Doc: Essay work, optical sizes and BWV861

[lilypond.git] / Documentation / essay / engraving.itely

@c -*- coding: utf-8; mode: texinfo; -*-

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

    When revising a translation, copy the HEAD committish of the
    version that you are working on.  See TRANSLATION for details.
@end ignore

@c \version "2.13.4"

@node Music engraving
@chapter Music engraving

This section covers the overall goals and architecture of
LilyPond.

@menu
* Engraving::                   
* Automated engraving::         
* What symbols to engrave?::    
* Music representation::        
* Example applications::
* Appendix
@end menu


@node Engraving
@unnumberedsec Engraving

@cindex engraving
@cindex typography, music
@cindex music typography
@cindex plate engraving
@cindex music engraving

The art of music typography is called @emph{(plate) engraving}. The term
derives from the traditional process of music printing. Just a few
decades ago, sheet music was made by cutting and stamping the music into
a zinc or pewter plate in mirror image. The plate would be inked, and
the depressions caused by the cutting and stamping would hold ink. An
image was formed by pressing paper to the plate. The stamping and
cutting was done completely by hand. Making a correction was cumbersome,
so the engraving had to be nearly perfect in one go. Engraving was a
highly specialized skill; a craftsman had to complete around five years
of training before earning the title of master engraver, and another
five years of experience were necessary to become truly skilled.

@quotation
@iftex
@image{pictures/hader-slaan,,7cm}
@end iftex
@ifnottex
@sourceimage{hader-slaan,,,jpg}
@end ifnottex
@end quotation

Nowadays, all newly printed music is produced with computers. This has
obvious advantages: prints are cheaper to make, editorial work can be
delivered by email, and the original data can be easily stored.
Unfortunately, computer-generated scores rarely match the quality of
hand-engraved scores. Instead, computer printouts have a bland,
mechanical look, which makes them unpleasant to play from.

From the beginning, we designed LilyPond to mimic the beauty of
traditional music engraving. Along the way, we have learned a great deal
about the work that goes into a well-engraved score. Below we describe
several of those aspects that we have tried to imitate in LilyPond.

@menu
* Music fonts::
* Optical spacing::
* Ledger lines::
* Optical sizes::
* Why work so hard?::           
@end menu

@node Music fonts
@unnumberedsubsec Music fonts

The images below illustrate some differences between traditional
engraving and typical computer output. The left picture shows a scan of
a flat symbol from a hand-engraved Bärenreiter edition, while the right
picture depicts a symbol from an edition of the same music published in
2000. Although both images are printed in the shame shade of ink, the
earlier version looks darker: the staff lines are heavier, and the
Bärenreiter flat has a bold, almost voluptuous rounded look. The right
scan, on the other hand, has thinner lines and a straight layout with
sharp corners.

@multitable @columnfractions .25 .25 .25 .25
@item @tab
@ifnotinfo
@iftex
@image{pictures/baer-flat-gray,,4cm}
@end iftex
@ifnottex
@image{pictures/baer-flat-gray,,,png}
@end ifnottex

@tab
@iftex
@image{pictures/henle-flat-gray,,4cm}
@end iftex
@ifnottex
@image{pictures/henle-flat-gray,,,png}
@end ifnottex

@end ifnotinfo
@ifinfo
@image{lilypond/pictures/henle-flat-bw,,,,png}
@image{lilypond/pictures/baer-flat-bw,,,,png}
@image{lilypond/pictures/lily-flat-bw,,,,png}
@end ifinfo


@item @tab
Bärenreiter (1950)
@tab
Henle (2000)

@end multitable

@cindex musical symbols
@cindex font
@cindex blackness
@cindex balance

When we wanted to write a computer program to create music typography,
there were no musical fonts freely available that could match the
elegance of our favorite scores. Not let down, we created a font of
musical symbols, relying on nice printouts of hand-engraved music. The
experience helped develop a typographical taste, and it made us
appreciate subtle design details. Without that experience, we would not
have realized how ugly the fonts were that we admired at first.

Below is a sample of two music fonts: the upper set is the default font
in the Sibelius software (the @emph{Opus} font), and the lower set is
our own LilyPond font.

