Le 20/09/2021 à 15:36, Lukas-Fabian Moser a écrit :
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?
That is also my understanding. The rotation property works on a stencil
using its own extents, performing the equivalent of ly:stencil-rotate,
not ly:stencil-rotate-absolute. After that, LilyPond places the stencil
using the newly created extents. The placement on the staff can depend
or not on these extents, e.g. that of a Clef won't but that of a Hairpin
will.
The term “reference point” is a bit ambiguous. LilyPond places every
object relative to its parents, displacing it by X-offset and Y-offset.
If you are writing an [XY]-offset callback, you might think of the
reference point as being the point where the object would be placed
if its X-offset and Y-offset were 0. On top of that, the object is
drawn using a stencil, which carries its own extents. If you are writing
a stencil callback, you can think of the reference point as being
the point where a point stencil (having extents '(0 . 0) and '(0 . 0))
would be placed, i.e. taking X-offset and Y-offset into account
this time. It is the latter notion that applies here.
Best,
Jean
