docs NR 5: Reorganise sections

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

@c -*- coding: utf-8; mode: texinfo; -*-
@c This file is part of lilypond.tely
@ignore
    Translation of GIT committish: FILL-IN-HEAD-COMMITTISH

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

@c \version "2.11.55"

@node Changing defaults
@chapter Changing defaults

@strong{N.B. This Chapter is still being developed at present.}

The purpose of LilyPond's design is to provide the finest quality
output by default.  Nevertheless, it may happen that you need to
change this default layout.  The layout is controlled through a large
number of @q{knobs and switches} collectively called @q{properties}.
A tutorial introduction to accessing and modifying these properties
can be found in the Learning Manual, see @rlearning{Tweaking output}.
This should be read first.  This chapter covers similar ground, but
in a style more appropriate to a reference manual.

@cindex Internals Reference

The definitive description of the controls available for tuning can
be found in a separate document: @rinternalsnamed{Top,the Internals
Reference}.  That manual lists all the variables, functions and
options available in LilyPond.  It is written as a HTML document,
which is available
@c leave the @uref as one long line.
@uref{http://@/lilypond@/.org/@/doc/@/stable/@/Documentation/@/user/@/lilypond@/-internals/,on@/-line},
and is also included with the LilyPond documentation package.

@c TODO The following is at variance to what actually follows.  Fix -td

@ignore
There are four areas where the default settings may be changed:

@itemize
@item
Automatic notation: changing the automatic creation of notation
elements.  For example, changing the beaming rules.

@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
Page layout: changing the appearance of the spacing, line
breaks, and page dimensions.  These modifications are discussed
@c in @ref{notation}, and @ref{Spacing issues}.
@end itemize

Internally, LilyPond uses Scheme (a LISP dialect) to provide
infrastructure.  Overriding layout decisions in effect accesses the
program internals, which requires Scheme input.  Scheme elements are
introduced in a @code{.ly} file with the hash mark
@code{#}.@footnote{@rlearning{Scheme tutorial}, contains a short tutorial
on entering numbers, lists, strings, and symbols in Scheme.}

@end ignore


@menu
* Interpretation contexts::
* Explaining the Internals Reference::
* Modifying properties::
* Useful concepts and properties::
* Advanced tweaks::
* Discussion of specific tweaks::
@end menu


@node Interpretation contexts
@section Interpretation contexts

This section describes what contexts are, and how to modify them.

@menu
* Contexts explained::          
* Creating contexts::           
* Modifying context plug-ins::  
* Changing context default settings::  
* Defining new contexts::       
* Aligning contexts::           
@end menu


@node Contexts explained
@subsection Contexts explained

@ignore
@c TODO Rethink and rewrite

>> > > - list of contexts: my *danger unmaintainable*
>> > > alarm just went off.  I'm

I knew it would... And leaving out some of them is perfectly fine
with me.
I do think that a list like this, with the main contexts and a
brief
description of  what they do (perhaps also with a note about what
default
behavior is associated with each of them, but this may be
unmanageable),
should be there, and then we could simply list the remaining ones
without
further explanation and with links to the IR.
@end ignore

@c TODO Improve layout, order and consistency of wording -td

@c TODO Add introduction which explains contexts in generality  -td

@c TODO Describe propagation of property values -td

Contexts are arranged heirarchically:

@menu
* Score - the master of all contexts::
* Top-level contexts - staff containers::
* Intermediate-level contexts - staves::
* Bottom-level contexts - voices::
@end menu

@node Score - the master of all contexts
@unnumberedsubsubsec Score - the master of all contexts

This is the top level notation context. No other context can
contain a Score context. By default the Score context handles
the administration of time signatures and makes sure that items
such as clefs, time signatures, and key-signatures are aligned
across staves.

A Score context is instantiated implicitly when a
@code{\score @{@dots{}@}} or @code{\layout @{@dots{}@}} block is
processed, or explicitly when a @code{\new Score} command is
executed.

@node Top-level contexts - staff containers
@unnumberedsubsubsec Top-level contexts - staff containers

@strong{@emph{StaffGroup}}

Groups staves while adding a bracket on the left side, grouping
the staves together. The bar lines of the contained staves are
connected vertically. StaffGroup only consists of a collection
of staves, with a bracket in front and spanning bar lines.

@strong{@emph{ChoirStaff}}

Identical to StaffGroup except that the bar lines of the contained
staves are not connected vertically.

@strong{@emph{GrandStaff}}

A group of staves, with a brace on the left side, grouping
the staves together. The bar lines of the contained staves are
connected vertically.

