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<p>
<a name="Defining-new-contexts"></a>
Next: <a rel="next" accesskey="n" href="Aligning-contexts.de.html#Aligning-contexts">Aligning contexts</a>,
Previous: <a rel="previous" accesskey="p" href="Changing-context-default-settings.de.html#Changing-context-default-settings">Changing context default settings</a>,
Up: <a rel="up" accesskey="u" href="Interpretation-contexts.de.html#Interpretation-contexts">Interpretation contexts</a>
<hr>
</div>
<h4 class="subsection">5.1.5 Defining new contexts</h4>
<p>Specific contexts, like <code>Staff</code> and <code>Voice</code>, are made of
simple building blocks. It is possible to create new types of
contexts with different combinations of engraver plug-ins.
<p>The next example shows how to build a different type of
<code>Voice</code> context from scratch. It will be similar to
<code>Voice</code>, but only prints centered slash note heads. It can be used
to indicate improvisation in jazz pieces,
<blockquote>
<p>
<a href="../1c/lily-19988917.ly">
<img align="middle"
border="0" src="../1c/lily-19988917.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p>These settings are defined within a <code>\context</code> block inside a
<code>\layout</code> block,
<pre class="example">\layout {
\context {
...
}
}
</pre>
<p>In the following discussion, the example input shown should go in place
of the <small class="dots">...</small> in the previous fragment.
<p>First it is necessary to define a name for the new context:
<pre class="example">\name ImproVoice
</pre>
<p>Since it is similar to the <code>Voice</code>, we want commands that work
on (existing) <code>Voice</code>s to remain working. This is achieved by
giving the new context an alias <code>Voice</code>,
<pre class="example">\alias Voice
</pre>
<p>The context will print notes and instructive texts, so we need to add
the engravers which provide this functionality,
<pre class="example">\consists Note_heads_engraver
\consists Text_engraver
</pre>
<p>but we only need this on the center line,
<pre class="example">\consists Pitch_squash_engraver
squashedPosition = #0
</pre>
<p>The <a name="index-Pitch_005fsquash_005fengraver-2636"></a><a href="../lilypond-internals/Pitch_005fsquash_005fengraver.de.html#Pitch_005fsquash_005fengraver">Pitch_squash_engraver</a> modifies note heads (created
by <a name="index-Note_005fheads_005fengraver-2637"></a><a href="../lilypond-internals/Note_005fheads_005fengraver.de.html#Note_005fheads_005fengraver">Note_heads_engraver</a>) and sets their vertical
position to the value of <code>squashedPosition</code>, in this case <code>0</code>,
the center line.
<p>The notes look like a slash, and have no stem,
<pre class="example">\override NoteHead #'style = #'slash
\override Stem #'transparent = ##t
</pre>
<p>All these plug-ins have to cooperate, and this is achieved with a
special plug-in, which must be marked with the keyword <code>\type</code>.
This should always be <code>Engraver_group</code>.
<pre class="example">\type "Engraver_group"
</pre>
<p>Put together, we get
<pre class="example">\context {
\name ImproVoice
\type "Engraver_group"
\consists "Note_heads_engraver"
\consists "Text_engraver"
\consists Pitch_squash_engraver
squashedPosition = #0
\override NoteHead #'style = #'slash
\override Stem #'transparent = ##t
\alias Voice
}
</pre>
<p><a name="index-g_t_005caccepts-2638"></a><a name="index-g_t_005caccepts-2639"></a>
Contexts form hierarchies. We want to hang the <code>ImproVoice</code>
under <code>Staff</code>, just like normal <code>Voice</code>s. Therefore, we
modify the <code>Staff</code> definition with the <code>\accepts</code>
command,
<pre class="example">\context {
\Staff
\accepts ImproVoice
}
</pre>
<p><a name="index-g_t_005cdenies-2640"></a><a name="index-g_t_005cdenies-2641"></a>
The opposite of <code>\accepts</code> is <code>\denies</code>,
which is sometimes needed when reusing existing context definitions.
<p>Putting both into a <code>\layout</code> block, like
<pre class="example">\layout {
\context {
\name ImproVoice
...
}
\context {
\Staff
\accepts "ImproVoice"
}
}
</pre>
<p>Then the output at the start of this subsection can be entered as
<pre class="example">\relative c'' {
a4 d8 bes8
\new ImproVoice {
c4^"ad lib" c
c4 c^"undress"
c c_"while playing :)"
}
a1
}
</pre>
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