Vertical spacing of footnotes

Right now, LilyPond's abilities to format footnote text strings are quite limited: each footnote on a page is processed and emitted separately, with some whitespace (footnote-padding) inbetween. footnote-padding is also inserted between the footnote separator markup and the first footnote. A consequence of this very simplistic output model (i.e., using padding and not baseline-to-baseline distances) is uneven vertical spacing if some footnote text doesn't contain both ascenders (like letter ‘d’) and descenders (like letter ‘p’).

This snippet fixes the vertical spacing of footnotes. It defines a new command \Footnote that accepts either a markup (for single-line footnotes) or a markup list (for multi-line footnotes). The baseline-to-baseline distance of the lines in multi-line footnotes is controlled by the top-level baseline skip in the paper variable text-font-defaults.baseline-skip.

\version "2.24.0"

#(define-markup-command (footnote-strut layout props depth)
   (boolean?)
   #:properties ((baseline-skip))
   "If `depth` is set to `#t`, insert a depth strut.  If set to `#f`,
insert a height strut.  The strut size is derived from the baseline
skip value."
   (let ((yext (if depth
                   (cons (* -0.3 baseline-skip) 0)
                   (cons 0 (* 0.7 baseline-skip)))))
     (ly:make-stencil (ly:stencil-expr
                       (make-transparent-box-stencil '(0 . 0.05) yext))
                      empty-interval
                      yext)))

#(define-markup-command (footnote-struts layout props elts)
   (markup-list?)
   #:properties ((baseline-skip))
   "Insert a height strut at the beginning of the first line and a depth
strut at the end of the last line of a footnote `elts` (which is a
markup list holding the lines)."
   (let ((stils (interpret-markup-list layout props elts)))
     (if (null? (cdr stils))
         ;; We have a single line.
         (let ((stil (car stils)))
           (interpret-markup layout props
                             #{ \markup {
                                  \override #`(baseline-skip . ,baseline-skip)
                                  \footnote-strut ##f
                                  \stencil #stil
                                  \footnote-strut ##t } #}))
         ;; We have multiple lines.
         (let* ((first-stil (car stils))
                (first-mkup #{ \markup {
                                 \override #`(baseline-skip . ,baseline-skip)
                                 \footnote-strut ##f
                                 \stencil #first-stil } #})
                (last-stil (last stils))
                (last-mkup #{ \markup {
                                \override #`(baseline-skip . ,baseline-skip)
                                \stencil #last-stil
                                \footnote-strut ##t } #})
                (rest-stils (cdr stils))
                (rest-stils (drop-right rest-stils 1))
                (rest-mkups
                 (map (lambda (stil)
                        #{ \markup {
                             \override #`(baseline-skip . ,baseline-skip)
                             \stencil #stil } #})
                      rest-stils)))
           (interpret-markup layout props
                             #{ \markup {
                                  \override #'(baseline-skip . 0)
                                  \column {
                                    #first-mkup
                                    #rest-mkups
                                    #last-mkup } } #})))))

#(define (markup-list-or-markup? x)
   (or (markup-list? x) (markup? x)))

% This command accepts either a markup (for a single-line footnote) or a
% markup list (for a multi-line footnote).  The baseline skip between the
% lines of a multi-line footnote is controlled by the paper variable
% `text-font-defaults.baseline-skip`, the distance between footnotes
% by the paper variable `footnote-padding`.
Footnote =
  #(define-music-function (mark offset text item)
     ((markup?) number-pair? markup-list-or-markup? symbol-list-or-music?)
     (let ((text (if (markup? text) (list text) text)))
       (if mark
           #{ \footnote #mark #offset
                \markup \footnote-struts #text #item #}
           #{ \footnote #offset
                \markup \footnote-struts #text #item #})))


%
% Example
%

#(set-default-paper-size "a7landscape")

\book {
  \header { tagline = ##f }

  \paper {
    text-font-defaults.baseline-skip = 1.7

    footnote-padding = 3\mm
    footnote-separator-markup =
      \markup { \override #'(span-factor . 1/3) \draw-hline }
  }

  \markup "time-based footnotes"

  \score {
    \relative c'' {
      r1 |
      \Footnote #'(-0.5 . -1)
        \markuplist {
          \override #'(line-width . 40)
          \wordwrap-lines {
	    Meter change. This is a multi-line footnote grob. }
        } Staff.TimeSignature
        \time 3/4
      \Footnote #'(1 . -1)
        \markuplist {
          \override #'(line-width . 40)
          \wordwrap-lines {
            Note stem. This is another multi-line footnote grob with a lot of
            senseless text. }
        } Stem
        <c e g>4 q q |
      \Footnote #'(-0.5 . 2) "Bar line." Staff.BarLine
        q q
      \Footnote #'(0.5 . -1) "Key change." Staff.KeySignature
        \key c \minor q |
    }
  }
}