Re: Draw a line on the X reference point of a grob

[Aaron Hill]

On 2020-01-19 8:17 am, Paolo Prete wrote:
> Hello. IIUC, the reference point for the X-offset property (or any 
> other
> x-shifting property that preserves the avoid-collisions algo) is on the 
> X-
> parent grob of the grob I want to modify.
>
> Is there a way to draw a line on this grob, so that I can visually well 
> see
> the distance that I'm going to modify?
> I tried to use the ly:grob-parent function, but there's obviously some
> error in the below code, as it doesn't show anything:
>
> %%%%%%%%%%%%%%%
>
> {
> \tweak DynamicText.after-line-breaking
> #(lambda (grob)
> (let* ((grob-x-parent (ly:grob-parent grob 0)))
>     (ly:grob-set-property! grob-x-parent 'stencil
> #{ \markup { \override #'(thickness . 3) \draw-line #'(0 . 3) } #})))
> c'\mf
> }

There are two issues, although one is hiding due to the other.

The \tweak command needs to be used immediately before whatever you are 
tweaking.  Since you want to modify the \mf, you must use the post-event 
version of \tweak.  Consider the following uses (and misuse) of \tweak:

%%%%
\version "2.18.2"
{
  % This will \tweak the NoteHead.
  \tweak color #red c'\mf

  % This will \tweak the Accidental.
  \tweak Accidental.color #red dis'\mf

  % !! This will have no effect. !!
  \tweak DynamicText.color #red ees'\mf

  % This will \tweak the DynamicText.
  f' -\tweak color #red \mf
}
%%%%

Using post-event \tweak or using an equivalent \once \override (as shown 
below), you will then be able to see the second problem--the stencil 
provided is the wrong type.  \markup is not a stencil, so you will need 
to convert it to one:

%%%%
\version "2.18.2"
{
  \once \override DynamicText.after-line-breaking = #(lambda (grob)
    (let* ((x-parent (ly:grob-parent grob X))
           (orig-sten (ly:grob-property x-parent 'stencil)))
      (ly:grob-set-property! x-parent 'stencil
        (grob-interpret-markup x-parent #{
           \markup \combine \stencil #orig-sten
             \with-dimensions #'(0 . 0) #'(0 . 0) \vcenter
             \with-color #red \draw-line #'(0 . 6) #}))))
  c'\mf
}
%%%%


-- Aaron Hill