@strong{@emph{PianoStaff}}

TODO No longer correct?  Check.  -td

Just like GrandStaff but with a forced distance between the
staves, so cross staff beaming and slurring can be used.

@strong{@emph{InnerStaffGroup}}

TODO  -td

@strong{@emph{InnerChoirStaff}}

TODO  -td

@node Intermediate-level contexts - staves
@unnumberedsubsubsec Intermediate-level contexts - staves

@strong{@emph{Staff}}

Handles clefs, bar lines, keys, accidentals. It can contain
Voice contexts.

@strong{@emph{RhythmicStaff}}

Like Staff but for printing rhythms. Pitches are ignored;
the notes are printed on one line.

@strong{@emph{TabStaff}}

Context for generating tablature. By default lays the music
expression out as a guitar tablature, printed on six lines.

@strong{@emph{DrumStaff}}

Handles typesetting for percussion. Can contain DrumVoice

@strong{@emph{VaticanaStaff}}

Same as Staff, except that it is designed for typesetting
a piece in gregorian style.

@strong{@emph{MensuralStaff}}

Same as Staff, except that it is designed for typesetting
a piece in mensural style.

@node Bottom-level contexts - voices
@unnumberedsubsubsec Bottom-level contexts - voices

Voice-level contexts initialise certain properties and start
appropriate engravers.  Being bottom-level contexts, they cannot
contain other contexts.

@strong{@emph{Voice}}

Corresponds to a voice on a staff. This context handles the
conversion of dynamic signs, stems, beams, super- and sub-scripts,
slurs, ties, and rests.  You have to instantiate this explicitly
if you require multiple voices on the same staff.

@strong{@emph{VaticanaVoice}}

Same as Voice, except that it is designed for typesetting a piece
in gregorian style.

@strong{@emph{MensuralVoice}}

Same as Voice, with modifications for typesetting a piece in
mensural style.

@strong{@emph{Lyrics}}

Corresponds to a voice with lyrics. Handles the printing of a
single line of lyrics.

@strong{@emph{DrumVoice}}

The voice context used in a percussion staff.

@strong{@emph{FiguredBass}}

The context in which BassFigure objects are created from
input entered in @code{\figuremode} mode.

@strong{@emph{TabVoice}}

The voice context used within a TabStaff context.  Usually left to
be created implicitly.

@strong{@emph{ChordNames}}

Typesets chord names.

------------------------------

TODO

Then the following, which I don't know what to do with:

    * GregorianTranscriptionVoice
    * GregorianTranscriptionStaff

    * FretBoards
        Engraves fretboards from chords. Not easy... Not
documented.
        There is now some documentation on FretBoards in the NR, under
         instrument-specific notation -- cds.

    * NoteNames

    * CueVoice Not documented
    * Global
        Hard coded entry point for LilyPond. Cannot be tuned.
    * Devnull
        Silently discards all musical information given to this
context.


@node Creating contexts
@subsection Creating contexts

@c TODO \new Score and \score
@c TODO more complete descriptions rather than learning style

For scores with only one voice and one staff, contexts are
created automatically.  For more complex scores, it is necessary to
create them by hand.  There are three commands that do this.

@itemize

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

@funindex \new
@cindex new contexts
@cindex Context, creating

@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 the @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, is preceded with
@code{\new Staff}.

@lilypond[quote,verbatim,relative=2,ragged-right,fragment]
<<
  \new Staff { c4 c }
  \new Staff { d4 d }
>>
@end lilypond

The @code{\new} command may also give a name to the context,

@example
\new @var{type} = @var{id} @var{music}
@end example
However, this user specified name is only used if there is no other
context already earlier with the same name.


@funindex \context

@item
Like @code{\new}, the @code{\context} command also directs a music
expression to a context object, but gives the context an explicit 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, a new
context with the specified name is created.  This is useful if
the context is 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 possible use of named contexts is funneling two different
music expressions into one context.  In the following example,
articulations and notes are entered separately,

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

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

@example
<<
  \new Staff \context Voice = "A" \music
  \context Voice = "A" \arts
>>
@end example
@lilypond[quote,ragged-right]
music = { c4 c4 }
arts = { s4-. s4-> }
\relative c'' <<
  \new Staff \context Voice = "A" \music
  \context Voice = "A" \arts
>>
@end lilypond

With this mechanism, it is possible to define an Urtext (original
edition), with the option to put several distinct articulations on the
same notes.

