[lilypond.git] / Documentation / user / changing-defaults.itely

@c -*-texinfo-*-
@node Changing defaults
@chapter Changing defaults


The purpose of LilyPond's design is to provide the finest output
quality as a default. Nevertheless, it may happen that you need to
change that default layout.  The layout is controlled through a large
number of proverbial ``knobs and switches.''  This chapter does not
list each and every knob. Rather, it outlines what groups of controls
are available, and how to tune them.

Which controls are available for tuning is described in a separate
document, the @internalsref{Program reference} manual. This manual
lists all different variables, functions and options available in
LilyPond. It is available as a HTML document, which is available
@uref{http://lilypond.org/doc/Documentation/user/out-www/lilypond-internals/,on-line},
but is also included with the LilyPond documentation package.

There are X areas where the default settings may be changed:

@itemize @bullet
@item Output: changing the appearance of individual
  objects. For example, changing stem directions, or the location of
  subscripts.
  
@item Context: changing aspects of the translation from music events to
  notation. For example, giving each staff a separate time signature. 
  
@item Global layout: changing the appearance of the spacing, line
  breaks and page dimensions.
@end itemize

Then, there are separate systems for typesetting text (like
@emph{ritardando}) and selecting different fonts. This chapter also
discusses these.

Internally, LilyPond uses Scheme (a LISP dialect) to provide
infrastructure.  Overriding layout decisions in effect accesses the
program internals, so it is necessary to learn a (very small) subset
of Scheme. That is why this chapter starts with a short tutorial on
entering numbers, lists, strings and symbols in Scheme.


@menu
* Scheme tutorial::             
* Context ::                    
* Fine tuning layout::          
* Tuning output::               
* Text markup::                 
* Global layout::               
* Interpretation context::      
* Output details::              
@end menu

@node Scheme tutorial
@section Scheme tutorial

@cindex Scheme
@cindex GUILE
@cindex Scheme, in-line code
@cindex accessing Scheme
@cindex evaluating Scheme
@cindex LISP

LilyPond uses the Scheme programming language, both as part of the
input syntax, and as internal mechanism to glue together modules of
the program. This section is a very brief overview of entering data in
Scheme.@footnote{If you want to know more about Scheme, see
@uref{http://www.schemers.org}.}

The most basic thing of a language is data: numbers, character
strings, lists, etc. Here is a list of data types that are relevant to
LilyPond input.

@table @asis
@item Booleans
  Boolean values are True or False. The Scheme for True is @code{#t}
  and False is @code{#f}.
@item Numbers
  Numbers are entered in the standard fashion,
  @code{1} is the (integer) number one, while @code{-1.5} is a
  floating point number (a non-integer number). 
@item Strings
  Strings are enclosed in double quotes,
  @example
    "this is a string"
  @end example

  Strings may span several lines
  @example
    "this
    is
    a string"
  @end example

  Quotation marks and newlines can also be added with so-called escape
  sequences. The string @code{a said "b"} is entered as
  @example
    "a said \"b\""
  @end example

  Newlines and backslashes are escaped with @code{\n} and @code{\\}
respectively.
@end table


In a music file, snippets of Scheme code are introduced with the hash
mark @code{#}. So, the previous examples translated in LilyPondese are

@example
  ##t ##f 
  #1 #1.5
  #"this is a string"
  #"this
  is
  a string"
@end example

For the rest of this section, we will assume that the data is entered
in a music file, so we add @code{#}s everywhere.

Scheme can be used to do calculations. It uses @emph{prefix}
syntax. Adding 1 and 2 is written as @code{(+ 1 2)} rather than the
traditional 1+2.

@lisp
  #(+ 1 2)
   @result{} #3 
@end lisp

The arrow @result{} shows that the result of evaluating @code{(+ 1 2)}
is @code{3}.  Calculations may be nested: the result of a function may
be used for another calculation.

@lisp
  #(+ 1 (* 3 4))
   @result{} #(+ 1 12) 
   @result{} #13
@end lisp

These calculations are examples of evaluations: an expression (like
@code{(* 3 4)} is replaced by its value @code{12}. A similar thing
happens with variables. After defining a variable  

@example
  twelve = #12 
@end example

variables can also be used in expressions, here

@example
  twentyFour = #(* 2 twelve) 
@end example 

the number 24 is stored in the variable @code{twentyFour}.

The @emph{name} of a variable is also an expression, similar to a
number or a string. It is entered as

@example
  #'twentyFour
@end example

The quote mark @code{'} prevents Scheme interpreter from substituting
@code{24} for the @code{twentyFour}. Instead, we get the name
@code{twentyFour}.

This syntax will be used very frequently, since many of the layout
tweaks involve assigning (Scheme) values to internal variables, for
example

@example
  \override Stem #'thickness = #2.6
@end example

This instruction adjusts the appearance of stems. The value @code{2.6}
is put into a the @code{thickness} variable of a @code{Stem}
object. This makes stems almost twice as thick as their normal size.
To distinguish between variables defined in input files (like
@code{twentyFour} in the example above), and internal variables, we
will call the latter ``properties.'' So, the stem object has a
@code{thickness} property.

Two-dimensional offsets (X and Y coordinates) as well as object sizes
(intervals with a left and right point) are entered as @code{pairs}. A
pair@footnote{In Scheme terminology, the pair is called @code{cons},
and its two elements are called car and cdr respectively.}  is entered
as @code{(first . second)}, and like symbols, they must be quoted,

@example
  \override TextScript #'extra-offset = #'(1 . 2)  
@end example 

This assigns the pair (1, 2) to @code{extra-offset} variable of the
TextScript object. This moves the object 1 staff space to the right,
and 2 spaces up.

The two elements of a pair may be arbitrary values, for example

@example
  #'(1 . 2)
  #'(#t . #f)
  #'("blah-blah" . 3.14159265)
@end example

A list is entered by enclosing its elements in parentheses, and adding
a quote. For example,
@example
  #'(1 2 3)
  #'(1 2 "string" #f)
@end example

We have been using lists all along.  A calculation, like @code{(+ 1
2)} is also a list (containing the symbol @code{+} and the numbers 1
and 2). For entering lists, use a quote @code{'} and for
calculations, do not use a quote. 

Inside a quoted list or pair, there is no need to quote anymore.  The
following is a pair of symbols, a list of symbols and a list of lists
respectively,

@example
  #'(stem . head)
  #'(staff clef key-signature)
  #'((1) (2))
@end example


@node Context 
@section Context

When music is printed, a lot of things notation elements must be added
to the input, which is often bare bones.  For example, compare the
input and output of the following example

@lilypond[verbatim,relative=2]
  cis4 cis2. g4
@end lilypond

The input is rather sparse, but in the output, bar lines, accidentals,
clef and time signature are added. LilyPond @emph{interprets} the
input. During this step, the musical information is inspected in time
order, similar to reading a score from left to right. While reading,
the program remembers where measure boundaries are, and what pitches
need explicit accidentals. 

