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<p>
<a name="Overview-of-modifying-properties"></a>
Next: <a rel="next" accesskey="n" href="The-set-command.fr.html#The-set-command">The set command</a>,
Up: <a rel="up" accesskey="u" href="Modifying-properties.fr.html#Modifying-properties">Modifying properties</a>
<hr>
</div>
<h4 class="subsection">5.3.1 Overview of modifying properties</h4>
<p>Each context is responsible for creating certain types of graphical
objects. The settings used for printing these objects are also stored by
context. By changing these settings, the appearance of objects can be
altered.
<p>The syntax for this is
<pre class="example">\override <var>context</var>.<var>name</var> #'<var>property</var> = #<var>value</var>
</pre>
<p>Here <var>name</var> is the name of a graphical object, like
<code>Stem</code> or <code>NoteHead</code>, and <var>property</var> is an internal
variable of the formatting system (‘grob property’ or ‘layout
property’). The latter is a symbol, so it must be quoted. The
subsection <a href="Modifying-properties.fr.html#Modifying-properties">Modifying properties</a>, explains what to fill in
for <var>name</var>, <var>property</var>, and <var>value</var>. Here we only
discuss the functionality of this command.
<p>The command
<pre class="verbatim">
\override Staff.Stem #'thickness = #4.0
</pre>
<p class="noindent">makes stems thicker (the default is 1.3, with staff line thickness as a
unit). Since the command specifies <code>Staff</code> as context, it only
applies to the current staff. Other staves will keep their normal
appearance. Here we see the command in action:
<blockquote>
<pre class="verbatim">
c4
\override Staff.Stem #'thickness = #4.0
c4
c4
c4
</pre>
<p>
<a href="../b2/lily-11336034.ly">
<img align="middle"
border="0" src="../b2/lily-11336034.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p>The <code>\override</code> command changes the definition of the <code>Stem</code>
within the current <code>Staff</code>. After the command is interpreted
all stems are thickened.
<p>Analogous to <code>\set</code>, the <var>context</var> argument may be left out,
causing the default context <code>Voice</code> to be used. Adding
<code>\once</code> applies the change during one timestep only.
<blockquote>
<pre class="verbatim">
c4
\once \override Stem #'thickness = #4.0
c4
c4
</pre>
<p>
<a href="../5b/lily-03ed1c48.ly">
<img align="middle"
border="0" src="../5b/lily-03ed1c48.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p>The <code>\override</code> must be done before the object is
started. Therefore, when altering <em>Spanner</em> objects such as slurs
or beams, the <code>\override</code> command must be executed at the moment
when the object is created. In this example,
<blockquote>
<pre class="verbatim">
\override Slur #'thickness = #3.0
c8[( c
\override Beam #'thickness = #0.6
c8 c])
</pre>
<p>
<a href="../af/lily-67845031.ly">
<img align="middle"
border="0" src="../af/lily-67845031.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p class="noindent">the slur is fatter but the beam is not. This is because the command for
<code>Beam</code> comes after the Beam is started, so it has no effect.
<p>Analogous to <code>\unset</code>, the <code>\revert</code> command for a context
undoes an <code>\override</code> command; like with <code>\unset</code>, it only
affects settings that were made in the same context. In other words, the
<code>\revert</code> in the next example does not do anything.
<pre class="example">\override Voice.Stem #'thickness = #4.0
\revert Staff.Stem #'thickness
</pre>
<p>Some tweakable options are called ‘subproperties’ and reside inside
properties. To tweak those, use commands of the form
<!-- leave this as a long long -->
<pre class="example">\override <var>context</var>.<var>name</var> #'<var>property</var> #'<var>subproperty</var> = #<var>value</var>
</pre>
<p class="noindent">such as
<pre class="example">\override Stem #'details #'beamed-lengths = #'(4 4 3)
</pre>
<p class="noindent">
<h5 class="subsubheading">See also</h5>
<p class="indent">
<p>Internals: <a name="index-OverrideProperty-2678"></a><a href="../lilypond-internals/OverrideProperty.fr.html#OverrideProperty">OverrideProperty</a>, <a name="index-RevertProperty-2679"></a><a href="../lilypond-internals/RevertProperty.fr.html#RevertProperty">RevertProperty</a>,
<a name="index-PropertySet-2680"></a><a href="../lilypond-internals/PropertySet.fr.html#PropertySet">PropertySet</a>, <a name="index-Backend-2681"></a><a href="../lilypond-internals/Backend.fr.html#Backend">Backend</a>, and
<a name="index-All-layout-objects-2682"></a><a href="../lilypond-internals/All-layout-objects.fr.html#All-layout-objects">All layout objects</a>.
<p class="noindent">
<h5 class="subsubheading">Known issues and warnings</h5>
<p>The back-end is not very strict in type-checking object properties.
Cyclic references in Scheme values for properties can cause hangs
or crashes, or both.
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<p>
Next: <a rel="next" accesskey="n" href="The-set-command.fr.html#The-set-command">The set command</a>,
Up: <a rel="up" accesskey="u" href="Modifying-properties.fr.html#Modifying-properties">Modifying properties</a>
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