(0 -2.5 -1 3.5 0 0 0 -2.5)
  d4( d' b g g,8 f' e d c2)
  \bar "||"
  
  %% first "piece" only is modified; second piece is left alone
  %% including () means "do not touch" and has the same effect as (0 0 0 0 0 0 0 0)
  \shapeSlur #'((0 -2.5 0 1.5 0 1 0 -1) ())
  d4(^"(1st half only)" d' b g
  \break
  g,8 f' e d c2)
  \bar "||"
  
  %% borh halves of the slur are modified
  \shapeSlur #'((0 -2.5 0 1.5 0 1 0 -1) (1 2 0 1 0 1 0 0))
  d4(^"(both halves)" d' b g
  \break
  g,8 f' e d c2)
}  

\paper {
  indent = 0
  ragged-right = ##t
}

\header {
  tagline = ""
}