Re: Extending the width of a glissando

[Lukas-Fabian Moser]

> > 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.

Yes, exactly. But the one thing the term "reference point" should never 
mean is "just the object's center".

So if the rotation is done with respect to the object's center (and in 
scaled coordinates such that the object's extents are from -1 to 1) - 
and this is what happens, as can be seen in the attached file where blue 
crosses mark the centers of objects, red crosses their respective '(0 . 
0) point -, then to describe this as

> and the x and y coordinates of the point relative to the object’s 
> reference point about which the rotation is to be performed
> (http://lilypond.org/doc/v2.23/Documentation/notation/rotating-objects.html#rotating-layout-objects)
is simply wrong in my opinion.

If you agree, I'll try and prepare a MR to change the documentation.

Maybe one might consider if it would be nice to have both behaviours: 
.rotation with respect to the object's center (current behaviour), but 
also with respect to the the stencil's '(0 . 0). Of course, one can 
achieve the latter with a bit of scheme code, but maybe a grob property 
of the form

\override Hairpin.rotation-system = #'absolute % offset (0,0) is the 
stencil's reference point, offset units are in staff spaces
\override Hairpin.rotation-system = #'relative % offset (0,0) is the 
object's center, offset units are such that the object's edges are at +- 1.

might be useful?

Lukas

stencil-points.ly
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