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5.1.4 Modifying context plug-ins

Notation contexts (like Score and Staff) not only store properties, they also contain plug-ins called ‘engravers’ that create notation elements. For example, the Voice context contains a Note_heads_engraver and the Staff context contains a Key_engraver.

For a full a description of each plug-in, see Engravers and Performers. Every context described in Contexts lists the engravers used for that context.

It can be useful to shuffle around these plug-ins. This is done by starting a new context with \new or \context, and modifying it,

\new context \with {
  \consists …
  \consists …
  \remove …
  \remove …
  etc.
}
{
  ..music..
}

where the … should be the name of an engraver. Here is a simple example which removes Time_signature_engraver and Clef_engraver from a Staff context,

<<
  \new Staff {
    f2 g
  }
  \new Staff \with {
     \remove "Time_signature_engraver"
     \remove "Clef_engraver"
  } {
    f2 g2
  }
>>

In the second staff there are no time signature or clef symbols. This is a rather crude method of making objects disappear since it will affect the entire staff. This method also influences the spacing, which may or may not be desirable. More sophisticated methods of blanking objects are shown in Visibility and color of objects.

The next example shows a practical application. Bar lines and time signatures are normally synchronized across the score. This is done by the Timing_translator and Default_bar_line_engraver. This plug-in keeps an administration of time signature, location within the measure, etc. By moving these engraver from Score to Staff context, we can have a score where each staff has its own time signature.

\score {
  <<
    \new Staff \with {
      \consists "Timing_translator"
      \consists "Default_bar_line_engraver"
    } {
        \time 3/4
        c4 c c c c c
    }
  \new Staff \with {
    \consists "Timing_translator"
    \consists "Default_bar_line_engraver"
  } {
      \time 2/4
      c4 c c c c c
  }
>>
\layout {
  \context {
    \Score
    \remove "Timing_translator"
    \remove "Default_bar_line_engraver"
    }
  }
}

Known issues and warnings

Usually the order in which the engravers are specified does not matter, but in a few special cases the order is important, for example where one engraver writes a property and another reads it, or where one engraver creates a grob and another must process it. The order in which the engravers are specified is the order in which they are called to carry out their processing.

The following orderings are important: the Bar_engraver must normally be first, and the New_fingering_engraver must come before the Script_column_engraver. There may be others with ordering dependencies.

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