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Generating custom flags

The stencil property of the Flag grob can be set to a custom scheme function to generate the glyph for the flag.

#(define-public (weight-flag grob)
   (let* ((stem-grob (ly:grob-parent grob X))
          (log (- (ly:grob-property stem-grob 'duration-log) 2))
          (is-up? (eqv? (ly:grob-property stem-grob 'direction) UP))
          (yext (if is-up? (cons (* log -0.8) 0) (cons 0 (* log 0.8))))
          (flag-stencil (make-filled-box-stencil '(-0.4 . 0.4) yext))
          (stroke-style (ly:grob-property grob 'stroke-style))
          (stroke-stencil (if (equal? stroke-style "grace")
                              (make-line-stencil 0.2 -0.9 -0.4 0.9 -0.4)
                              empty-stencil)))
     (ly:stencil-add flag-stencil stroke-stencil)))


% Create a flag stencil by looking up the glyph from the font
#(define (inverted-flag grob)
   (let* ((stem-grob (ly:grob-parent grob X))
          (dir (if (eqv? (ly:grob-property stem-grob 'direction) UP) "d" "u"))
          (flag (retrieve-glyph-flag "" dir "" grob))
          (line-thickness (ly:staff-symbol-line-thickness grob))
          (stem-thickness (ly:grob-property stem-grob 'thickness))
          (stem-width (* line-thickness stem-thickness))
          (stroke-style (ly:grob-property grob 'stroke-style))
          (stencil (if (null? stroke-style)
                       flag
                       (add-stroke-glyph flag stem-grob dir stroke-style "")))
          (rotated-flag (ly:stencil-rotate-absolute stencil 180 0 0)))
     (ly:stencil-translate rotated-flag (cons (- (/ stem-width 2)) 0))))

snippetexamplenotes =
{
  \autoBeamOff c'8 d'16 c'32 d'64 \acciaccatura {c'8} d'64
}

{
  \time 1/4
  \textMark "Normal flags"
  \snippetexamplenotes

  \textMark "Custom flag: inverted"
  \override Flag.stencil = #inverted-flag
  \snippetexamplenotes

  \textMark "Custom flag: weight"
  \override Flag.stencil = #weight-flag
  \snippetexamplenotes

  \textMark "Revert to normal"
  \revert Flag.stencil
  \snippetexamplenotes
}