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Displaying grob ancestry

When working with grob callbacks, it can be helpful to understand a grob’s ancestry. Most grobs have parents which influence the positioning of the grob. X- and Y-parents influence the horizontal and vertical positions for the grob, respectively. Additionally, each parent may have parents of its own.

Unfortunately, there are several aspects of a grob’s ancestry that can lead to confusion:

• The types of parents a grob has may depend on context.
• For some grobs, the X- and Y-parents are the same.
• A particular ancestor may be related to a grob in multiple ways.
• The concept of generations is misleading.

For example, the System grob can be both parent (on the Y-side) and grandparent (twice on the X-side) to a VerticalAlignment grob.

This macro prints (to the console) a textual representation of a grob’s ancestry.

When called this way:

{ \once \override NoteHead.before-line-breaking = #display-ancestry c }

The following output is generated:

NoteHead
X,Y: NoteColumn
     X: PaperColumn
        X,Y: System
     Y: VerticalAxisGroup
        X: NonMusicalPaperColumn
           X,Y: System
        Y: VerticalAlignment
           X: NonMusicalPaperColumn
              X,Y: System
           Y: System

%% http://lsr.di.unimi.it/LSR/Item?id=622

#(define (get-ancestry grob)
  (if (not (null? (ly:grob-parent grob X)))
      (list (grob::name grob)
            (get-ancestry (ly:grob-parent grob X))
            (get-ancestry (ly:grob-parent grob Y)))
      (grob::name grob)))

#(define (format-ancestry lst padding)
   (string-append
    (symbol->string (car lst))
    "\n"
    (let ((X-ancestry
           (if (list? (cadr lst))
               (format-ancestry (cadr lst) (+ padding 3))
               (symbol->string (cadr lst))))
          (Y-ancestry
           (if (list? (caddr lst))
               (format-ancestry (caddr lst) (+ padding 3))
               (symbol->string (caddr lst)))))
      (if (equal? X-ancestry Y-ancestry)
          (string-append
           (format #f "~&")
           (make-string padding #\space)
           "X,Y: "
           (if (list? (cadr lst))
               (format-ancestry (cadr lst) (+ padding 5))
               (symbol->string (cadr lst))))
          (string-append
           (format #f "~&")
           (make-string padding #\space)
           "X: " X-ancestry
           "\n"
           (make-string padding #\space)
           "Y: " Y-ancestry
           (format #f "~&"))))
    (format #f "~&")))

#(define (display-ancestry grob)
   (format (current-error-port)
      "~3&~a~2%~a~&"
      (make-string 36 #\-)
      (if (ly:grob? grob)
          (format-ancestry (get-ancestry grob) 0)
          (format #f "~a is not a grob" grob))))

\relative c' {
  \once \override NoteHead.before-line-breaking = #display-ancestry
  f4
  \once \override Accidental.before-line-breaking = #display-ancestry
  \once \override Arpeggio.before-line-breaking = #display-ancestry
  <f as c>4\arpeggio
}