LilyPond, square notation: add shade to selected staff lines

shaded_staff_lines.ly

\version "2.24.1"

\include "gregorian.ly"

#(set-global-staff-size 32)

\layout {
  \override VaticanaStaff.StaffSymbol.color = "black"
  \override VaticanaStaff.LedgerLineSpanner.color = "black"

  \override VaticanaLyrics.LyricText.font-size = #-2
}

\paper {
  #(define fonts
     (set-global-fonts
      #:roman "Junicode"
      #:factor (/ staff-height pt 20)))
}


%% Gray shades of selected staff lines to be combined with the actual staff symbol
%% via ly:stencil-add.
%%
%% (shade-staff-lines 1 2 3 4)
%% (shade-staff-lines 1 3)
%% The numbers specify staff lines to be provided with a shade.
%% Line numbering is as in Gregorio (1 is the bottom line).
%%
%% Based on http://lsr.di.unimi.it/LSR/Item?id=700
#(define ((shade-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))
          (width (ly:grob-property grob 'width))
          (line-positions (ly:grob-property grob 'line-positions))
          (staff-space (ly:grob-property grob 'staff-space 1))
          (thickness (/ staff-space 4))
          (line-stencil #f)
          (total-lines empty-stencil)
          (shaded-lines 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)))))))
      (list LEFT RIGHT))

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

         (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))
               (when (memq (- 4 i) shaded-lines)
                 (set! total-lines (ly:stencil-add
                                    total-lines
                                    (ly:stencil-translate-axis
                                     (ly:stencil-in-color line-stencil "#e1e1e1")
                                     (- height (* i staff-space) thickness) Y))))))
     total-lines))



% with lilygabc
\include "lilygabc.ily"
\score {
  \gabc-vaticana
    "(c4) Tres(g) pú(g')e(g)ri(gh) jus(hv_GF)su(gh) re(h)gis(g'_) (,)
    in(f) for(hj~)ná(j)cem(i) mis(jk)si(h) sunt,(g.) (;)
    non(j) ti(i')mén(j)tes(h') flam(j)mam(g') i(h)gnis,(g_f_) (;)
    di(d)cén(f_g)tes:(g_f) Be(h)ne(ji)dí(hg)ctus(fg) De(g)us,(gf) al(gh)le(h)lú(g.)ia.(g.) (::)
    E(j) u(j) o(i) u(j) a(h) e.(g) (::)"
  \layout {
    \override VaticanaStaff.StaffSymbol.stencil =
      #(lambda (grob)
         (ly:stencil-add
          ((shade-staff-lines 2 4) grob)
          (ly:staff-symbol::print grob)))
  }
}

% vanilla LilyPond
\score {
  <<
  \new VaticanaVoice = "v" {
    \clef "vaticana-do3"
    g g\ictus g
    \[ g\melisma \pes a\melismaEnd \]
    \[ \virga a\episemInitium\episemFinis\melisma \inclinatum g \inclinatum f\melismaEnd \]
    \[ g\melisma \pes a\melismaEnd \]
    a g\episemInitium\episemFinis\ictus \divisioMinima
  }
  \new VaticanaLyrics \lyricsto "v" {
    Tres pú -- e -- ri jus -- su re -- gis
  }
  >>
  \layout {
    \override VaticanaStaff.StaffSymbol.stencil =
      #(lambda (grob)
         (ly:stencil-add
          ((shade-staff-lines 2 4) grob)
          (ly:staff-symbol::print grob)))
  }
}