Generate special note head shapes

When a note head with a special shape cannot easily be generated with graphic markup, PostScript code can be used to generate the shape. This example shows how a parallelogram-shaped note head is generated.

\version "2.24"

%% Updaters remark:
%% For unkown reasons this snippet returns a gs-error, but only, if compiled
%% with multiple others like: lilypond *.ly
%% Thus changing to a path-stencil.
%% TODO description needs to get adjusted  --harm
    
parallelogram =
  #(ly:make-stencil
    '(path 0.1
        (rmoveto 0 0.25
         lineto 1.3125 0.75 
         lineto 1.3125 -0.25 
         lineto 0 -0.75)
         round
         round
         #t)
    (cons 0 1.3125)
    (cons -.75 .75))

myNoteHeads = \override NoteHead.stencil = \parallelogram
normalNoteHeads = \revert NoteHead.stencil

\relative c'' {
  \myNoteHeads
  g4 d'
  \normalNoteHeads
  <f, \tweak stencil \parallelogram b e>4 d
}