@quotation
@iftex
@image{pictures/pdf/OpusAndFeta,,,}
@end iftex
@ifnottex
@sourceimage{OpusAndFeta,,,png}
@end ifnottex
@end quotation

The LilyPond symbols are heavier and their weight is more consistent,
which makes them easier to read. Fine endings, such as the one on the
bottom of the quarter rest, should not end in sharp points, but rather
in rounded shapes. This is because sharp corners of the punching dies
are fragile and quickly wear out when stamping in metal. Taken together,
the blackness of the font must be carefully tuned together with the
thickness of lines, beams and slurs to give a strong yet balanced
overall impression.

Also, notice that our half-notehead is not elliptic but slightly diamond
shaped. The vertical stem of a flat symbol is slightly brushed, becoming
wider at the top. The sharp and the natural are easier to distinguish
from a distance because their angled lines have different slopes and the
vertical strokes are heavier.

@node Optical spacing
@unnumberedsubsec Optical spacing

In spacing, the distribution of space should reflect the durations
between notes.  However, many modern scores adhere to the
durations with mathematical precision, which leads to poor
results.  In the next example a motive is printed twice: the first time
using exact mathematical spacing, and the second with corrections.
Which do you prefer?

@cindex optical spacing

@lilypond
\paper {
  ragged-right = ##t
  indent = #0.0
}

music = {
   c'4 e''4 e'4 b'4 |
   \stemDown
   b'8[ e'' a' e'']
   \stemNeutral
   e'8[ e'8 e'8 e'8]
}
\score
{
  \music
  \layout {
    \context {
      \Staff
      \override NoteSpacing #'stem-spacing-correction = #0.0
      \override NoteSpacing #'same-direction-correction = #0.0
      \override StaffSpacing #'stem-spacing-correction = #0.0
    }
  }
}
@end lilypond

@lilypond
\paper {
  ragged-right = ##t
  indent = #0.0
}

music = {
   c'4 e''4 e'4 b'4 |
   \stemDown
   b'8[ e'' a' e'']
   \stemNeutral
   e'8[ e'8 e'8 e'8]
}

\score
{
  \music
  \layout {
    \context {
      \Staff
      \override NoteSpacing #'stem-spacing-correction = #0.6
    }
  }
}
@end lilypond

@cindex regular rhythms
@cindex regular spacing
@cindex spacing, regular

Each bar in the fragment only uses notes that are played in a
constant rhythm.  The spacing should reflect that.  Unfortunately,
the eye deceives us a little; not only does it notice the distance
between note heads, it also takes into account the distance
between consecutive stems.  As a result, the notes of an
up-stem/@/down-stem combination should be put farther apart, and
the notes of a down-stem/@/up-stem combination should be put
closer together, all depending on the combined vertical positions
of the notes.  The upper two measures are printed with this
correction, the lower two measures, however, form down-stem/@/up-stem
clumps of notes. A master engraver would adjust the spacing as needed to
please the eye.

Another example of optical spacing is the visual interplay between the
stems and the bar lines. When an up-stem precedes the bar line, a little
more space is needed to keep it from feeling crowded:

@lilypond
\paper {
  ragged-right = ##t
}

\score {
  {
    c''8 c'' c'' c'' c'' c'' c'' c'' \break
    a' a' a' a' a' a' a' a'
  }
  \layout {
    \context {
      \Staff
      \remove "Time_signature_engraver"
      \override NoteSpacing #'stem-spacing-correction = #0.7
    }
  }
}
@end lilypond

@node Ledger lines
@unnumberedsubsec Ledger lines

@cindex ledger lines
@cindex collisions

Ledger lines present a typographical challenge: they make it more
difficult to space musical symbols close together and they must be clear
enough to identify the pitch at a glance. In the example below, we see
that ledger lines should be thicker than normal staff lines and that an
expert engraver will shorten a ledger line to allow closer spacing with
accidentals. We have included this feature in LilyPond's engraving.

@multitable @columnfractions .25 .25 .25 .25
@item @tab

@iftex
@image{pictures/baer-ledger,3cm}
@end iftex
@ifnottex
@sourceimage{baer-ledger,,,png}
@end ifnottex

@tab

@iftex
@image{pictures/lily-ledger,3cm}
@end iftex
@ifnottex
@sourceimage{lily-ledger,,,png}
@end ifnottex

@end multitable


@node Optical sizing
@unnumberedsubsec Optical sizing

Music may need to be printed in a range of sizes. Originally, this was
accomplished by creating punching dies in each of the required sizes,
which meant that each die was designed to look its best at that size.
With the advent of digital fonts, a single outline can be mathematically
scaled to any size, which is very convenient, but at the smaller sizes
the glyphs will appear very light.

