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5.1.6 Defining new contexts
Specific contexts, like Staff and Voice, are made from
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 context, we want commands that
work in (existing) Voice contexts to continue working. This is
achieved by giving the new context an alias of Voice,
\alias Voice
The context will print notes and instructive texts, so we need to add the engravers which provide this functionality, plus the engraver which groups notes, stems and rests which occur at the same musical moment into columns,
\consists "Note_heads_engraver" \consists "Text_engraver" \consists "Rhythmic_column_engraver"
The note heads should all be placed on the center line,
\consists "Pitch_squash_engraver" squashedPosition = #0
The Pitch_squash_engraver modifies note heads (created
by the 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 \hide Stem
All these plug-ins have to communicate under the control of the
context. The mechanisms with which contexts communicate are
established by declaring the context \type. Within a
\layout block, most contexts will be of type
Engraver_group. Some special contexts and contexts in
\midi blocks use other context types. Copying and
modifying an existing context definition will also fill in the
type. Since this example creates a definition from scratch, it
needs to be specified explicitly.
\type "Engraver_group"
Put together, we get
\context {
\name ImproVoice
\type "Engraver_group"
\consists "Note_heads_engraver"
\consists "Text_engraver"
\consists "Rhythmic_column_engraver"
\consists "Pitch_squash_engraver"
squashedPosition = #0
\override NoteHead.style = #'slash
\hide Stem
\alias Voice
}
Contexts form hierarchies. We want to place the ImproVoice
context within the Staff context, just like normal Voice
contexts. Therefore, we modify the Staff definition with the
\accepts command,
\context {
\Staff
\accepts ImproVoice
}
Often when reusing an existing context definition, the resulting context can be used anywhere where the original context would have been useful.
\layout {
…
\inherit-acceptability to from
}
will arrange to have contexts of type to accepted by all contexts also accepting from. For example, using
\layout {
…
\inherit-acceptability "ImproVoice" "Voice"
}
will add an \accepts for ImproVoice to both
Staff and RhythmicStaff definitions.
The opposite of \accepts is \denies,
which is sometimes needed when reusing existing context definitions.
Arranging the required pieces into a \layout block leaves
us with
\layout {
\context {
\name ImproVoice
…
}
\inherit-acceptability "ImproVoice" "Voice"
}
Then the output at the start of this subsection can be entered as
\relative {
a'4 d8 bes8
\new ImproVoice {
c4^"ad lib" c
c4 c^"undress"
c c_"while playing :)"
}
a1
}
To complete this example, changes affecting the context hierarchy
should be repeated in a \midi block so that Midi output
depends on the same context relations.
See also
Internals Reference: Note_heads_engraver, Text_engraver, Rhythmic_column_engraver, Pitch_squash_engraver.
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