This information can be present on several levels.  For example, the
effect of an accidental is limited to a single stave, while a bar line
must be synchronized across the entire score.  To match this
hierarchy, LilyPond's interpretation step is hierarchical.  There are
interpretation contexts, like @context{Voice}, Staff and Score, and each level
can maintain its own properties.

Full description of all available contexts is in the program
reference, see
@ifhtml
@internalsref{Contexts}
@end ifhtml.
@ifnothtml 
Translation @arrow{} Context.
@end ifnothtml

@menu
* Creating contexts::           
* Changing context properties on the fly ::  
* Modifying context plug-ins::  
* Defining context defaults ::  
* which properties to change::  
@end menu

@node Creating contexts
@subsection Creating contexts

For simple scores, the correct contexts are created automatically. For
more complex scores, it is necessary to instantiate them by hand.
There are three commands to do this.

The easiest command is @code{\new}, and it also the quickest to type.
It is prepended to a  music expression, for example

@example
  \new @var{type} @var{music expression}
@end example

@noindent
where @var{type} is a context name (like @code{Staff} or
@code{Voice}).  This command creates a new context, and starts
interpreting @var{music expression} with that.

A practical application of @code{\new} is a score with many
staves. Each part that should be on its own staff, gets a @code{\new
Staff}.

@lilypond[verbatim,relative=2,raggedright]
  << \new Staff { c4 c }
     \new Staff { d4 d }
  >>
@end lilypond


The @code{\context} command also directs a music expression to a
context object, but gives the context an extra name. The syntax is

@example
  \context @var{type} = @var{id} @var{music}
@end example

This form will search for an existing context of type @var{type}
called @var{id}. If that context does not exist yet, it is created.
This is useful if the context referred to later on. For example, when
setting lyrics the melody is in a named context

@example
 \context Voice = "@b{tenor}" @var{music}
@end example

@noindent
so the texts can be properly aligned to its notes,

@example
\new Lyrics \lyricsto "@b{tenor}" @var{lyrics} 
@end example

@noindent

Another possibility is funneling two different music expressions into
one context. In the following example, articulations and notes are
entered separately,

@example
music = \notes { c4 c4 }
arts = \notes  { s4-. s4-> }
@end example

They are combined by sending both to the same @context{Voice} context,

@example
  << \new Staff \context Voice = "A" \music
     \context Voice = "A" \arts
  >>
@end example
@lilypond[raggedright]
music = \notes \context Voice = "one" { c4 c4 }
arts = \notes \context Voice = "one"  { s4-. s4-> }
\score {
  << \new Staff \context Voice = "A" \music
     \context Voice = "A" \arts
  >>
} 
@end lilypond



The third command for creating contexts is
@example
  \context @var{type} @var{music}
@end example


@noindent
It is similar to @code{\context} with @code{= @var{id}}, but now it
matches any context of type @var{type}, regardless of its given name.

This variant is used with music expressions that can be interpreted at
several levels. For example, the @code{\applyoutput} command (see
@ref{Running a function on all layout objects}). Without an explicit
@code{\context}, it is usually is applied to @context{Voice}

@example
  \applyoutput #@var{function}   % apply to Voice
@end example

To have it interpreted at @context{Score} or @context{Staff} level use
these forms

@example
  \context Score \applyoutput #@var{function}
  \context Staff \applyoutput #@var{function}
@end example


@node Changing context properties on the fly 
@subsection Changing context properties  on the fly

Each context can have different @emph{properties}, variables contained
in that context. They can be changed during the interpretation step.
This is achieved by inserting the @code{\set} command in the music,

@quotation
  @code{\set }[@var{context}]@code{.}@var{prop}@code{ = #}@var{value} 
@end quotation

For example,
@lilypond[verbatim,relative=2]
  R1*2 
  \set Score.skipBars = ##t
  R1*2
@end lilypond

This command skips measures that have no notes. The result is that
multi rests are condensed.  The value assigned is a Scheme object. In
this case, it is @code{#t}, the boolean True value.

If the @var{context} argument is left out, then the current
bottom-most context (typically ChordNames, @context{Voice} or Lyrics) is used.
In this example,

@lilypond[verbatim,relative=2]
  c8 c c c
  \set autoBeaming = ##f
  c8 c c c
@end lilypond

@noindent
the @var{context} argument to @code{\set} is left out, and the current
@internalsref{Voice} is used.  Contexts are hierarchical, so if a
bigger context was specified, for example @code{Staff}, then the
change would apply to all Voices in the current stave. The change is
applied `on-the-fly', during the music, so that the setting only
affects the second group of eighth notes.

There is also an @code{\unset} command,
@quotation
  @code{\set }[@var{context}]@code{.}@var{prop}
@end quotation

@noindent
which removes the definition of @var{prop}. This command only removes
the definition if it is set in @var{context}. In

@example
  \set Staff.autoBeaming = ##f
  \unset Voice.autoBeaming
@end example

@noindent
the current voice does not have the property, and the definition at
staff level remains intact.

Settings that should only apply to a single time-step can be entered
easily with @code{\once}, for example in

@lilypond[verbatim,relative=2]
  c4
  \once \set fontSize = #4.7
  c4
  c4
@end lilypond

@code{fontSize} is unset after the third note.

A full description of all available context properties is in the
program reference, see
@ifhtml
@internalsref{Tunable-context-properties}.
@end ifhtml
@ifnothtml
Translation @arrow{} Tunable context properties.
@end ifnothtml


@node Modifying context plug-ins
@subsection 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
@code{Note_head_engraver} and the Staff context contains a
@code{Key_signature_engraver}.

For a full a description of each plug-in, see 
@ifhtml
@internalsref{Engravers}
@end ifhtml
@ifnothtml
Program reference @arrow Translation @arrow{} Engravers.
@end ifnothtml
Every context described in
@ifhtml
@internalsref{Contexts}
@end ifhtml.
@ifnothtml 
Program reference @arrow Translation @arrow{} Context.
@end ifnothtml
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 @code{\new} or @code{\context}, and
modifying it like this, 

@example
 \new @var{context} \with @{
   \consists @dots{}
   \consists @dots{}
   \remove  @dots{}
   \remove @dots{}
   @emph{etc.}
 @}
 @var{..music..}
@end example

where the @dots{} should be the name of an engraver. Here is a simple
example which removes @code{Time_signature_engraver} and
@code{Clef_engraver} from a @code{Staff} context,

@lilypond[relative=1, verbatim]
<< \new Staff {
    f2 g
  }
  \new Staff \with {
     \remove Time_signature_engraver
     \remove Clef_engraver
  } {
    f2 g2
  }
>>
@end example

In the second stave there are no time signature or clef symbols.  This
is a rather crude method of making objects disappear, it will affect
the entire staff. More sophisticated method is shown in TODO.