In LilyPond, we have created fonts in a range of weights, corresponding
to a range of music sizes. This is a LilyPond engraving at staff size
26:

@quotation
@iftex
@image{pictures/pdf/size26,,23mm}
@end iftex
@ifnottex
@sourceimage{size26,,,png}
@end ifnottex
@end quotation

@noindent
and this is the same engraving set at staff size 11, then
magnified by 236% to print at the same size as the previous example:

@quotation
@iftex
@image{pictures/pdf/size11,,23mm}
@end iftex
@ifnottex
@sourceimage{size11,,,png}
@end ifnottex
@end quotation

At smaller sizes, LilyPond uses proportionally heavier lines and so the
music will still read well. This also allows staves of different
sizes to coexist peacefully when used together on the same page:

@c Grieg's Violin Sonata Op. 45
@lilypond[indent=1.5cm]
global = {
  \time 6/8
  \key c \minor
}

\new Score <<
  \new Staff \with {
      fontSize = #-4
      \override StaffSymbol #'staff-space = #(magstep -4)
      \override StaffSymbol #'thickness = #(magstep -3)
    }
    \relative c' {
      \global
      \set Staff.instrumentName = #"Violin"
      c8.(\f^> b16 c d) ees8.(^> d16 c b)
      g8.(^> b16 c ees) g8-.^> r r
      R2.
    }
  \new PianoStaff <<
    \set PianoStaff.instrumentName = #"Piano"
    \new Staff \relative c' {
      \global
      s2.
      s4. s8 r8 r16 <c f aes c>
      <c f aes c>4.^> <c ees g>8 r r
    }
    \new Staff \relative c {
      \global
      \clef "bass"
      << {
        \once \override DynamicText #'X-offset = #-3
        <ees g c>2.~->^\f
        <ees g c>4.~ <ees g c>8
      } \\ {
        <c g c,>2.~
        <c g c,>4.~ <c g c,>8
      } >>
      r8 r16 <f, c' aes'>16
      <f c' aes'>4.-> <c' g'>8 r r
    }
  >>
>>
@end lilypond

@node Why work so hard?
@unnumberedsubsec Why work so hard?

Musicians are usually more absorbed with performing than with studying
the looks of a piece of music, so nitpicking typographical details may
seem academic. But it is not. Sheet music is performance material:
everything is done to aid the musician in letting her perform better,
and anything that is unclear or unpleasant to read is a hinderance.

Traditionally engraved music uses bold symbols on heavy staff to create
a strong, well-balanced look that stands out well when the music is far
away from the reader: for example, if it is on a music stand. A careful
distribution of white space allows music to be set very tightly without
crowding symbols together. The result minimizes the number of page
turns, which is a great advantage.

This is a common characteristic of typography. Layout should be pretty,
not only for its own sake, but especially because it helps the reader in
his task. For sheet music this is of double importance because musicians
have a limited amount of attention. The less attention they need for
reading, the more they can focus on playing the music. In other words,
better typography translates to better performances.

These examples demonstrate that music typography is an art that is
subtle and complex, and that producing it requires considerable
expertise, which musicians usually do not have.  LilyPond is our
effort to bring the graphical excellence of hand-engraved music to
the computer age, and make it available to normal musicians.  We
have tuned our algorithms, font-designs, and program settings to
produce prints that match the quality of the old editions we love
to see and love to play from.


@node Automated engraving
@unnumberedsec Automated engraving

@cindex engraving, automated
@cindex automated engraving

@menu
* Beauty contests::             
* Notation benchmarking::       
* Flexible architecture::       
@end menu

@node Beauty contests
@unnumberedsubsec Beauty contests

Rather than trying to write rules that describe how to draw an
attractive slur for every possible scenario, we have written rules that
describe an attractive slur and LilyPond finds the nicest possibility.
For example, here are three possible slur configurations, and LilyPond
has given each one a score in `ugly points':

@lilypond
%\layout { #(define debug-slur-scoring #t) }

\relative c {
    \clef bass
    \once \override Slur #'positions = #'(1.5 . 1)
    e8[(_"15.39" f] g[ a b d,)] r4
    \once \override Slur #'positions = #'(2 . 3)
    e8[(_"13.08" f] g[ a b d,)] r4
    e8[(_"12.04" f] g[ a b d,)] r4
}
@end lilypond

@noindent

The first example gets 15.39 points for grazing one of the notes. The
second one is nicer, but the slur doesn't start or end on the noteheads.
It gets 1.71 points for the left side and 9.37 points for the right
side, plus another 2 points because the slur ascends while the melody
descends for a total of 13.08 ulgy points. The final slur gets 10.04
points for the gap on the right and 2 points for the upward slope, but
it is the most attractive of the three configurations, so LilyPond
selects that one.