@cindex creating contexts

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


@noindent
This is similar to @code{\context} with @code{= @var{id}}, but 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 applied to @code{Voice}

@example
\applyOutput #'@var{context} #@var{function}   % apply to Voice
@end example

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

@example
\applyOutput #'Score #@var{function}
\applyOutput #'Staff #@var{function}
@end example

@end itemize


@node Modifying context plug-ins
@subsection Modifying context plug-ins

@c TODO Should this be Modifying engravers or Modifying contexts?

Notation contexts (like @code{Score} and @code{Staff}) not only
store properties,
they also contain plug-ins called @q{engravers} that create notation
elements.  For example, the @code{Voice} context contains a
@code{Note_head_engraver} and the @code{Staff} context contains a
@code{Key_signature_engraver}.

For a full a description of each plug-in, see
@ifhtml
@rinternals{Engravers and Performers}.
@end ifhtml
@ifnothtml
Internals Reference @expansion{} Translation @expansion{} Engravers.
@end ifnothtml
Every context described in
@ifhtml
@rinternals{Contexts}
@end ifhtml
@ifnothtml
Internals Reference @expansion{} Translation @expansion{} 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,

@funindex \with

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

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

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

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

@cindex polymetric scores
@cindex Time signatures, multiple

@lilypond[quote,relative=1,ragged-right,verbatim,fragment]
\new Score \with {
  \remove "Timing_translator"
  \remove "Default_bar_line_engraver"
} <<
  \new Staff \with {
    \consists "Timing_translator"
    \consists "Default_bar_line_engraver"
  } {
      \time 3/4
      c4 c c c c c
  }
  \new Staff \with {
    \consists "Timing_translator"
    \consists "Default_bar_line_engraver"
  } {
       \time 2/4
       c4 c c c c c
  }
>>
@end lilypond


@node Changing context default settings
@subsection Changing context default settings

The adjustments of the previous subsections (
@ref{The set command}, @ref{Modifying context plug-ins}, and
@ref{Overview of modifying properties}) can also be entered
separately from the music in the @code{\layout} block,

@example
\layout @{
  @dots{}
  \context @{
    \Staff

    \set fontSize = #-2
    \override Stem #'thickness = #4.0
    \remove "Time_signature_engraver"
  @}
@}
@end example

The @code{\Staff} command brings in the existing definition of the
staff context so that it can be modified.

The statements
@example
\set fontSize = #-2
\override Stem #'thickness = #4.0
\remove "Time_signature_engraver"
@end example

@noindent
affect all staves in the score.  Other contexts can be modified
analogously.

The @code{\set} keyword is optional within the @code{\layout} block, so

@example
\context @{
  @dots{}
  fontSize = #-2
@}
@end example

@noindent
will also work.



@knownissues

It is not possible to collect context changes in a variable and apply
them to a @code{\context} definition by referring to that variable.

The @code{\RemoveEmptyStaffContext} will overwrite your current
@code{\Staff} settings.  If you wish to change the defaults for a
staff which uses @code{\RemoveEmptyStaffContext}, you must do so
after calling @code{\RemoveEmptyStaffContext}, ie

@example
\layout @{
  \context @{
    \RemoveEmptyStaffContext

    \override Stem #'thickness = #4.0
  @}
@}
@end example

TODO: add \with in here.



@node Defining new contexts
@subsection Defining new contexts

Specific contexts, like @code{Staff} and @code{Voice}, are made of
simple building blocks.  It is possible to create new types of
contexts with different combinations of engraver plug-ins.

The next example shows how to build a different type of
@code{Voice} context from scratch.  It will be similar to
@code{Voice}, but only prints centered slash note heads.  It can be used
to indicate improvisation in jazz pieces,

@lilypond[quote,ragged-right]
\layout { \context {
  \name ImproVoice
  \type "Engraver_group"
  \consists "Note_heads_engraver"
  \consists "Text_engraver"
  \consists Pitch_squash_engraver
  squashedPosition = #0
  \override NoteHead #'style = #'slash
  \override Stem #'transparent = ##t
  \alias Voice
}
\context { \Staff
  \accepts "ImproVoice"
}}

\relative c'' {
  a4 d8 bes8 \new ImproVoice { c4^"ad lib" c
   c4 c^"undress" c_"while playing :)" c }
  a1
}
@end lilypond


These settings are defined within a @code{\context} block inside a
@code{\layout} block,

@example
\layout @{
  \context @{
    @dots{}
  @}
@}
@end example

In the following discussion, the example input shown should go in place
of the @dots{} in the previous fragment.

