Re: Extending the width of a glissando

[Jean Abou Samra]

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