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

@cindex polymetric scores


@lilypond[relative=1,raggedright,verbatim]
\new Score \with {
  \remove Timing_engraver
} <<
  \new Staff \with {
    \consists Timing_engraver
  } {
      \time 3/4
      c4 c c c c c
  }
  \new Staff \with {
    \consists Timing_engraver {
  } {
       \time 2/4
       c4 c c c c c
  }
  >>
@end lilypond


@node Defining context defaults 
@subsection Defining context defaults

Context properties can be set as defaults, within the
@code{\paper} block. For example, 

@verbatim
\paper {
  \context {
    \ScoreContext
    skipBars = ##t
  }
}
@end verbatim

@noindent
will set skipBars default 

When This    score-wide


@node which properties to change
@subsection which properties to change


There are many different properties.  Not all of them are listed in
this manual. However, the program reference lists them all in the
section @internalsref{Context-properties}, and most properties are
demonstrated in one of the
@ifhtml
@uref{../../../input/test/out-www/collated-files.html,tips-and-tricks}
@end ifhtml
@ifnothtml
tips-and-tricks
@end ifnothtml
examples.


@node Fine tuning layout
@section Fine tuning layout

Sometimes it is necessary to change music layout by hand.  When music
is formatted, layout objects are created for each symbol.  For
example, every clef and every note head is represented by a layout
object.  These layout objects also carry variables, which we call
@emph{layout properties}. By changing these variables from their
values, we can alter the look of a formatted score:

@lilypond[verbatim,relative]
  c4
  \override Stem #'thickness = #3.0
  c4 c4 c4 
@end lilypond

@noindent
In the example shown here, the layout property @code{thickness} (a
symbol) is set to 3 in the @code{Stem} layout objects of the current
As a result, the notes following @code{\override} have thicker
stems.

For the most part, a manual override is needed only on a case by
case basis and not for all subsequent instances of the altered
property. To accomplish this, simply prefix @code{\once} to the
@code{\override} statement and the override will apply only once,
immediately reverting to its default setting, i.e.

@example
 \once \override Stem #'thickness = #3.0
@end example

@lilypond[relative]
  c4
  \once \override Stem #'thickness = #3.0
  c4 c4 c4 
@end lilypond

@noindent
Some overrides are so common that predefined commands are provided as
a short cut.  For example, @code{\slurUp} and @code{\stemDown}. These
commands are described in
@ifhtml
the
@end ifhtml
@ref{Notation manual}, under the sections for slurs and stems
respectively.

The exact tuning possibilities for each type of layout object are
documented in the program reference of the respective
object. However, many layout objects share properties, which can be
used to apply generic tweaks.  We mention a couple of these:

@itemize @bullet
@item The @code{extra-offset} property, which
@cindex @code{extra-offset}
has a pair of numbers as value, moves around objects in the printout.
The first number controls left-right movement; a positive number will
move the object to the right.  The second number controls up-down
movement; a positive number will move it higher.  The units of these
offsets are staff-spaces.  The @code{extra-offset} property is a
low-level feature: the formatting engine is completely oblivious to
these offsets.

In the following example, the second fingering is moved a little to
the left, and 1.8 staff space downwards:

@cindex setting object properties

@lilypond[relative=1,verbatim]
\stemUp
f-5
\once \override Fingering
    #'extra-offset = #'(-0.3 . -1.8) 
f-5
@end lilypond

@item
Setting the @code{transparent} property will cause an object to be printed
in `invisible ink': the object is not printed, but all its other
behavior is retained. The object still takes up space, it takes part in
collisions, and slurs, and ties and beams can be attached to it.

@cindex transparent objects
@cindex removing objects
@cindex invisible objects
The following example demonstrates how to connect different voices
using ties. Normally, ties only connect two notes in the same
voice. By introducing a tie in a different voice, and blanking a stem
in that voice, the tie appears to cross voices:

@lilypond[fragment,relative=1,verbatim]
  c4 << {
      \once \override Stem #'transparent = ##t
      b8~ b8
  } \\ {
       b[ g8]
  } >>
@end lilypond

@item
The @code{padding} property for objects with
@cindex @code{padding}
@code{side-position-interface} can be set to increase distance between
symbols that are printed above or below notes. We only give an
example; a more elaborate explanation is in @ref{Constructing a
tweak}:

@lilypond[relative=1,verbatim]
  c2\fermata
  \override Script #'padding = #3
  b2\fermata
@end lilypond

@end itemize

More specific overrides are also possible.  The notation manual
discusses in depth how to figure out these statements for yourself, in
@ref{Tuning output}.




@node Tuning output
@section Tuning output

There are situations where default layout decisions are not
sufficient.  In this section we discuss ways to override these
defaults.

Formatting is internally done by manipulating so called objects
(graphic objects). Each object carries with it a set of properties
(object or layout properties) specific to that object.  For example, a
stem object has properties that specify its direction, length and
thickness.

The most direct way of tuning the output is by altering the values of
these properties. There are two ways of doing that: first, you can
temporarily change the definition of one type of object, thus
affecting a whole set of objects.  Second, you can select one specific
object, and set a layout property in that object.

Do not confuse layout properties with translation
properties. Translation properties always use a mixed caps style
naming, and are manipulated using @code{\set} and @code{\unset}: 
@example
  \set Context.propertyName = @var{value}
@end example

Layout properties are use Scheme style variable naming, i.e.  lower
case words separated with dashes. They are symbols, and should always
be quoted using @code{#'}.  For example, this could be an imaginary
layout property name:
@example
  #'layout-property-name
@end example


@menu
* Tuning objects::              
* Constructing a tweak::        
* Selecting font sizes::        
* Font selection::              
@end menu



@node Tuning objects
@subsection Tuning objects 

@cindex object description

The definition of an object is a list of default object
properties. For example, the definition of the Stem object (available
in @file{scm/define-grobs.scm}), includes the following definitions
for @internalsref{Stem}:

@example
        (thickness . 1.3)
        (beamed-lengths . (3.5 3.5 3.5 4.5 5.0))
        (Y-extent-callback . ,Stem::height)
        @var{...}
@end example


Adding variables on top of this existing definition overrides the
system default, and alters the resulting appearance of the layout
object.

@syntax


Changing a variable for only one object is commonly achieved with
@code{\once}:

@example
\once \override @var{context}.@var{objectname}
    @var{symbol} = @var{value}
@end example
Here @var{symbol} is a Scheme expression of symbol type, @var{context}
and @var{objectname} is a string and @var{value} is a Scheme expression.
This command applies a setting only during one moment in the score.

In the following example, only one @internalsref{Stem} object is
changed from its original setting:

@lilypond[verbatim,fragment,relative=1]
  c4 
  \once \override Voice.Stem #'thickness = #4
  c4
  c4
@end lilypond
@cindex @code{\once}

For changing more objects, the same command, without @code{\once} can
be used:
@example
\override @var{context}.@var{objectname}   @var{symbol} = @var{value}
@end example
This command adds @code{@var{symbol} = @var{value}} to the definition
of @var{objectname} in the context @var{context}, and this definition
stays in place until it is removed.