This technique is quite general, and is used to select beam
configurations, ties and dots in chords, line breaks, and page breaks.

@node Notation benchmarking
@unnumberedsubsec Notation benchmarking

We have tuned LilyPond's layout rules by comparing its output to
hand-engraved scores. Here is one line of a benchmark piece from a
hand-engraved edition (Bärenreiter BA320), and as engraved by LilyPond
1.4:

@iftex
@image{pictures/baer-sarabande-hires,16cm}
@end iftex
@ifnottex
@sourceimage{baer-sarabande,,,png}
@end ifnottex

@iftex
@image{pictures/lily14-sarabande,16cm}
@end iftex
@ifnottex
@sourceimage{lily14-sarabande,,,png}
@end ifnottex

@noindent
On careful inspection, there are a number of errors in the LilyPond 1.4
output:

@itemize @bullet
@item most of the stems are too long
@item the trill marks are too big
@item the second and fourth measures are too narrow
@item the slur is awkward-looking
@item the stems are too thin
@end itemize

@noindent
(There were also two missing notes, and one wrong one!)

By adjusting the layout rules and font design, the output has improved
considerably. This is the same piece, engraved by the current version of
LilyPond (@version{}):

@lilypond[staffsize=19,line-width=15.9\cm]
\relative c {
  \clef "bass"
  \key d \minor
  \time 3/4
  \mergeDifferentlyDottedOn
  << {\slurDashed d8.-\flageolet( e16) e4.-\trill( d16 e)}
     \\ {d4_2 a2}
  >>
  \slurDashed
  <f' a, d,>4. e8( d c)
  \slurSolid
  bes g' f e16( f g_1 a_2 bes_3 d,_2)
  \slurDashed
  cis4.-\trill b8_3( a g)
  << {\slurDashed d'8.( e16) e4.-\trill( d16 e)}
     \\ {<f, a>4 a2}
  >>
}
@end lilypond

[AH: I have not written or edited beyond this point. Here I want to do a
comparison of the last seven measures of Bach's Fugue in G minor from
the Well-Tempered Clavier, Book I, BWV 861. The appendix has all of the
source material, but I have some writing to do. This should demonstrate
LilyPond's excellent output, particularly compared to the default Finale
output, but more fairly than was done before. This will also show that
there are always situations that can be improved with some human
intervention, leading nicely into the next section.]


@node Flexible architecture
@unnumberedsubsec Flexible architecture

How do we go about implementing typography?  If craftsmen need
over ten years to become true masters, how could we simple hackers
ever write a program to take over their jobs?

The answer is: we cannot.  Typography relies on human judgment of
appearance, so people cannot be replaced completely.  However,
much of the dull work can be automated.  If LilyPond solves most
of the common situations correctly, this will be a huge
improvement over existing software.  The remaining cases can be
tuned by hand.  Over the course of years, the software can be
refined to do more and more things automatically, so manual
overrides are less and less necessary.

When we started, we wrote the LilyPond program entirely in the C++
programming language; the program's functionality was set in stone
by the developers.  That proved to be unsatisfactory for a number
of reasons:

@itemize

@item When LilyPond makes mistakes, users need to override
formatting decisions.  Therefore, the user must have access to the
formatting engine.  Hence, rules and settings cannot be fixed by
us at compile-time but must be accessible for users at run-time.

@item Engraving is a matter of visual judgment, and therefore a
matter of taste.  As knowledgeable as we are, users can disagree
with our personal decisions.  Therefore, the definitions of
typographical style must also be accessible to the user.

@item Finally, we continually refine the formatting algorithms, so
we need a flexible approach to rules.  The C++ language forces a
certain method of grouping rules that cannot readily be applied to
formatting music notation.

