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<p>
<a name="Properties-of-layout-objects"></a>
Next: <a rel="next" accesskey="n" href="Properties-found-in-interfaces.fr.html#Properties-found-in-interfaces">Properties found in interfaces</a>,
Up: <a rel="up" accesskey="u" href="The-Internals-Reference-manual.fr.html#The-Internals-Reference-manual">The Internals Reference manual</a>
<hr>
</div>
<h4 class="subsection">4.2.1 Properties of layout objects</h4>
<p><a name="index-properties-of-layout-objects-353"></a><a name="index-properties-of-grobs-354"></a><a name="index-grobs_002c-properties-of-355"></a><a name="index-layout-objects_002c-properties-of-356"></a>
Suppose you have a slur in a score which, to your mind,
appears too thin and you'd like to draw it a little heavier.
How do you go about doing this? You know from the statements
earlier about the flexibility of LilyPond that such a thing
should be possible, and you would probably guess that an
<code>\override</code> command would be needed. But is there a
heaviness property for a slur, and if there is, how might it
be modified? This is where the Internals Reference manual
comes in. It contains all the information you might need to
construct this and all other <code>\override</code> commands.
<p>Before we look at the Internals Reference a word of warning.
This is a <strong>reference</strong> document, which means there is
little or no explanation contained within it: its purpose is
to present information precisely and concisely. This
means it might look daunting at first sight. Don't worry!
The guidance and explanation presented here will enable you
to extract the information from the Internals Reference for
yourself with just a little practice.
<p><a name="index-override-example-357"></a><a name="index-Internals-Reference_002c-example-of-using-358"></a>
Let's use a concrete example with a simple fragment of real
music:
<blockquote>
<pre class="verbatim">
{
\time 6/8
{
r4 b8 b[( g]) g |
g[( e]) e d[( f]) a |
a g
}
\addlyrics {
The man who feels love's sweet e -- mo -- tion
}
}
</pre>
<p>
<a href="../67/lily-9ef082e1.ly">
<img align="middle"
border="0" src="../67/lily-9ef082e1.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p>Suppose now that we decide we would like the slurs to be a
little heavier. Is this possible? The slur is certainly a
layout object, so the question is, ‘Is there a property
belonging to a slur which controls the heaviness?’ To answer
this we must look in the Internals Reference, or IR for short.
<p>The IR for the version of LilyPond you are using may be found
on the LilyPond website at <a href="http://lilypond.org">http://lilypond.org</a>. Go to the
documentation page and click on the Internals Reference link.
For learning purposes you should use the standard HTML version,
not the ‘one big page’ or the PDF. For the next few
paragraphs to make sense you will need to actually do this
as you read.
<p>Under the heading <strong>Top</strong> you will see five links. Select
the link to the <em>Backend</em>, which is where information about
layout objects is to be found. There, under the heading
<strong>Backend</strong>, select the link to <em>All layout objects</em>.
The page that appears lists all the layout objects used in your
version of LilyPond, in alphabetic order. Select the link to
Slur, and the properties of Slurs are listed.
<p>An alternative way of finding this page is from the Notation
Reference. On one of the pages that deals with slurs you may find a
link to the Internals Reference. This link will take you directly to
this page, but if you have an idea about the name of the layout object
to be tweaked, it is easier to go straight to the IR and search there.
<p>This Slur page in the IR tells us first that Slur objects are created
by the Slur_engraver. Then it lists the standard settings. Note
these are <strong>not</strong> in alphabetic order. Browse down them looking
for a property that might control the heaviness of slurs, and you
should find
<pre class="example"><code>thickness</code> (number)
<code>1.2</code>
Line thickness, generally measured in <code>line-thickness</code>
</pre>
<p>This looks a good bet to change the heaviness. It tells us that
the value of <code>thickness</code> is a simple <em>number</em>,
that the default value is 1.2, and that the units are
in another property called <code>line-thickness</code>.
<p>As we said earlier, there are few to no explanations in the IR,
but we already have enough information to try changing the
slur thickness. We see that the name of the layout object
is <code>Slur</code>, that the name of the property to change is
<code>thickness</code> and that the new value should be a number
somewhat larger than 1.2 if we are to make slurs thicker.
<p>We can now construct the <code>\override</code> command by simply
substituting the values we have found for the names, omitting
the context. Let's use a very large value for the thickness
at first, so we can be sure the command is working. We get:
<pre class="example">\override Slur #'thickness = #5.0
</pre>
<p>Don't forget the <code>#'</code> preceding the
property name and and <code>#</code> preceding the new value!
<p>The final question is, ‘Where should this command be
placed?’ While you are unsure and learning, the best
answer is, ‘Within the music, before the first slur and
close to it.’ Let's do that:
<blockquote>
<pre class="verbatim">
{
\time 6/8
{
% Increase thickness of all following slurs from 1.2 to 5.0
\override Slur #'thickness = #5.0
r4 b8 b[( g]) g |
g[( e]) e d[( f]) a |
a g
}
\addlyrics {
The man who feels love's sweet e -- mo -- tion
}
}
</pre>
<p>
<a href="../0d/lily-65bfbd2e.ly">
<img align="middle"
border="0" src="../0d/lily-65bfbd2e.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p class="noindent">and we see that the slur is indeed heavier.