An existing definition may be removed by the following command:
@c
@example
\property @var{context}.@var{objectname} \revert @var{symbol}
@end example
@c

Some examples: 
@lilypond[verbatim]
c'4 \override Stem   #'thickness = #4.0
c'4
c'4 \revert Stem #'thickness
c'4
@end lilypond



Reverting a setting which was not set in the first place has no
effect.


@seealso

Internals: @internalsref{OverrideProperty}, @internalsref{RevertProperty},
@internalsref{PropertySet}, @internalsref{All-backend-properties}, and
@internalsref{All-layout-objects}.


@refbugs

The back-end is not very strict in type-checking object properties.
Cyclic references in Scheme values for properties can cause hangs
and/or crashes.


@node Constructing a tweak
@subsection Constructing a tweak


@cindex internal documentation
@cindex finding graphical objects
@cindex graphical object descriptions 
@cindex tweaking
@cindex @code{\override}
@cindex @code{\set}
@cindex internal documentation



Three pieces of information are required to use @code{\override} and
@code{\set}: the name of the layout object, the context and the name
of the property.  We demonstrate how to glean this information from
the notation manual and the program reference.

The generated documentation is a set of HTML pages which should be
included if you installed a binary distribution, typically in
@file{/usr/share/doc/lilypond}.  They are also available on the web:
go to the @uref{http://lilypond.org,LilyPond website}, click
``Documentation'', select the correct version, and then click
``Program reference.'' It is advisable to bookmark the local HTML
files. They will load faster than the ones on the web and matches the
version of LilyPond you are using.
 


@c  [TODO: revise for new site.]

Suppose we want to move the fingering indication in the fragment
below:

@lilypond[relative=2,verbatim]
c-2
\stemUp
f
@end lilypond

If you visit the documentation of @code{Fingering} (in @ref{Fingering
instructions}), you will notice that there is written:

@quotation
@seealso

Internals: @internalsref{FingerEvent} and @internalsref{Fingering}.

@end quotation

@separate

@noindent
In other words, the fingerings once entered, are internally stored as
@code{FingerEvent} music objects. When printed, a @code{Fingering}
layout object is created for every @code{FingerEvent}.

The Fingering object has a number of different functions, and each of
those is captured in an interface. The interfaces are listed under
@internalsref{Fingering} in the program reference.



The @code{Fingering} object has a fixed size
(@internalsref{item-interface}), the symbol is a piece of text
(@internalsref{text-interface}), whose font can be set
(@internalsref{font-interface}).  It is centered horizontally
(@internalsref{self-alignment-interface}), it is placed vertically
next to other objects (@internalsref{side-position-interface}), and
its placement is coordinated with other scripts
(@internalsref{text-script-interface}).  It also has the standard
@internalsref{grob-interface} (grob stands for Graphical object)
@cindex grob
@cindex graphical object
@cindex layout object
@cindex object, layout 
with all the variables that come with
it.  Finally, it denotes a fingering instruction, so it has
@internalsref{finger-interface}.

For the vertical placement, we have to look under
@code{side-position-interface}:
@quotation
@code{side-position-interface}

  Position a victim object (this one) next to other objects (the
  support).  In this case, the property @code{direction} signifies where to put the
  victim object relative to the support (left or right, up or down?)
@end quotation

@cindex padding
@noindent
below this description, the variable @code{padding} is described as
@quotation
@table @code
@item padding
 (dimension, in staff space)

   add this much extra space between objects that are next to each
other. Default value: @code{0.6}
@end table
@end quotation

By increasing the value of @code{padding}, we can move away the
fingering.  The following command inserts 3 staff spaces of white
between the note and the fingering:
@example
\once \override Fingering   #'padding = #3
@end example

Inserting this command before the Fingering object is created,
i.e. before @code{c2}, yields the following result:

@lilypond[relative=2,fragment,verbatim]
\once \override Fingering
    #'padding = #3
c-2
\stemUp
f
@end lilypond

The context name @code{Voice} in the example above can be determined
as follows. In the documentation for @internalsref{Fingering}, it says
@quotation
Fingering grobs are created by: @internalsref{Fingering_engraver} @c
@end quotation

Clicking @code{Fingering_engraver} shows the documentation of
the module responsible for interpreting the fingering instructions and
translating them to a @code{Fingering} object.  Such a module is called
an @emph{engraver}.  The documentation of the @code{Fingering_engraver}
says
@example
Fingering_engraver is part of contexts: Voice 
@end example
so tuning the settings for Fingering should be done with
@example
  \override Fingering @dots{}
@end example

Of course, the tweak may also done in a larger context than
@code{Voice}, for example, @internalsref{Staff} or
@internalsref{Score}.

@seealso

Internals: the program reference also contains alphabetical lists of
@internalsref{Contexts}, @internalsref{All-layout-objects} and
@internalsref{Music-expressions}, so you can also find which objects
to tweak by browsing the internals document.


@node Selecting font sizes
@subsection Selecting font sizes

The most common thing to change about the appearance of fonts is their
size. The font size of any context can be easily changed by setting
the @code{fontSize} property for that context.  Its value is a number:
negative numbers make the font smaller, positive numbers larger. An
example is given below:
@c
@lilypond[fragment,relative=1,verbatim]
  c4 c4 \set fontSize = #-3
  f4 g4
@end lilypond
This command will set @code{font-size} (see below) in all layout
objects in the current context. It does not change the size of
variable symbols, such as beams or slurs.

The font size is set by modifying the @code{font-size} property.  Its
value is a number indicating the size relative to the standard size.
Each step up is an increase of approximately 12% of the font size. Six
steps is exactly a factor two. The Scheme function @code{magstep}
converts a @code{font-size} number to a scaling factor.

LilyPond has fonts in different design sizes: the music fonts for
smaller sizes are chubbier, while the text fonts are relatively wider.
Font size changes are achieved by scaling the design size that is
closest to the desired size.

The @code{font-size} mechanism does not work for fonts selected
through @code{font-name}. These may be scaled with
@code{font-magnification}.


One of the uses of @code{fontSize} is to get smaller symbols for cue
notes. An elaborate example of those is in
@inputfileref{input/test,cue-notes.ly}.

@cindex @code{font-style}

@refcommands

The following commands set @code{fontSize} for the current voice.

@cindex @code{\tiny}
@code{\tiny}, 
@cindex @code{\small}
@code{\small}, 
@cindex @code{\normalsize}
@code{\normalsize}.



@cindex magnification
@cindex cue notes


@node Font selection
@subsection Font selection

Font selection for the standard fonts, @TeX{}'s Computer Modern fonts,
can also be adjusted with a more fine-grained mechanism.  By setting
the object properties described below, you can select a different font;
all three mechanisms work for every object that supports
@code{font-interface}:


@itemize @bullet
@item @code{font-encoding}
is a symbol that sets layout of the glyphs. Choices include
@code{text} for normal text, @code{braces} (for piano staff braces),
@code{music} (the standard music font, including ancient glyphs),
@code{dynamic} (for dynamic signs) and @code{number} for the number
font.


