Fixing LSR 888: center-on-words ignoring punctuation

[Alexander Kobel]

Hi all,

yet another semi-feature request where I'm running out of knowledge...
I happily used LSR 888 (center lyrics without taking punctuation into 
account) for years and only now found out that the alignment is off 
under certain circumstances: The default algorithm seems to align lyrics 
on the center of the note*head* of the associated voice. Which seems the 
right thing to do. However, center-on-words aligns on the center of the 
*entire note* - in particular, this yields different results on quavers 
with stems up and "outward flags". At least, neither dots nor 
accidentals are taken into account...

Even more ugly, the alignment is totally off on chords with two adjacent 
notes (shifting one note). I know that this should be a forbidden 
situation for vocal music, but sometimes there is a reason for writing 
such chords instead of two separate voices (e.g., soprano and alto 
notated on a common staff in a homorhythmic piece, with one of the 
voices splitting for a cluster on just a few notes).
In such a rare case, I do not like the default algorithm's choices 
either (and I'd prefer alignment on the center of the noteheads of the 
chord); but center-on-words does even /way/ worse.

I attach a "test case" (including a full copy of the core of 
http://lsr.di.unimi.it/LSR/Snippet?id=888), where one would expect 
center-on-words to do exactly nothing, but you can notice the slight 
offsets on the "nonius scale" of lyrics.

Anyone got an idea where to start to fix that? I think one would have to 
figure how to get the note-column-extent without flags; but I have no 
clue how to approach that.


Thanks in advance,
Alexander
\version "2.18.0"  %% and higher

%% http://lsr.di.unimi.it/LSR/Item?id=888
%% [...]

#(define space-set
  (list->char-set 
    (string->list "—.?-;,:“”‘’–— */()[]{}|<>!`~&…")))

#(define (width grob text)
  (let* ((X-extent 
           (ly:stencil-extent (grob-interpret-markup grob text) X)))
   (if (interval-empty? X-extent)
       0 
       (cdr X-extent))))

#(define (center-on-word grob)
  (let* ((text (ly:grob-property-data grob 'text))
         (syllable (markup->string text))
         (word-position
           (if (string-skip syllable space-set)
               (string-skip syllable space-set)
               0))
         (word-end
           (if (string-skip-right syllable space-set)
               (+ (string-skip-right syllable space-set) 1)
               (string-length syllable)))
         (preword (substring syllable 0 word-position))
         (word (substring syllable word-position word-end))
         (preword-width (width grob preword))
         (word-width (width grob (if (string-null? syllable) text word)))
         (note-column (ly:grob-parent grob X))
         (note-column-extent (ly:grob-extent note-column note-column X))
         (note-column-width (interval-length note-column-extent)))
         
  (- 
    (*
      (/ (- note-column-width word-width) 2)  
      (1+ (ly:grob-property-data grob 'self-alignment-X)))
    preword-width)))

melodies = \relative c'' {
  \autoBeamOff
  g8 a b c
  << { \voiceTwo c } \new Voice { \voiceOne d8 } >>
  << { \voiceOne c } \new Voice { \voiceTwo d8 } >>
  << { \voiceTwo c } \new Voice { \voiceOne e8 } >>
  << { \voiceOne c } \new Voice { \voiceTwo e8 } >>
  \oneVoice
  <c b> <b a> <a g> <g f> <f e>8.*2/3 <e d> <d c> <c c>
}

melody = \relative c'' {
  \autoBeamOff
  g8 a b c
  \voiceTwo c
  \voiceOne c
  \voiceTwo c
  \voiceOne c
  \oneVoice
  <c b> <b a> <a g> <g f> <f e>8.*2/3 <e d> <d c> <c c>
}

lyr = \lyricmode { \repeat unfold 16 "|" }

\score {
  <<
    \new Staff << \new Voice = "cows" \melodies >>
    \new Lyrics \with { \override LyricText.X-offset = #center-on-word } \lyricsto "cows" { \set stanza = "cow" \lyr }
    \new Staff << \new Voice = "ncows" \transpose c c' \melodies >>
    \new Lyrics \lyricsto "ncows" { \set stanza = "default" \lyr }
    \new Staff << \new Voice = "cow^8s" \transpose c c' \melodies >>
    \new Lyrics \with { \override LyricText.X-offset = #center-on-word } \lyricsto "cow^8s" { \set stanza = "cow" \lyr }
    \new Staff << \new Voice = "cow" \melody >>
    \new Lyrics \with { \override LyricText.X-offset = #center-on-word } \lyricsto "cow" { \set stanza = "cow" \lyr }
    \new Staff << \new Voice = "ncow" \transpose c c' \melody >>
    \new Lyrics \lyricsto "ncow" { \set stanza = "default" \lyr }
    \new Staff << \new Voice = "cow^8" \transpose c c' \melody >>
    \new Lyrics \with { \override LyricText.X-offset = #center-on-word } \lyricsto "cow^8" { \set stanza = "cow" \lyr }
  >>
  \layout {
    \context {
      \Score
      % just for easier comparison
      proportionalNotationDuration = #(ly:make-moment 1 16)
      \override SpacingSpanner.strict-note-spacing = ##t
    }
  }
}
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