Hiding parts of slurs to avoid collisions with other objects

Curved objects like slurs sometimes collide with key signatures, time signatures, accidentals, or other objects.

This snippet shows how to hide the part of the slur that is colliding with something.

\version "2.24.0"

%% http://lsr.di.unimi.it/LSR/Item?id=613


#(define (parse-grob-sym grob-sym)
   (let* ((grob-str (symbol->string grob-sym))
          (dot-index (string-index grob-str #\.))
          (context (if dot-index
                       (string-take grob-str dot-index)
                       "Voice"))
          (grob (if dot-index
                    (substring grob-str (+ dot-index 1))
                    grob-str)))
     (cons context grob)))

hideCurvesFrom =
#(define-music-function
   (grob-sym x-padding y-padding)
   (symbol? pair? pair?)
   (let* ((context (car (parse-grob-sym grob-sym)))
          (top-grob (cdr (parse-grob-sym grob-sym))))
     #{
        \override Tie.layer = #-2
        \override Slur.layer = #-2
        \override PhrasingSlur.layer = #-2
        \override $context . $top-grob . avoid-slur = #'ignore
        \override $context . $top-grob . layer = #-1
        \override $context . $top-grob . stencil =
        #(lambda (grob)
           (define (get-stil-proc alist)
             ;; removes the first 'stencil entry from the grob-property
             ;; alist, implicitly created by this callback:
             ;;   (stencil . #<procedure #f (grob)>)
             ;; and then retrieves the original 'stencil procedure.
             (let ((stil-proc (ly:assoc-get 'stencil alist)))
               (if (procedure-name stil-proc)
                   stil-proc
                   (begin (set! alist (assoc-remove! alist 'stencil))
                          (get-stil-proc alist)))))
           (let* ((basic-props (ly:grob-basic-properties grob))
                  (stil-proc (get-stil-proc basic-props))
                  (this-stil (stil-proc grob)))
              (if (ly:stencil? this-stil)
                  (let* ((stil-x-ext (ly:stencil-extent this-stil 0))
                         (stil-y-ext (ly:stencil-extent this-stil 1))
                         (box-x-ext (cons (- (car stil-x-ext) (car x-padding))
                                          (+ (cdr stil-x-ext) (cdr x-padding))))
                         (box-y-ext (cons (- (car stil-y-ext) (car y-padding))
                                          (+ (cdr stil-y-ext) (cdr y-padding))))
                         (box-w (- (cdr box-x-ext) (car box-x-ext)))
                         (box-h (- (cdr box-y-ext) (car box-y-ext))))
                    (ly:stencil-add
                     (ly:make-stencil
                      (list 'embedded-ps
                            (ly:format
                             (string-append "gsave\n"
                                            "currentpoint translate\n"
                                            "1 setgray\n"
                                            "~4f ~4f ~4f ~4f rectfill\n"
                                            "grestore\n")
                             (car box-x-ext)
                             (car box-y-ext)
                             box-w
                             box-h))
                      stil-x-ext
                      stil-y-ext)
                     this-stil))
                  '())))
     #}))

revertHideCurvesFrom =
#(define-music-function (grob-sym) (symbol?)
   (let ((context (car (parse-grob-sym grob-sym)))
         (top-grob (cdr (parse-grob-sym grob-sym))))
     #{
        \revert Tie.layer
        \revert Slur.layer
        \revert PhrasingSlur.layer
        \revert $context . $top-grob . avoid-slur
        \revert $context . $top-grob . layer
        \revert $context . $top-grob . stencil
     #}))

%%%%%%%%%%%%%%%%%% EXAMPLE %%%%%%%%%%%%%%%%%%

\relative c {
  \repeat volta 2 {
    
    %% syntax: \hideCurvesFrom [grob] [x-padding] [y-padding]
    
    % always call \hideCurvesFrom before the curve starts:
    \hideCurvesFrom #'Fingering #'(0.3 . 0.3) #'(0 . 0)
    \hideCurvesFrom #'Staff.KeySignature #'(0.3 . 0.3) #'(0 . 0)
    \hideCurvesFrom #'Staff.TimeSignature #'(0.3 . 0.3) #'(0 . 0)
    
    % a negative padding value can prevent whiteout near an edge:
    \hideCurvesFrom #'Staff.Clef #'(0.3 . 0.3) #'(0 . -0.5)
    
    \clef bass
    \once \override TextScript.extra-offset = #'(-8 . 0)
    e2.\(^\markup \fontsize #-1 \italic {
      tacet la \concat { 1 \super \lower #0.5 ma } volta:
    }
    d4( |
    
    \clef treble \key g \major
    c''4)^2 c2 b4~ |
    \time 3/4
    b2\) 
    % updating a previously entered command:
    \hideCurvesFrom #'Staff.Clef #'(0.3 . 0.3) #'(0 . 0)
    fis4( |
    
    \clef bass \time 2/4
    d,4)
    % always call \hideCurvesFrom before the curve starts:
    \hideCurvesFrom #'Score.VoltaBracket #'(0.3 . 0.8) #'(3 . 0)
    e4( |
  }
  
  \alternative {
    { r2 | }
    {
      % revert commands independently as needed:
      \revertHideCurvesFrom #'Score.VoltaBracket
      d2)
    }
  }
  % revert commands when no longer needed:
  \revertHideCurvesFrom #'Fingering
  \revertHideCurvesFrom #'Staff.Clef
  \revertHideCurvesFrom #'Staff.KeySignature
  \revertHideCurvesFrom #'Staff.TimeSignature
}