@end itemize

@cindex Scheme programming language

These problems have been addressed by integrating an interpreter
for the Scheme programming language and rewriting parts of
LilyPond in Scheme.  The current formatting architecture is built
around the notion of graphical objects, described by Scheme
variables and functions.  This architecture encompasses formatting
rules, typographical style and individual formatting decisions.
The user has direct access to most of these controls.

Scheme variables control layout decisions.  For example, many
graphical objects have a direction variable that encodes the
choice between up and down (or left and right).  Here you see two
chords, with accents and arpeggios.  In the first chord, the
graphical objects have all directions down (or left).  The second
chord has all directions up (right).

@lilypond[quote,ragged-right]
\new Score \with {
   \override SpacingSpanner #'spacing-increment = #3
   \override TimeSignature #'transparent = ##t
} \relative c' {
   \stemDown <e g b>4_>-\arpeggio
   \override Arpeggio #'direction = #RIGHT
   \stemUp <e g b>4^>-\arpeggio
}
@end lilypond

@cindex score formatting
@cindex formatting a score
@cindex formatting rules

@noindent
The process of formatting a score consists of reading and writing
the variables of graphical objects.  Some variables have a preset
value.  For example, the thickness of many lines -- a
characteristic of typographical style -- is a variable with a
preset value.  You are free to alter this value, giving your score
a different typographical impression.

@lilypond[quote,ragged-right]
fragment = {
   \clef bass f8 as8
   c'4-~ c'16 as g f e16 g bes c' des'4
}
<<
   \new Staff \fragment
   \new Staff \with {
      \override Beam #'beam-thickness = #0.3
      \override Stem #'thickness = #0.5
      \override Bar #'thickness = #3.6
      \override Tie #'thickness = #2.2
      \override StaffSymbol #'thickness = #3.0
      \override Tie #'extra-offset = #'(0 .  0.3)
      }
      \fragment
>>
@end lilypond

Formatting rules are also preset variables: each object has
variables containing procedures.  These procedures perform the
actual formatting, and by substituting different ones, we can
change the appearance of objects.  In the following example, the
rule governing which note head objects are used to produce the
note head symbol is changed during the music fragment.

@lilypond[quote,ragged-right]
#(set-global-staff-size 30)

#(define (mc-squared grob orig current)
  (let* ((interfaces (ly:grob-interfaces grob))
         (pos (ly:grob-property grob 'staff-position)))
    (if (memq 'note-head-interface interfaces)
        (begin
          (ly:grob-set-property! grob 'stencil
            (grob-interpret-markup grob
              (make-lower-markup 0.5
                (case pos
                  ((-5) "m")
                  ((-3) "c ")
                  ((-2) (make-smaller-markup (make-bold-markup "2")))
                  (else "bla")))))))))

\new Voice \relative c' {
  \stemUp
  \set autoBeaming = ##f
  \time 2/4
  <d f g>4
  \once \override NoteHead #'stencil = #note-head::brew-ez-stencil
  \once \override NoteHead #'font-size = #-7
  \once \override NoteHead #'font-family = #'sans
  \once \override NoteHead #'font-series = #'bold
  <d f g>4
  \once \override NoteHead #'style = #'cross
  <d f g>4
  \applyOutput #'Voice #mc-squared
  <d f g>4
  <<
    { d8[ es-( fis^^ g] fis2-) }
    \repeat unfold 5 { \applyOutput #'Voice #mc-squared s8 }
  >>
}
@end lilypond


@node What symbols to engrave?
@unnumberedsec What symbols to engrave?

@cindex engraving
@cindex typography
@cindex engraver
@cindex plug-in

The formatting process decides where to place symbols.  However,
this can only be done once it is decided @emph{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
@code{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 @code{Note_heads_engraver}.

@lilypond[quote,ragged-right]
\include "engraver-example.ily"

\score {
  \topVoice
  \layout {
    \context {
      \Voice
      \remove "Stem_engraver"
      \remove "Phrasing_slur_engraver"
      \remove "Slur_engraver"
      \remove "Script_engraver"
      \remove "Beam_engraver"
      \remove "Auto_beam_engraver"
    }
    \context {
      \Staff
      \remove "Accidental_engraver"
      \remove "Key_engraver"
      \remove "Clef_engraver"
      \remove "Bar_engraver"
      \remove "Time_signature_engraver"
      \remove "Staff_symbol_engraver"
      \consists "Pitch_squash_engraver"
    }
  }
}
@end lilypond