<p>So this is the basic way of constructing <code>\override</code>
commands. There are a few more complications that we
shall meet in later sections, but you now know all the
essentials required to make up your own – but you will
still need some practice. This is provided in the examples
which follow.
<h4 class="subheading">Finding the context</h4>
<p><a name="index-context_002c-finding-359"></a>
But first, what if we had needed to specify the Context?
What should it be? We could guess that slurs are in
the Voice context, as they are clearly closely associated
with individual lines of music, but can we be sure? To
find out, go back to the top of the IR page describing the
Slur, where it says ‘Slur objects are created by: Slur
engraver’. So slurs will be created in whichever context
the <code>Slur_engraver</code> is in. Follow the link to the
<code>Slur_engraver</code> page. At the very bottom it tells
us that <code>Slur_engraver</code> is part of five Voice contexts,
including the standard voice context, <code>Voice</code>, so our
guess was correct. And because <code>Voice</code> is one of the
lowest level contexts which is implied unambiguously by
the fact that we are entering notes, we can omit it in this
location.
<h4 class="subheading">Overriding once only</h4>
<p><a name="index-overriding-once-only-360"></a><a name="index-once-override-361"></a><a name="index-g_t_005conce-362"></a><a name="index-g_t_005conce-363"></a>
<p>As you can see, <em>all</em> the slurs are thicker in the
final example above. But what if we
wanted just the first slur to be thicker? This is achieved
with the <code>\once</code> command. Placed immediately before
the <code>\override</code> command it causes it to change only the
slur which begins on the <strong>immediately following</strong> note.
If the
immediately following note does not begin a slur the command
has no effect at all – it is not remembered until a slur
is encountered, it is simply discarded. So the command with
<code>\once</code> must be
repositioned as follows:
<blockquote>
<pre class="verbatim">
{
\time 6/8
{
r4 b8
% Increase thickness of immediately following slur only
\once \override Slur #'thickness = #5.0
b[( g]) g |
g[( e]) e d[( f]) a |
a g
}
\addlyrics {
The man who feels love's sweet e -- mo -- tion
}
}
</pre>
<p>
<a href="../79/lily-e3e2db37.ly">
<img align="middle"
border="0" src="../79/lily-e3e2db37.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p class="noindent">Now only the first slur is made heavier.
<p>The <code>\once</code> command can also be used before the <code>\set</code>
command.
<h4 class="subheading">Reverting</h4>
<p><a name="index-revert-364"></a><a name="index-default-properties_002c-reverting-365"></a><a name="index-g_t_005crevert-366"></a><a name="index-g_t_005crevert-367"></a>
<p>Finally, what if we wanted just the first two slurs to be
heavier? Well, we could use two commands, each preceded by
<code>\once</code> placed immediately before each of the notes where
the slurs begin:
<blockquote>
<pre class="verbatim">
{
\time 6/8
{
r4 b8
% Increase thickness of immediately following slur only
\once \override Slur #'thickness = #5.0
b[( g]) g |
% Increase thickness of immediately following slur only
\once \override Slur #'thickness = #5.0
g[( e]) e d[( f]) a |
a g
}
\addlyrics {
The man who feels love's sweet e -- mo -- tion
}
}
</pre>
<p>
<a href="../6d/lily-6e0309f7.ly">
<img align="middle"
border="0" src="../6d/lily-6e0309f7.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p class="noindent">or we could omit the <code>\once</code> command and use the
<code>\revert</code> command
to return the <code>thickness</code> property to its default value
after the second slur:
<blockquote>
<pre class="verbatim">
{
\time 6/8
{
r4 b8
% Increase thickness of all following slurs from 1.2 to 5.0
\override Slur #'thickness = #5.0
b[( g]) g |
g[( e])
% Revert thickness of all following slurs to default of 1.2
\revert Slur #'thickness
e d[( f]) a |
a g
}
\addlyrics {
The man who feels love's sweet e -- mo -- tion
}
}
</pre>
<p>
<a href="../dc/lily-4f0d3500.ly">
<img align="middle"
border="0" src="../dc/lily-4f0d3500.png" alt="[image of music]">
</a>
</p>
</blockquote>
<p class="noindent">The <code>\revert</code> command can be used to return any property
changed with <code>\override</code> back to its default value.
You may use whichever method best suits what you want to do.
<p>That concludes our introduction to the IR, and the basic
method of tweaking. Several examples follow in the later
sections of this Chapter, partly to introduce you to some of the
additional features of the IR, and partly to give you more
practice in extracting information from it. These examples will
contain progressively fewer words of guidance and explanation.
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Up: <a rel="up" accesskey="u" href="The-Internals-Reference-manual.fr.html#The-Internals-Reference-manual">The Internals Reference manual</a>
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