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Specific contexts, like Staff
and Voice, are made of
simple building blocks, and it is possible to compose engraver
plug-ins in different combinations, thereby creating new types of
contexts.
The next example shows how to build a different type of
Voice
context from scratch. It will be similar to
Voice, but print centered slash noteheads only. It can be used
to indicate improvisation in Jazz pieces,
These settings are again done within a \context block inside a
\paper block,
\paper {
\context {
...
}
}
In the following discussion, the example input shown should go on the ... in the previous fragment.
First, name the context gets a name. Instead of Voice
it
will be called ImproVoice
,
\name ImproVoice
Since it is similar to the Voice
, we want commands that work
on (existing) Voice
s to remain working. This is achieved by
giving the new context an alias Voice
,
\alias Voice
The context will print notes, and instructive texts
\consists Note_heads_engraver \consists Text_engraver
but only 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, without a 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_engraver,
\type "Engraver_group_engraver"
Putting together, we get
\context {
\name ImproVoice
\type "Engraver_group_engraver"
\consists "Note_heads_engraver"
\consists "Text_script_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,1
\context {
\StaffContext
\accepts ImproVoice
}
Putting both into a \paper block, like
\paper {
\context {
\name ImproVoice
...
}
\context {
\StaffContext
\accepts "ImproVoice"
}
}
Then the output at the start of this subsection can be entered as
\score {
\notes \relative c'' {
a4 d8 bes8
\new ImproVoice {
c4^"ad lib" c
c4 c^"undress"
c c_"while playing :)"
}
a1
}
}
[1] The opposite of \accepts is \denies,
which is sometimes when reusing existing context definitions.
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