@noindent
Then a @code{Staff_symbol_engraver} adds the staff,

@lilypond[quote,ragged-right]
\include "engraver-example.ily"

\score {
  \topVoice
  \layout {
    \context {
      \Voice
      \remove "Stem_engraver"
      \remove "Phrasing_slur_engraver"
      \remove "Slur_engraver"
      \remove "Script_engraver"
      \remove "Beam_engraver"
      \remove "Auto_beam_engraver"
    }
    \context {
      \Staff
      \remove "Accidental_engraver"
      \remove "Key_engraver"
      \remove "Clef_engraver"
      \remove "Bar_engraver"
      \consists "Pitch_squash_engraver"
      \remove "Time_signature_engraver"
    }
  }
}
@end lilypond

@noindent
the @code{Clef_engraver} defines a reference point for the staff,

@lilypond[quote,ragged-right]
\include "engraver-example.ily"

\score {
  \topVoice
  \layout {
    \context {
      \Voice
      \remove "Stem_engraver"
      \remove "Phrasing_slur_engraver"
      \remove "Slur_engraver"
      \remove "Script_engraver"
      \remove "Beam_engraver"
      \remove "Auto_beam_engraver"
    }
    \context {
      \Staff
      \remove "Accidental_engraver"
      \remove "Key_engraver"
      \remove "Bar_engraver"
      \remove "Time_signature_engraver"
    }
  }
}
@end lilypond

@noindent
and the @code{Stem_engraver} adds stems.

@lilypond[quote,ragged-right]
\include "engraver-example.ily"

\score {
  \topVoice
  \layout {
    \context {
      \Voice
      \remove "Phrasing_slur_engraver"
      \remove "Slur_engraver"
      \remove "Script_engraver"
      \remove "Beam_engraver"
      \remove "Auto_beam_engraver"
    }
    \context {
      \Staff
      \remove "Accidental_engraver"
      \remove "Key_engraver"
      \remove "Bar_engraver"
      \remove "Time_signature_engraver"
    }
  }
}
@end lilypond

@noindent
The @code{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.

@lilypond[quote,ragged-right]
\include "engraver-example.ily"
\score { \topVoice }
@end lilypond

@cindex polyphony
@cindex engraving multiple voices
@cindex contexts

This system works well for monophonic music, but what about
polyphony?  In polyphonic notation, many voices can share a staff.

@lilypond[quote,ragged-right]
\include "engraver-example.ily"
\new Staff << \topVoice \\ \botVoice >>
@end lilypond

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 @q{Voice context,} while the
engravers for key, accidental, bar, etc., go into a group called
@q{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.


@seealso
Internals Reference: @rinternals{Contexts}.


@lilypond[quote,ragged-right]
\include "engraver-example.ily"
\score {
   <<
      \new Staff << \topVoice \\ \botVoice >>
      \new Staff << \pah \\ \hoom >>
   >>
}
@end lilypond


@node Music representation
@unnumberedsec Music representation

@cindex syntax
@cindex recursive structures

Ideally, the input format for any high-level formatting system is
an abstract description of the content.  In this case, that would
be the music itself.  This poses a formidable problem: how can we
define what music really is? Instead of trying to find an answer,
we have reversed the question.  We write a program capable of
producing sheet music, and adjust the format to be as lean as
possible.  When the format can no longer be trimmed down, by
definition we are left with content itself.  Our program serves as
a formal definition of a music document.

The syntax is also the user-interface for LilyPond, hence it is
easy to type:

@example
@{
  c'4 d'8
@}
@end example

@noindent
to create a quarter note on middle C (C1) and an eighth note on
the D above middle C (D1).

@lilypond[quote]
{
  c'4 d'8
}
@end lilypond

On a microscopic scale, such syntax is easy to use.  On a larger
scale, syntax also needs structure.  How else can you enter
complex pieces like symphonies and operas?  The structure is
formed by the concept of music expressions: by combining small
fragments of music into larger ones, more complex music can be
expressed.  For example

@lilypond[quote,verbatim,fragment,relative=1]
f4
@end lilypond

@noindent
Simultaneous notes can be constructed by enclosing them with
@code{<<} and @code{>>}:

@example
<<c4 d4 e4>>
@end example

@lilypond[quote,fragment,relative=1]
\new Voice { <<c4 d4 e>> }
@end lilypond

@noindent
This expression is put in sequence by enclosing it in curly braces
@code{@{@tie{}@dots{}@tie{}@}}:

