Hi Peter,
One thing puzzles me about the documentation when it comes to
placement - it continually refers to the 'reference point' for an
object. I sort of understand what it means, but it doesn't seem to be
defined anywhere. For instance - to rotate a hairpin you have to
specify the co-ordinates of the centre of rotation relative to its
reference point. But where is the reference point for a hairpin? The
start, the middle, the end? (The phrase is also used in other
contests, such as octave placement in NR section 1.1.1, to add to the
confusion).
If I understand it correctly, the "reference point" of a layout object
should be its relative (0,0) coordinate. It is possible to display that
point using a small function:
\version "2.23.4"
showReferencePoint =
#(define-music-function (path) (symbol-list?)
#{
\override $path .stencil =
#(grob-transformer
'stencil
(lambda (grob orig)
(ly:stencil-outline
(ly:stencil-add
orig
(stencil-with-color (make-line-stencil 0.2 -0.5 -0.5 0.5
0.5) red)
(stencil-with-color (make-line-stencil 0.2 -0.5 0.5 0.5
-0.5) red))
orig)))
#})
\layout {
\showReferencePoint Score.TimeSignature
\showReferencePoint Score.Clef
}
{
\showReferencePoint Hairpin
a4\< d'4\!
\once \showReferencePoint NoteHead
a4\< d'4\!
\bar "||"
\showReferencePoint NoteHead
\showReferencePoint DynamicText
c'1\f
\once \override DynamicText.X-offset = 0
% Now the reference point of the "f" is aligned with the reference point
% of the notehead.
c'1\f
\bar "||"
\showReferencePoint Rest
r8
\tweak Y-offset 1 r
}
But I'm a bit wary of the statement in
http://lilypond.org/doc/v2.23/Documentation/notation/rotating-objects.html#rotating-layout-objects
that the .rotate property uses x/y-coordinates relative to the object's
reference point. It rather seems to me (if I read grob.cc and stencil.cc
in the source correctly) that the .rotation property uses the same
coordinate system as ly:stencil-rotate does, namely
(0,0) = center of object
(-1,-1) = lower left corner
(1,1) = upper right corner.
This also explains the example regarding rotating hairpins: -1 0 is
center-left (which also happens to be the reference point of a Hairpin).
(Of course, studying the effect of setting .rotation for a layout object
is complicated by the fact that LilyPond's spacing engine might move the
object around after rotating.)
Question to the experts: Am I right in thinking that the documentation
is misleading here?
Lukas