@item @code{font-family}
 is a symbol indicating the general class of the typeface.  Supported are
@code{roman} (Computer Modern), @code{sans} and @code{typewriter}
  
@item @code{font-shape}
  is a symbol indicating the shape of the font, there are typically
several font shapes available for each font family. Choices are
@code{italic}, @code{caps} and @code{upright}.

@item @code{font-series}
is a  symbol indicating the series of the font. There are typically several
font series for each font family and shape. Choices are @code{medium}
and @code{bold}. 

@end itemize

Fonts selected in the way sketched above come from a predefined style
sheet.

 The font used for printing a object can be selected by setting
@code{font-name}, e.g.
@example
  \override Staff.TimeSignature
      #'font-name = #"cmr17"
@end example

@noindent
Any font can be used, as long as it is available to @TeX{}. Possible
fonts include foreign fonts or fonts that do not belong to the
Computer Modern font family.  The size of fonts selected in this way
can be changed with the @code{font-magnification} property.  For
example, @code{2.0} blows up all letters by a factor 2 in both
directions.

@cindex font size
@cindex font magnification



@seealso

Init files: @file{ly/declarations-init.ly} contains hints how new
fonts may be added to LilyPond.

@refbugs

No style sheet is provided for other fonts besides the @TeX{}
Computer Modern family.

@cindex font selection
@cindex font magnification
@cindex @code{font-interface}


@node Text markup
@section Text markup
@cindex text markup
@cindex markup text


@cindex typeset text

LilyPond has an internal mechanism to typeset texts. You can access it
with the keyword @code{\markup}. Within markup mode, you can enter texts
similar to lyrics: simply enter them, surrounded by spaces:
@cindex markup

@lilypond[verbatim,fragment,relative=1]
 c1^\markup { hello }
 c1_\markup { hi there }
 c1^\markup { hi \bold there, is \italic anyone home? }
@end lilypond

@cindex font switching

The markup in the example demonstrates font switching commands.  The
command @code{\bold} and @code{\italic} only apply to the first
following word; enclose a set of texts with braces to apply a command
to more words:
@example
  \markup @{ \bold @{ hi there @} @}
@end example

@noindent
For clarity, you can also do this for single arguments, e.g.

@verbatim
  \markup { is \italic { anyone } home }
@end verbatim

@cindex font size, texts


In markup mode you can compose expressions, similar to mathematical
expressions, XML documents and music expressions.  The braces group
notes into horizontal lines. Other types of lists also exist: you can
stack expressions grouped with @code{<}, and @code{>} vertically with
the command @code{\column}. Similarly, @code{\center-align} aligns
texts by their center lines:

@lilypond[verbatim,fragment,relative=1]
 c1^\markup { \column < a bbbb c > }
 c1^\markup { \center-align < a bbbb c > }
 c1^\markup { \line < a b c > }
@end lilypond


Markups can be stored in variables, and these variables
may be attached to notes, like
@verbatim
allegro = \markup { \bold \large { Allegro } }
\notes { a^\allegro b c d }
@end verbatim


Some objects have alignment procedures of their own, which cancel out
any effects of alignments applied to their markup arguments as a
whole.  For example, the @internalsref{RehearsalMark} is horizontally
centered, so using @code{\mark \markup @{ \left-align .. @}} has no
effect.

Similarly, for moving whole texts over notes with
@code{\raise}, use the following trick:
@example
  "" \raise #0.5 raised
@end example

The text @code{raised} is now raised relative to the empty string
@code{""} which is not visible.  Alternatively, complete objects can
be moved with layout properties such as @code{padding} and
@code{extra-offset}.



@seealso

Init files:  @file{scm/new-markup.scm}.


@refbugs

Text layout is ultimately done by @TeX{}, which does kerning of
letters.  LilyPond does not account for kerning, so texts will be
spaced slightly too wide.

Syntax errors for markup mode are confusing.

Markup texts cannot be used in the titling of the @code{\header}
field. Titles are made by La@TeX{}, so La@TeX{} commands should be used
for formatting.



@menu
* Overview of text markup commands::  
@end menu

@node  Overview of text markup commands
@subsection Overview of text markup commands

@include markup-commands.tely


@node Global layout
@section Global layout

The global layout determined by three factors: the page layout, the
line breaks and the spacing. These all influence each other. The
choice of spacing determines how densely each system of music is set,
which influences where line breaks breaks are chosen, and thus
ultimately how many pages a piece of music takes. This section
explains how to tune the algorithm for spacing.

Globally spoken, this procedure happens in three steps: first,
flexible distances (``springs'') are chosen, based on durations. All
possible line breaking combination are tried, and the one with the
best results---a layout that has uniform density and requires as
little stretching or cramping as possible---is chosen. When the score
is processed by @TeX{}, each page is filled with systems, and page breaks
are chosen whenever the page gets full.



@menu
* Vertical spacing::            
* Horizontal spacing::          
* Font Size::                   
* Line breaking::               
* Page layout::                 
@end menu


@node Vertical spacing
@subsection Vertical spacing

@cindex vertical spacing
@cindex distance between staves
@cindex staff distance
@cindex between staves, distance
@cindex staves per page
@cindex space between staves

The height of each system is determined automatically by LilyPond, to
keep systems from bumping into each other, some minimum distances are
set.  By changing these, you can put staves closer together, and thus
put more  systems onto one page.

Normally staves are stacked vertically. To make
staves maintain a distance, their vertical size is padded. This is
done with the property @code{minimumVerticalExtent}. It takes a pair
of numbers, so if you want to make it smaller from its, then you could
set
@example
  \set Staff.minimumVerticalExtent = #'(-4 . 4)
@end example
This sets the vertical size of the current staff to 4 staff spaces on
either side of the center staff line.  The argument of
@code{minimumVerticalExtent} is interpreted as an interval, where the
center line is the 0, so the first number is generally negative.  The
staff can be made larger at the bottom by setting it to @code{(-6
. 4)}.

The piano staves are handled a little differently: to make cross-staff
beaming work correctly, it is necessary that the distance between staves
is fixed beforehand.  This is also done with a
@internalsref{VerticalAlignment} object, created in
@internalsref{PianoStaff}. In this object the distance between the
staves is fixed by setting @code{forced-distance}. If you want to
override this, use a @code{\context} block as follows:
@example
  \paper @{
    \context @{
      \PianoStaffContext
      \override VerticalAlignment #'forced-distance = #9
    @}
    @dots{}
  @}
@end example
This would bring the staves together at a distance of 9 staff spaces,
measured from the center line of each staff.

@seealso

Internals: Vertical alignment of staves is handled by the
@internalsref{VerticalAlignment} object.




