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    Translation of GIT committish: FILL-IN-HEAD-COMMITTISH

    When revising a translation, copy the HEAD committish of the
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@end ignore



@node implementing-notation 

@unnumberedsec Music notation

Common music notation encompasses some 500 years of music. Its
applications range from monophonic melodies to monstruous counterpoint
for large orchestras.  How can we get a grip on such a many-headed
beast?  Our solution is to make a strict distinction between notation,
@emph{what} symbols to use, and engraving, @emph{where} to put
them.  For tackling notation, we have broken up the problem into
digestible (and programmable) chunks: every type of symbol is handled
by a separate plugin.  All plugins cooperate through the LilyPond
architecture.  They are completely modular and independent, so each
can be developed and improved separately.

@itemize
@item
The most basic plug-in creates Note-heads:

@divClass{float-center}
@divEnd
@sourceimage{engraver-noteheads,,,.png}

This plug-in creates graphical objects from musical events.  People
that put graphics to musical ideas are called copyists or engravers,
so by analogy, this plug-in is called @code{Note_head_engraver}.


@item
 The @code{Staff_symbol_engraver} generates the object
representing the staff lines.

@divClass{float-center}
@divEnd
@sourceimage{engraver-staff,,,.png}

@item

 The @code{Clef_engraver} tells @code{Note_head_engraver} how high
each head should be placed.

@divClass{float-center}
@divEnd
@sourceimage{engraver-clef,,,.png}



@item

For the flags and stems we add  a @code{Stem_engraver}:

@divClass{float-center}
@divEnd
@sourceimage{engraver-stem,,,.png}

This engraver is notified of any note head coming along.  Every time
one (or more, for a chord) note head is seen, a stem object is
created, and attached to the note head.

@item

Beams, slurs, accents are handled by separate engravers. Like the
@code{Stem_engraver}, they create objects and connect them to stems,
note heads, etc.:

@divClass{float-center}
@divEnd
@sourceimage{engraver-slur,,,.png}



@item

Accidentals, bar lines, time signature, and key signature each have a
separate
engraver.

@divClass{float-center}
@divEnd
@sourceimage{engraver-acc,,,.png}

The @code{Accidental_engraver} is the most complex plug-in: it has
to look at the key signature, note pitches, ties, and bar lines to
decide when to print accidentals.


@end itemize


@c @unnumberedsec  Polyphonic notation
@heading  Polyphonic notation

The system shown in the last section works well for monophonic music,
but what about polyphony?  In polyphonic notation, many voices can
share a staff:

@divClass{float-center}
@divEnd
@sourceimage{engraver-final,,,.png}

In this situation, the accidentals and staff are shared, but the
stems, slurs, beams, etc. are private to each voice. Hence, engravers
should be grouped.  The engravers for note head, stems, slurs, etc. go
into a group called "Voice context," while the engravers for key,
accidental, bar, etc. go into a group called "Staff context."  In the
case of polyphony, a single Staff context contains more than one Voice
context.  Similarly, more Staff contexts can be put into a single
Score context:

@divClass{float-center}
@divEnd
@sourceimage{engraver-score,,,.png}

@divClass{float-right}
@divEnd
Next: @ref{engraving,The art of stamping}:
how @emph{did} they make hand-made music? 

@bye