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Previous: <a rel="previous" accesskey="p" href="Avoiding-tweaks-with-slower-processing.html#Avoiding-tweaks-with-slower-processing">Avoiding tweaks with slower processing</a>,
Up: <a rel="up" accesskey="u" href="Further-tweaking.html#Further-tweaking">Further tweaking</a>
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<h4 class="subsection">4.6.5 Advanced tweaks with Scheme</h4>
<p>Although many things are possible with the <code>\override</code> and
<code>\tweak</code> commands, an even more powerful way of modifying
the action of LilyPond is available through a programmable
interface to the LilyPond internal operation. Code written in
the Scheme programming language can be incorporated directly in
the internal operation of LilyPond. Of course, at least a basic
knowledge of programming in Scheme is required to do this, and an
introduction is provided in the <a href="Scheme-tutorial.html#Scheme-tutorial">Scheme tutorial</a>.
<p>As an illustration of one of the many possibilities, instead of
setting a property to a constant it can be set to a Scheme
procedure which is then called whenever that property is accessed
by LilyPond. The property can then be set dynamically to a value
determined by the procedure at the time it is called. In this
example we color the note head in accordance with its position on
the staff.
<blockquote>
<pre class="verbatim">
#(define (color-notehead grob)
"Color the notehead according to its position on the staff."
(let ((mod-position (modulo (ly:grob-property grob 'staff-position) 7)))
(case mod-position
;; Return rainbow colors
((1) (x11-color 'red )) ; for C
((2) (x11-color 'orange )) ; for D
((3) (x11-color 'yellow )) ; for E
((4) (x11-color 'green )) ; for F
((5) (x11-color 'blue )) ; for G
((6) (x11-color 'purple )) ; for A
((0) (x11-color 'violet )) ; for B
)
)
)
\relative c' {
% Arrange to obtain color from color-notehead procedure
\override NoteHead #'color = #color-notehead
c2 c' |
b4 g8 a b4 c |
c,2 a' |
g1 |
}
\addlyrics {
Some -- where o -- ver the Rain -- bow, way up high,
}
</pre>
<p>
<a href="../f5/lily-96df4015.ly">
<img align="middle"
border="0" src="../f5/lily-96df4015.png" alt="[image of music]">
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</p>
</blockquote>
<p>Further examples showing the use of these programmable interfaces
can be found in <a href="Tweaking-with-Scheme.html#Tweaking-with-Scheme">Tweaking with Scheme</a>.
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