@example
@{ f4 <<c4 d4 e4>> @}
@end example

@lilypond[quote,relative=1,fragment]
{ f4 <<c d e4>> }
@end lilypond

@noindent
The above is also an expression, and so it may be combined again
with another simultaneous expression (a half note) using
@code{<<}, @code{\\}, and @code{>>}:

@example
<< g2 \\ @{ f4 <<c4 d4 e4>> @} >>
@end example

@lilypond[quote,fragment,relative=2]
\new Voice { << g2 \\ { f4 <<c d e>> } >> }
@end lilypond

Such recursive structures can be specified neatly and formally in
a context-free grammar.  The parsing code is also generated from
this grammar.  In other words, the syntax of LilyPond is clearly
and unambiguously defined.

User-interfaces and syntax are what people see and deal with most.
They are partly a matter of taste, and also the subject of much
discussion.  Although discussions on taste do have their merit,
they are not very productive.  In the larger picture of LilyPond,
the importance of input syntax is small: inventing neat syntax is
easy, while writing decent formatting code is much harder.  This
is also illustrated by the line-counts for the respective
components: parsing and representation take up less than 10% of
the source code.


@node Example applications
@unnumberedsec Example applications

@cindex simple examples
@cindex examples, simple

We have written LilyPond as an experiment of how to condense the
art of music engraving into a computer program.  Thanks to all
that hard work, the program can now be used to perform useful
tasks.  The simplest application is printing notes.

@lilypond[quote,relative=1]
{
  \time 2/4
  c4 c g'4 g a4 a g2
}
@end lilypond

@noindent
By adding chord names and lyrics we obtain a lead sheet.

@lilypond[quote,ragged-right]
<<
   \chords { c2 c f2 c }
   \new Staff
   \relative c' {
     \time 2/4
     c4 c g' g a a g2
   }
   \addlyrics { twin -- kle twin -- kle lit -- tle star }
>>
@end lilypond

Polyphonic notation and piano music can also be printed.  The
following example combines some more exotic constructs.

@lilypond[quote]
\header {
  title = "Screech and boink"
  subtitle = "Random complex notation"
  composer = "Han-Wen Nienhuys"
}

\score {
  \context PianoStaff <<
    \new Staff = "up" {
      \time 4/8
      \key c \minor
      << {
        \revert Stem #'direction
        \change Staff = down
        \set subdivideBeams = ##t
        g16.[
          \change Staff = up
          c'''32
          \change Staff = down
          g32
          \change Staff = up
          c'''32
          \change Staff = down
          g16]
        \change Staff = up
        \stemUp
        \set followVoice = ##t
        c'''32([ b''16 a''16 gis''16 g''32)]
      } \\ {
        s4 \times 2/3 { d'16[ f' g'] } as'32[ b''32 e'' d'']
      } \\ {
        s4 \autoBeamOff d''8.. f''32
      } \\ {
        s4 es''4
      } >>
    }

    \new Staff = "down" {
      \clef bass
      \key c \minor
      \set subdivideBeams = ##f
      \override Stem  #'french-beaming = ##t
      \override Beam  #'beam-thickness = #0.3
      \override Stem  #'thickness = #4.0
      g'16[ b16 fis16 g16]
      << \makeClusters {
        as16 <as b>
        <g b>
        <g cis>
      } \\ {
        \override Staff.Arpeggio  #'arpeggio-direction =#down
        <cis, e, gis, b, cis>4\arpeggio
      }
    >> }
  >>
  \midi {
    \context {
      \Score
      tempoWholesPerMinute = #(ly:make-moment 60 8)
    }
  }
  \layout {
    \context {
      \Staff
      \consists Horizontal_bracket_engraver
    }
  }
}
@end lilypond

The fragments shown above have all been written by hand, but that
is not a requirement.  Since the formatting engine is mostly
automatic, it can serve as an output means for other programs that
manipulate music.  For example, it can also be used to convert
databases of musical fragments to images for use on websites and
multimedia presentations.

This manual also shows an application: the input format is text,
and can therefore be easily embedded in other text-based formats
such as @LaTeX{}, HTML, or in the case of this manual, Texinfo.
By means of a special program, the input fragments can be replaced
by music images in the resulting PDF or HTML output files.  This
makes it easy to mix music and text in documents.


TODO: add extra chapter for computer aesthetics?

