lilypond-essay: What symbols to engrave?
What symbols to engrave?
------------------------
The formatting process decides where to place symbols. However, this
can only be done once it is decided _what_ symbols should be printed –
in other words, what notation to use.
Common music notation is a system of recording music that has evolved
over the past 1000 years. The form that is now in common use dates from
the early Renaissance. Although the basic form (i.e., note heads on a
5-line staff) has not changed, the details still evolve to express the
innovations of contemporary notation. Hence, common music notation
encompasses some 500 years of music. Its applications range from
monophonic melodies to monstrous counterpoints for a large orchestra.
How can we get a grip on such a seven-headed beast, and force it into
the confines of a computer program? Our solution is to break up the
problem of notation (as opposed to engraving, i.e., typography) into
digestible and programmable chunks: every type of symbol is handled by a
separate module, a so-called plug-in. Each plug-in is completely
modular and independent, so each can be developed and improved
separately. Such plug-ins are called ‘engraver’s, by analogy with
craftsmen who translate musical ideas to graphic symbols.
In the following example, we start out with a plug-in for note heads,
the ‘Note_heads_engraver’.
[image src="" alt="[image of music]" text="image of music" ]
Then a ‘Staff_symbol_engraver’ adds the staff,
[image src="" alt="[image of music]" text="image of music" ]
the ‘Clef_engraver’ defines a reference point for the staff,
[image src="" alt="[image of music]" text="image of music" ]
and the ‘Stem_engraver’ adds stems.
[image src="" alt="[image of music]" text="image of music" ]
The ‘Stem_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 connected to the note head. By adding engravers for beams,
slurs, accents, accidentals, bar lines, time signature, and key
signature, we get a complete piece of notation.
[image src="" alt="[image of music]" text="image of music" ]
This system works well for monophonic music, but what about
polyphony? In polyphonic notation, many voices can share a staff.
[image src="" alt="[image of music]" text="image of music" ]
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 heads, 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, multiple Staff contexts can be put into a single
Score context. The Score context is the top level notation context.
[image src="" alt="[image of music]" text="image of music" ]
See also
........
Internals Reference: 
(lilypond-internals)Contexts.
Info Catalog
lilypond-essay: Music representation
lilypond-essay: Building software
lilypond-essay: Flexible architecture