Re: Extending the width of a glissando

[Lukas-Fabian Moser]

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