Customizing individual staff lines

This snippet is a near-exact copy of snippet #700 (Coloring individual staff lines).

It allows you to provide separate functions for drawing each staff line. #f will draw the staff line normally.

\version "2.24.0"

%%http://lsr.di.unimi.it/LSR/Item?id=878

#(define-public ((custom-staff-lines . rest) grob)

   (define (index-cell cell dir)
     (if (equal? dir RIGHT)
         (cdr cell)
         (car cell)))

   (define (index-set-cell! x dir val)
     (case dir
       ((-1) (set-car! x val))
       ((1) (set-cdr! x val))))

   (let* ((common (ly:grob-system grob))
          (span-points '(0 . 0))
          (thickness (* (ly:grob-property grob 'thickness 1.0)
                        (ly:output-def-lookup (ly:grob-layout grob) 'line-thickness)))
          (width (ly:grob-property grob 'width))
          (line-positions (ly:grob-property grob 'line-positions))
          (staff-space (ly:grob-property grob 'staff-space 1))
          (line-stencil #f)
          (total-lines empty-stencil)
          ;; use a local copy of list, since stencil creation mutates list
          (functions rest))

     (for-each
      (lambda (dir)
        (if (and (= dir RIGHT)
                 (number? width))
            (set-cdr! span-points width)
            (let* ((bound (ly:spanner-bound grob dir))
                   (bound-ext (ly:grob-extent bound bound X)))
              
              (index-set-cell! span-points dir
                               (ly:grob-relative-coordinate bound common X))
              (if (and (not (ly:item-break-dir bound))
                       (not (interval-empty? bound-ext)))
                  (index-set-cell! span-points dir 
                                   (+ (index-cell span-points dir)
                                      (index-cell bound-ext dir))))))
        (index-set-cell! span-points dir (- (index-cell span-points dir)
                                            (* dir thickness 0.5))))
      (list LEFT RIGHT))

     (set! span-points
           (coord-translate span-points
                            (- (ly:grob-relative-coordinate grob common X))))
     (set! line-stencil
           (make-line-stencil thickness (car span-points) 0 (cdr span-points) 0))

     (if (pair? line-positions)
         (for-each (lambda (position)
                     (let ((func (if (pair? functions)
                                      (car functions)
                                      #f)))
                       (set! total-lines
                             (ly:stencil-add
                              total-lines
                              (ly:stencil-translate-axis
                               (if (procedure? func)
                                  (func grob thickness (car span-points) (cdr span-points))
                                  line-stencil)
                               (* position staff-space 0.5) Y)))))
                   line-positions)       
         (let* ((line-count (ly:grob-property grob 'line-count 5))
                (height (* (1- line-count) (/ staff-space 2))))
           (do ((i 0 (1+ i)))                      
               ((= i line-count))
             (let ((func (if (and (pair? functions)
                                   (> (length functions) i))
                              (list-ref functions i)
                              #f)))
               (set! total-lines (ly:stencil-add
                                  total-lines
                                  (ly:stencil-translate-axis
                                     (if (procedure? func)
                                        (func grob thickness (car span-points) (cdr span-points))
                                        line-stencil)
                                   (- height (* i staff-space)) Y)))))))
     total-lines))

% Example using markup paths
#(define (wavy-line grob thickness x1 x2)
   (grob-interpret-markup grob
     (markup #:path 0.1
            (let ((w (- x2 x1)))
              (list (list 'moveto 0 0)
                    (list 'curveto (* w 0.4) 1 (* w 0.6) -1 w 0))))))

\new Staff {
  c'1
  \stopStaff
  \override Staff.StaffSymbol.stencil = #(custom-staff-lines wavy-line wavy-line #f wavy-line wavy-line)
  \startStaff
  c'1
  \stopStaff
  \revert Staff.StaffSymbol.stencil
  \startStaff
  c'1
}