@node Horizontal spacing
@subsection Horizontal Spacing

The spacing engine translates differences in durations into
stretchable distances (``springs'') of differing lengths. Longer
durations get more space, shorter durations get less.  The shortest
durations get a fixed amount of space (which is controlled by
@code{shortest-duration-space} in the @internalsref{SpacingSpanner} object). 
The longer the duration, the more space it gets: doubling a
duration adds a fixed amount (this amount is controlled by
@code{spacing-increment}) of space to the note.

For example, the following piece contains lots of half, quarter and
8th notes, the eighth note is followed by 1 note head width (NHW). 
The quarter note is followed by 2 NHW, the half by 3 NHW, etc.
@lilypond[fragment,verbatim,relative=1] c2 c4. c8 c4. c8 c4. c8 c8
c8 c4 c4 c4
@end lilypond

Normally, @code{shortest-duration-space} is set to 1.2, which is the
width of a note head, and @code{shortest-duration-space} is set to
2.0, meaning that the shortest note gets 2 NHW (i.e. 2 times
@code{shortest-duration-space}) of space. For normal notes, this space
is always counted from the left edge of the symbol, so the shortest
notes are generally followed by one NHW of space.

If one would follow the above procedure exactly, then adding a single
32th note to a score that uses 8th and 16th notes, would widen up the
entire score a lot. The shortest note is no longer a 16th, but a 32nd,
thus adding 1 NHW to every note. To prevent this, the
shortest duration for spacing is not the shortest note in the score,
but the most commonly found shortest note.  Notes that are even
shorter this are followed by a space that is proportional to their
duration relative to the common shortest note.  So if we were to add
only a few 16th notes to the example above, they would be followed by
half a NHW:

@lilypond[fragment,verbatim,relative=2]
 c2 c4. c8 c4. c16[ c] c4. c8 c8 c8 c4 c4 c4
@end lilypond

The most common shortest duration is determined as follows: in every
measure, the shortest duration is determined. The most common short
duration, is taken as the basis for the spacing, with the stipulation
that this shortest duration should always be equal to or shorter than
1/8th note. The shortest duration is printed when you run lilypond
with @code{--verbose}.  These durations may also be customized. If you
set the @code{common-shortest-duration} in
@internalsref{SpacingSpanner}, then this sets the base duration for
spacing. The maximum duration for this base (normally 1/8th), is set
through @code{base-shortest-duration}.

@cindex @code{common-shortest-duration}
@cindex @code{base-shortest-duration}
@cindex @code{stem-spacing-correction}
@cindex @code{spacing}

In the introduction it was explained that stem directions influence
spacing. This is controlled with @code{stem-spacing-correction}
property in @internalsref{NoteSpacing}, which are generated for every
@internalsref{Voice} context. The @code{StaffSpacing} object
(generated at @internalsref{Staff} context) contains the same property
for controlling the stem/bar line spacing. The following example
shows these corrections, once with default settings, and once with
exaggerated corrections:

@lilypond
    \score { \notes {
      c'4 e''4 e'4 b'4 |
      b'4 e''4 b'4 e''4|
      \override Staff.NoteSpacing   #'stem-spacing-correction
   = #1.5
      \override Staff.StaffSpacing   #'stem-spacing-correction
   = #1.5
      c'4 e''4 e'4 b'4 |
      b'4 e''4 b'4 e''4|      
    }
    \paper { raggedright = ##t } }
@end lilypond

@cindex SpacingSpanner, overriding properties

Properties of the  @internalsref{SpacingSpanner} must be overridden
from the @code{\paper} block, since the @internalsref{SpacingSpanner} is
created before any property commands are interpreted.
@example
\paper @{ \context  @{
  \ScoreContext
  SpacingSpanner \override #'spacing-increment = #3.0
@} @}
@end example


@seealso

Internals: @internalsref{SpacingSpanner}, @internalsref{NoteSpacing},
@internalsref{StaffSpacing}, @internalsref{SeparationItem}, and
@internalsref{SeparatingGroupSpanner}.

@refbugs

Spacing is determined on a score wide basis. If you have a score that
changes its character (measured in durations) halfway during the
score, the part containing the longer durations will be spaced too
widely.

There is no convenient mechanism to manually override spacing.



@node Font Size
@subsection Font size

@cindex font size, setting
@cindex staff size, setting
@cindex @code{paper} file

The Feta font provides musical symbols at eight  different
sizes. Each font is tuned for a different staff size: at smaller sizes
the font gets heavier, to match the relatively heavier staff lines.
The recommended font sizes are listed in the following table:

@multitable @columnfractions  .25 .25 .25 .25

@item @b{font name}
@tab @b{staff height (pt)}
@tab @b{staff height (mm)}
@tab @b{use}

@item feta11
@tab 11.22
@tab 3.9 
@tab pocket scores

@item feta13
@tab 12.60
@tab 4.4
@tab

@item feta14
@tab 14.14
@tab 5.0
@tab 

@item feta16
@tab 15.87
@tab 5.6
@tab 

@item feta18
@tab 17.82
@tab 6.3
@tab song books

@item feta20
@tab 17.82
@tab 7.0
@tab standard parts 

@item feta23
@tab 22.45 
@tab 7.9
@tab 

@item feta20
@tab 25.2 
@tab 8.9
@tab
@c modern rental material  ?

@end multitable

These fonts are available in any sizes. The context property
@code{fontSize} and the layout property @code{staff-space} (in
@internalsref{StaffSymbol}) can be used to tune size for individual
staves. The size of individual staves are relative to the global size,
which can be set   in the following manner:

@example
  #(set-global-staff-size 14)
@end example

This sets the global default size to 14pt staff height, and scales all
fonts accordingly.

@seealso

This manual: @ref{Selecting font sizes}.


@node Line breaking
@subsection Line breaking

@cindex line breaks
@cindex breaking lines

Line breaks are normally computed automatically. They are chosen such
that lines look neither cramped nor loose, and that consecutive lines
have similar density.

Occasionally you might want to override the automatic breaks; you can
do this by  specifying @code{\break}. This will force a line break at
this point.  Line breaks can only occur at places where there are bar
lines.  If you want to have a line break where there is no bar line,
you can force an invisible bar line by entering @code{\bar
""}. Similarly, @code{\noBreak} forbids a line break at a 
point.


@cindex regular line breaks
@cindex four bar music. 

For line breaks at regular intervals  use @code{\break} separated by
skips and repeated with @code{\repeat}:
@example
<<  \repeat unfold 7 @{
         s1 \noBreak s1 \noBreak
         s1 \noBreak s1 \break  @}
   @emph{the real music}
>> 
@end  example

@noindent
This makes the following 28 measures (assuming 4/4 time) be broken every
4 measures, and only there.

@refcommands

@code{\break}, @code{\noBreak}
@cindex @code{\break}
@cindex @code{\noBreak}

@seealso

Internals: @internalsref{BreakEvent}.