@page
@node Appendix
@unnumberedsec Appendix

This appendix contaions four reference engravings and two
software-engraved versions of Bach's Fugue in G minor from the
Well-Tempered Clavier, Book I, BWV 861 (the last seven measures).

@noindent
Bärenreiter BA5070 (Neue Ausgabe Sämtlicher Werke, Serie V, Band 6.1,
1989):

@iftex
@image{pictures/bwv861-baer,16cm}
@end iftex
@ifnottex
@sourceimage{bwv861-baer-small,,,png}
@end ifnottex

@noindent
Bärenreiter BA5070 (Neue Ausgabe Sämtlicher Werke, Serie V, Band 6.1,
1989), an alternate musical source. Aside from the textual differences,
this demsontrates slight variations in the engraving decisions, even
from the same publisher and edition:

@iftex
@image{pictures/bwv861-baer-alt,16cm}
@end iftex
@ifnottex
@sourceimage{bwv861-baer-alt-small,,,png}
@end ifnottex

@noindent
Breitkopf & Härtel, edited by Ferruccio Busoni (Wiesbaden, 1894), also
available from the Petrucci Music Library (IMSLP #22081). The editorial
markings (fingerings, articulations, etc.) have been removed for clearer
comparison with the other editions here:

@iftex
@image{pictures/bwv861-breitkopf,16cm}
@end iftex
@ifnottex
@sourceimage{bwv861-breitkopf-small,,,png}
@end ifnottex

@noindent
Bach-Gessellschaft edition (Leipzig, 1866), available from the Petrucci
Music Library (IMSPL #02221):

@iftex
@image{pictures/bwv861-gessellschaft,16cm}
@end iftex
@ifnottex
@sourceimage{bwv861-baer-gessellschaft-small,,,png}
@end ifnottex

@noindent
Finale 2008:

@iftex
@image{pictures/pdf/bwv861-finale2008a}
@end iftex
@ifnottex
@sourceimage{bwv-finale2008a,,,png}
@end ifnottex

@noindent
LilyPond, version @version:

@lilypond[staffsize=14.3,line-width=15.9\cm]
global = {\key g \minor}

partI = \relative c' {
  \voiceOne
  fis8 d' ees g, fis4 g
  r8 a16 bes c8 bes16 a d8 r r4
  r2 r8 d16 ees f8 ees16 d
  ees4 ~ ees16 d c bes a4 r8 ees'16 d
  c8 d16 ees d8 e16 fis g8 fis16 g a4 ~
  a8 d, g f ees d c bes
  a2 g\fermata \bar "|."
}

partII = \relative c' {
  \voiceTwo
  d4 r4 r8 d'16 c bes8 c16 d
  ees8 d c ees a, r r4
  r8 fis16 g a8 g16 fis g2 ~
  g2 r8 d' ees g,
  fis4 g r8 a16 bes c8 bes16 a
  bes4. <g b>8 <a c> r <d, g> r
  <ees g>4 <d fis> d2
}
partIII = \relative c' {
  \voiceOne
  r2 r8 d ees g, fis4 g r8 a16 bes c8 bes16 a
  bes2 ~ bes8 b16 a g8 a16 b
  c4 r r2
  R1
  r8 d ees g, fis4 g
  r8 a16 bes c8 bes16 a b2
}
partIV = \relative c {
  \voiceTwo
  d4 r r2
  r8 d ees g, fis4 a
  d,8 d'16 c bes8 c16 d ees2 ~
  ees8 ees16 d c8 d16 ees fis,8 a16 g fis8 g16 a
  d,8 d'16 c bes8 c16 d ees8 c a fis'
  g f ees d c bes a g
  c a d d, g2\fermata
}

\score {
  <<
    % \set Score.barNumberVisibility = #all-bar-numbers-visible
    % required in 2.13
    \set Score.currentBarNumber = #28
    \bar ""
    \new PianoStaff <<
      \new Staff = "RH" <<
        \global
        \new Voice = "voiceI" { \partI }
        \new Voice = "voiceII" { \partII }
      >>

      \new Staff = "LH" <<
        \clef "bass"
        \global
        \new Voice = "voiceIII" { \partIII }
        \new Voice = "voiceIV" { \partIV }
      >>
    >>
  >>
  \layout {
    \context {
      \Staff
      \remove "Time_signature_engraver"
    }
  }
}
@end lilypond