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5.1.6 Defining new contexts
Specific contexts, like Staff and Voice, are made of
simple building blocks. It is possible to create new types of
contexts with different combinations of engraver plug-ins.
The next example shows how to build a different type of
Voice context from scratch. It will be similar to
Voice, but only prints centered slash note heads. It can be used
to indicate improvisation in jazz pieces,
These settings are defined within a \context block inside a
\layout block,
\layout {
\context {
…
}
}
In the following discussion, the example input shown should go in place of the … in the previous fragment.
First it is necessary to define a name for the new context:
\name ImproVoice
Since it is similar to the Voice, we want commands that work
on (existing) Voices to remain working. This is achieved by
giving the new context an alias Voice,
\alias Voice
The context will print notes and instructive texts, so we need to add the engravers which provide this functionality,
\consists Note_heads_engraver \consists Text_engraver
but we only need this on the center line,
\consists Pitch_squash_engraver squashedPosition = #0
The
Pitch_squash_engraver modifies note heads (created
by
Note_heads_engraver) and sets their vertical
position to the value of squashedPosition, in this case 0,
the center line.
The notes look like a slash, and have no stem,
\override NoteHead #'style = #'slash \override Stem #'transparent = ##t
All these plug-ins have to cooperate, and this is achieved with a
special plug-in, which must be marked with the keyword \type.
This should always be Engraver_group.
\type "Engraver_group"
Put together, we get
\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
}
Contexts form hierarchies. We want to hang the ImproVoice
under Staff, just like normal Voices. Therefore, we
modify the Staff definition with the \accepts
command,
\context {
\Staff
\accepts ImproVoice
}
The opposite of \accepts is \denies,
which is sometimes needed when reusing existing context definitions.
Putting both into a \layout block, like
\layout {
\context {
\name ImproVoice
…
}
\context {
\Staff
\accepts "ImproVoice"
}
}
Then the output at the start of this subsection can be entered as
\relative c'' {
a4 d8 bes8
\new ImproVoice {
c4^"ad lib" c
c4 c^"undress"
c c_"while playing :)"
}
a1
}
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