@node Page layout
@subsection Page layout

@cindex page breaks
@cindex breaking pages

@cindex @code{indent}
@cindex @code{linewidth}

The most basic settings influencing the spacing are @code{indent} and
@code{linewidth}. They are set in the @code{\paper} block. They
control the indentation of the first line of music, and the lengths of
the lines.

If  @code{raggedright} is set to true in the @code{\paper}
block, then the lines are justified at their natural length. This
useful for short fragments, and for checking how tight the natural
spacing is.

@cindex page layout
@cindex vertical spacing

The page layout process happens outside the LilyPond formatting
engine: variables controlling page layout are passed to the output,
and are further interpreted by @code{lilypond} wrapper program. It
responds to the following variables in the @code{\paper} block.  The
spacing between systems is controlled with @code{interscoreline}, its
default is 16pt.  The distance between the score lines will stretch in
order to fill the full page @code{interscorelinefill} is set to a
positive number.  In that case @code{interscoreline} specifies the
minimum spacing.

@cindex @code{textheight}
@cindex @code{interscoreline}
@cindex @code{interscorelinefill}

If the variable @code{lastpagefill} is defined,
@c fixme: this should only be done if lastpagefill= #t 
systems are evenly distributed vertically on the last page.  This
might produce ugly results in case there are not enough systems on the
last page.  The @command{lilypond-book} command ignores
@code{lastpagefill}.  See @ref{lilypond-book manual} for more
information.

@cindex @code{lastpagefill}

Page breaks are normally computed by @TeX{}, so they are not under
direct control of LilyPond.  However, you can insert a commands into
the @file{.tex} output to instruct @TeX{} where to break pages.  This
is done by setting the @code{between-systems-strings} on the
@internalsref{NonMusicalPaperColumn} where the system is broken.
An example is shown in @inputfileref{input/regression,between-systems.ly}.
The predefined command @code{\newpage} also does this.

@cindex paper size
@cindex page size
@cindex @code{papersize}

To change the paper size, use the following Scheme code:
@example
        \paper@{
           #(set-paper-size "a4")
        @}
@end example


@refcommands

@cindex @code{\newpage}
@code{\newpage}. 


@seealso

In this manual: @ref{Invoking lilypond}.

Examples: @inputfileref{input/regression,between-systems.ly}.

Internals: @internalsref{NonMusicalPaperColumn}.

@refbugs

LilyPond has no concept of page layout, which makes it difficult to
reliably choose page breaks in longer pieces.


@node Interpretation context
@section Interpretation context

@menu
* Creating contexts::           
* Default contexts::            
* Context properties::          
* Defining contexts::           
* Changing contexts locally::   
* Engravers and performers::    
* Defining new contexts::       
@end menu


Interpretation contexts are objects that only exist during program
run.  During the interpretation phase (when @code{interpreting music}
is printed on the standard output), the music expression in a
@code{\score} block is interpreted in time order, the same order in
which we hear and play the music.  During this phase, the interpretation
context holds the state for the current point within the music, for
example:
@itemize @bullet
@item What notes are playing at this point?

@item What symbols will be printed at this point?

@item What is the current key signature, time signature, point within
the measure, etc.?
@end itemize

Contexts are grouped hierarchically: A @internalsref{Voice} context is
contained in a @internalsref{Staff} context (because a staff can contain
multiple voices at any point), a @internalsref{Staff} context is contained in
@internalsref{Score}, @internalsref{StaffGroup}, or
@internalsref{ChoirStaff} context.

Contexts associated with sheet music output are called @emph{notation
contexts}, those for sound output are called @emph{performance
contexts}.  The default definitions of the standard notation and
performance contexts can be found in @file{ly/engraver-init.ly} and
@file{ly/performer-init.ly}, respectively.


@node Creating contexts
@subsection Creating contexts
@cindex @code{\context}
@cindex context selection

Contexts for a music expression can be selected manually, using one of
the following music expressions:

@example
\new @var{contexttype} @var{musicexpr}
\context @var{contexttype} [= @var{contextname}] @var{musicexpr}
@end example

@noindent
This means that @var{musicexpr} should be interpreted within a context
of type @var{contexttype} (with name @var{contextname} if specified).
If no such context exists, it will be created:

@lilypond[verbatim,raggedright]
\score {
  \notes \relative c'' {
    c4 <<d4 \context Staff = "another" e4>> f
  }
}
@end lilypond

@noindent
In this example, the @code{c} and @code{d} are printed on the default
staff.  For the @code{e}, a context @code{Staff} called @code{another}
is specified; since that does not exist, a new context is created.
Within @code{another}, a (default) Voice context is created for the
@code{e4}.  A context is ended when when all music referring it has
finished, so after the third quarter, @code{another} is removed.

The @code{\new} construction creates a context with a
generated, unique @var{contextname}. An expression with
@code{\new} always leads to a new context. This is convenient
for creating multiple staves, multiple lyric lines, etc.

When using automatic staff changes, automatic phrasing, etc., the
context names have special meanings, so @code{\new} cannot be
used.


@node Default contexts
@subsection Default contexts

Every top level music is interpreted by the @code{Score} context; in
other words, you may think of @code{\score} working like

@example
\score @{
  \context Score @var{music}
@}
@end example

Music expressions  inherit their context from the enclosing music
expression. Hence, it is not necessary to explicitly specify
@code{\context} for most expressions.  In
the following example, only the sequential expression has an explicit
context. The notes contained therein inherit the @code{goUp} context
from the enclosing music expression.

@lilypond[verbatim,raggedright]
  \notes \context Voice = goUp { c'4 d' e' }
@end lilypond


Second, contexts are created automatically to be able to interpret the
music expressions.  Consider the following example:

@lilypond[verbatim,raggedright]
  \score { \notes { c'4-( d' e'-) } }
@end lilypond

@noindent
The sequential music is interpreted by the Score context initially,
but when a note is encountered, contexts are setup to accept that
note.  In this case, a @code{Voice}, and @code{Staff}
context are created.  The rest of the sequential music is also
interpreted with the same @code{Voice}, and
@code{Staff} context, putting the notes on the same staff, in the same
voice.

@node Context properties
@subsection Context properties

Contexts have properties.  These properties are set from the @file{.ly}
file using the following expression:
@cindex context properties
@cindex properties, context

@example
\set @var{contextname}.@var{propname} = @var{value}
@end example

@noindent
Sets the @var{propname} property of the context @var{contextname} to
the specified Scheme expression @var{value}.  Both @var{propname} and
@var{contextname} are strings, which can often be written unquoted.

@cindex inheriting
Properties that are set in one context are inherited by all of the
contained contexts.  This means that a property valid for the
@internalsref{Voice} context can be set in the @internalsref{Score} context
(for example) and thus take effect in all @internalsref{Voice} contexts.

