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diff --git a/input/regression/stencil-hacking.ly b/input/regression/stencil-hacking.ly
new file mode 100644 (file)
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+++ b/input/regression/stencil-hacking.ly
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+
+\version "2.5.13"
+
+\header { texidoc=" You can write stencil callbacks in Scheme, thus
+providing custom glyphs for notation elements.  A simple example is
+adding parentheses to existing stencil callbacks.
+
+The parenthesized beam is less successful due to implementation of the
+Beam. The note head is also rather naive, since the extent of the
+parens are also not seen by accidentals.
+"
+       
+}
+
+#(define (parenthesize-callback callback)
+   "Construct a function that will do CALLBACK and add parentheses.
+Example usage:
+
+  \\property NoteHead \\override #'print-function
+                  =
+                     #(parenthesize-callback Note_head::print)
+                   
+"
+
+   
+   (define (parenthesize-stencil grob)
+     "This function adds parentheses to the original callback for
+GROB.  The dimensions of the stencil is not affected.
+"
+     
+     (let* ((fn (ly:get-default-font grob))
+           (pclose (ly:font-get-glyph fn "accidentals.rightparen"))
+           (popen (ly:font-get-glyph fn "accidentals.leftparen"))
+           (subject (callback grob))
+
+           ; remember old size
+           (subject-dim-x (ly:stencil-extent subject 0))
+           (subject-dim-y (ly:stencil-extent subject 1)))
+
+        ; add parens
+        (set! subject
+            (ly:stencil-combine-at-edge 
+             (ly:stencil-combine-at-edge subject 0 1 pclose 0.2)
+             0 -1 popen  0.2))
+
+       ; revert old size.
+       (ly:make-stencil
+        (ly:stencil-expr subject) subject-dim-x subject-dim-y)))
+   parenthesize-stencil)
+    
+
+\layout { raggedright = ##t }
+\relative c' {
+    c4 e
+
+    \override NoteHead  #'print-function
+    =
+    #(parenthesize-callback Note_head::print)
+    g bes
+    \revert NoteHead #'print-function
+    \override Beam  #'print-function
+    =
+    #(parenthesize-callback Beam::print)
+
+    a8 gis8 a2.
+    
+}
+
+
+