Re: C++ question:

[Knut Petersen]

>           // A markup property whiteout-markup-wzd is implemented.
>           // The following definition is used for that property:
>           // \markup { \with-dimensions #'(0 . 0) #'(0 . 0) {
>           //           \filled-box #'(0.0 . 1.0) #'(-0.5 . 0.5) #0.0 } }
>           SCM properties = Font_interface::text_font_alist_chain (me);
>           SCM ws_zd_mod = Text_interface::interpret_markup (
>                              me->layout ()->self_scm (),
>                              properties,
>                              me->get_property ("whiteout-markup-wzd"));
>           Stencil wsa = *unsmob<Stencil> (ws_zd_mod);

>           // The following three lines should give an equivalent definition:
>           Box wb (Interval (0.0, 1.0), Interval (-0.5, 0.5));
>           Stencil wsb (Lookup::round_filled_box (wb, 0));
>           wsb.set_empty(false); 

The difference between Stencil wsa and wsb is that \markup { \withdimensions 
#'(0 . 0) #'(0 . 0) ...}
not only sets the dimensions :

(define-markup-command (with-dimensions layout props x y arg)
  (number-pair? number-pair? markup?)
  #:category other
  "
@cindex setting extent of text objects

Set the dimensions of @var{arg} to @var{x} address@hidden@var{y}."
  (let* ((expr (ly:stencil-expr (interpret-markup layout props arg))))
    (ly:stencil-add
     (make-transparent-box-stencil x y)
     (ly:make-stencil
      `(delay-stencil-evaluation ,(delay expr))
      x y))))

Knut