Properties can be unset using the following statement.
@example
\unset @var{contextname}.@var{propname} 
@end example

@cindex properties, unsetting
@cindex @code{\unset}

@noindent
This removes the definition of @var{propname} in @var{contextname}.  If
@var{propname} was not defined in @var{contextname} (but was inherited
from a higher context), then this has no effect.

If @var{contextname} is left out, then it defaults to the current
``bottom'' context: this is a context like @internalsref{Voice} that
cannot contain any other contexts.


@node Defining contexts
@subsection Defining contexts

@cindex context definition
@cindex translator definition

The most common way to create a new context definition is by extending
an existing one.  An existing context from the paper block is copied
by referencing a context identifier:

@example
\paper @{
  \context @{
    @var{context-identifier}
  @}
@}
@end example

@noindent
Every predefined context has a standard identifier. For example, the
@code{Staff} context can be referred to as @code{\StaffContext}.

The context can then be modified by setting or changing properties,
e.g.
@example
\context @{
  \StaffContext
  Stem \set #'thickness = #2.0
  defaultBarType = #"||"
@}
@end example
These assignments happen before interpretation starts, so a property
command will override any predefined settings.

@cindex engraver

@refbugs

It is not possible to collect multiple property assignments in a
variable, and apply to one @code{\context} definition by
referencing that variable.

@node Changing contexts locally
@subsection Changing contexts locally


Extending an existing context can also be done locally. A piece of
music can be interpreted in a changed context by using the following syntax

@example
  \with @{
     @var{context modifications}
  @}
@end example

These statements comes between @code{\new} or @code{\context} and the
music to be interpreted. The @var{context modifications} property
settings and @code{\remove}, @code{\consists} and @code{\consistsend}
commands. The syntax is similar to the @code{\context} block.

The following example shows how a staff is created with bigger spaces,
and without a @code{Clef_engraver}.

@lilypond[relative=1,fragment,verbatim]
<<
  \new Staff { c4 es4 g2 }
  \new Staff \with {
        \override StaffSymbol #'staff-space = #(magstep 1.5)
        fontSize = #1.5
        \remove "Clef_engraver"
  } {
        c4 es4 g2
  } >>
@end lilypond

@refbugs

The command @code{\with} has no effect on contexts that already
exist. 


@node Engravers and performers
@subsection  Engravers and performers


Each context is composed of a number of building blocks, or plug-ins
called engravers.  An engraver is a specialized C++ class that is
compiled into the executable. Typically, an engraver is responsible
for one function: the @code{Slur_engraver} creates only @code{Slur}
objects, and the @code{Skip_event_swallow_translator} only swallows
(silently gobbles) @code{SkipEvent}s.



@cindex engraver
@cindex plug-in

An existing context definition can be changed by adding or removing an
engraver. The syntax for these operations is
@example
\consists @var{engravername}
\remove @var{engravername}
@end example

@cindex @code{\consists}
@cindex @code{\remove}

@noindent
Here @var{engravername} is a string, the name of an engraver in the
system. In the following example, the @code{Clef_engraver} is removed
from the Staff context. The result is a staff without a clef, where
the middle C is at its default position, the center line:

@lilypond[verbatim,raggedright]
\score {
  \notes {
    c'4 f'4
  }
  \paper {
    \context {
      \StaffContext
      \remove Clef_engraver
    }
  }
}
@end lilypond

A list of all engravers is in the internal documentation,
see @internalsref{Engravers}.

@node Defining new contexts
@subsection Defining new contexts


It is also possible to define new contexts from scratch.  To do this,
you must define give the new context a name.  In the following
example, a very simple Staff context is created: one that will put
note heads on a staff symbol.

@example
\context @{
  \type "Engraver_group_engraver"
  \name "SimpleStaff"
  \alias "Staff"
  \consists "Staff_symbol_engraver"
  \consists "Note_head_engraver"
  \consistsend "Axis_group_engraver"
@}
@end example

@noindent
The argument of @code{\type} is the name for a special engraver that
handles cooperation between simple engravers such as
@code{Note_head_engraver} and @code{Staff_symbol_engraver}.  This
should always be  @code{Engraver_group_engraver} (unless you are
defining a Score context from scratch, in which case
@code{Score_engraver}   must be used).

The complete list of context  modifiers is the following:
@itemize @bullet
@item @code{\alias} @var{alternate-name}:
This specifies a different name.  In the above example,
@code{\set Staff.X = Y} will also work on @code{SimpleStaff}s.

@item @code{\consistsend} @var{engravername}:
Analogous to @code{\consists}, but makes sure that
@var{engravername} is always added to the end of the list of
engravers.

Engravers that group context objects into axis groups or alignments
need to be at the end of the list. @code{\consistsend} insures that
engravers stay at the end even if a user adds or removes engravers.

@item @code{\accepts} @var{contextname}:
This context can contains @var{contextname} contexts.  The first
@code{\accepts} is created as a default context when events (e.g. notes
or rests) are encountered.

@item @code{\denies}:
The opposite of @code{\accepts}.

@item @code{\name} @var{contextname}:
This sets the type name of the context, e.g. @code{Staff},
@code{Voice}.  If the name is not specified, the translator will not
do anything.
@end itemize

@c EOF




@node Output details
@section Output details

The default output format is La@TeX{}, which should be run
through La@TeX{}.  Using the option @option{-f}
(or @option{--format}) other output formats can be selected also, but
 none of them work reliably.

Now the music is output system by system (a `system' consists of all
staves belonging together).  From @TeX{}'s point of view, a system is an
@code{\hbox} which contains a lowered @code{\vbox} so that it is centered
vertically on the baseline of the text.  Between systems,
@code{\interscoreline} is inserted vertically to have stretchable space.
The horizontal dimension of the @code{\hbox} is given by the
@code{linewidth} parameter from LilyPond's @code{\paper} block.

After the last system LilyPond emits a stronger variant of
@code{\interscoreline} only if the macro
@code{\lilypondpaperlastpagefill} is not defined (flushing the systems
to the top of the page).  You can avoid that by setting the variable
@code{lastpagefill} in LilyPond's @code{\paper} block.

It is possible to fine-tune the vertical offset further by defining the
macro @code{\lilypondscoreshift}:

@example
\def\lilypondscoreshift@{0.25\baselineskip@}
@end example

@noindent
where @code{\baselineskip} is the distance from one text line to the next.

Here an example how to embed a small LilyPond file @code{foo.ly} into
running La@TeX{} text without using the @code{lilypond-book} script
(@pxref{lilypond-book manual}):

@example
\documentclass@{article@}

\def\lilypondpaperlastpagefill@{@}
\lineskip 5pt
\def\lilypondscoreshift@{0.25\baselineskip@}

\begin@{document@}
This is running text which includes an example music file
\input@{foo.tex@}
right here.
\end@{document@}
@end example

The file @file{foo.tex} has been simply produced with

@example
  lilypond-bin foo.ly
@end example

The call to @code{\lineskip} assures that there is enough vertical space
between the LilyPond box and the surrounding text lines.