First it is necessary to define a name for the new context:

@example
\name ImproVoice
@end example

Since it is similar to the @code{Voice}, we want commands that work
on (existing) @code{Voice}s to remain working.  This is achieved by
giving the new context an alias @code{Voice},

@example
\alias Voice
@end example

The context will print notes and instructive texts, so we need to add
the engravers which provide this functionality,

@example
\consists Note_heads_engraver
\consists Text_engraver
@end example

but we only need this on the center line,

@example
\consists Pitch_squash_engraver
squashedPosition = #0
@end example

The @rinternals{Pitch_squash_engraver} modifies note heads (created
by @rinternals{Note_heads_engraver}) and sets their vertical
position to the value of @code{squashedPosition}, in this case@tie{}@code{0},
the center line.

The notes look like a slash, and have no stem,

@example
\override NoteHead #'style = #'slash
\override Stem #'transparent = ##t
@end example

All these plug-ins have to cooperate, and this is achieved with a
special plug-in, which must be marked with the keyword @code{\type}.
This should always be @code{Engraver_group}.

@example
\type "Engraver_group"
@end example

Put together, we get

@example
\context @{
  \name ImproVoice
  \type "Engraver_group"
  \consists "Note_heads_engraver"
  \consists "Text_engraver"
  \consists Pitch_squash_engraver
  squashedPosition = #0
  \override NoteHead #'style = #'slash
  \override Stem #'transparent = ##t
  \alias Voice
@}
@end example

@funindex \accepts
Contexts form hierarchies.  We want to hang the @code{ImproVoice}
under @code{Staff}, just like normal @code{Voice}s.  Therefore, we
modify the @code{Staff} definition with the @code{\accepts}
command,

@example
\context @{
  \Staff
  \accepts ImproVoice
@}
@end example

@funindex \denies
The opposite of @code{\accepts} is @code{\denies},
which is sometimes needed when reusing existing context definitions.

Putting both into a @code{\layout} block, like

@example
\layout @{
  \context @{
    \name ImproVoice
    @dots{}
  @}
  \context @{
    \Staff
    \accepts "ImproVoice"
  @}
@}
@end example

Then the output at the start of this subsection can be entered as

@example
\relative c'' @{
  a4 d8 bes8
  \new ImproVoice @{
    c4^"ad lib" c
    c4 c^"undress"
    c c_"while playing :)"
  @}
  a1
@}
@end example


@node Aligning contexts
@subsection Aligning contexts

New contexts may be aligned above or below existing contexts.  This
could be useful in setting up a vocal staff (@rlearning{Vocal ensembles}) and
in ossia,

@c TODO Better example needed.  Ref LM, and expand on it.

@cindex ossia
@findex alignAboveContext
@findex alignBelowContext

@lilypond[quote,ragged-right]
ossia = { f4 f f f }
\score{
  \relative c' \new Staff = "main" {
    c4 c c c
    <<
      \new Staff \with {alignAboveContext=main} \ossia
      { d8 f d f d f d f }
    >>
  }
}
@end lilypond


@node Explaining the Internals Reference
@section Explaining the Internals Reference


@menu
* Navigating the program reference::  
* Layout interfaces::           
* Determining the grob property::  
* Naming conventions::          
@end menu

@node Navigating the program reference
@subsection Navigating the program reference

@c TODO remove this (it's in the LM)
@c Replace with more factual directions

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

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

If you visit the documentation on fingering instructions (in
@ref{Fingering instructions}), you will notice:

@quotation
@strong{See also}

Internals Reference: @rinternals{Fingering}.

@end quotation


@c  outdated info; probably will delete.
@ignore
This fragment points to two parts of the program reference: a page
on @code{FingeringEvent} and one on @code{Fingering}.

The page on @code{FingeringEvent} describes the properties of the music
expression for the input @code{-2}.  The page contains many links
forward.  For example, it says

@quotation
Accepted by: @rinternals{Fingering_engraver},
@end quotation

@noindent
That link brings us to the documentation for the Engraver, the
plug-in, which says

@quotation
This engraver creates the following layout objects: @rinternals{Fingering}.
@end quotation

In other words, once the @code{FingeringEvent}s are interpreted, the
@code{Fingering_engraver} plug-in will process them.
@end ignore

@ignore
@c  I can't figure out what this is supposed to mean.  -gp

The @code{Fingering_engraver} is also listed to create
@rinternals{Fingering} objects,

@c  old info?  it doesn't make any sense to me with our current docs.
This is also the
second bit of information listed under @b{See also} in the Notation
manual.
@end ignore

@ifnothtml
The programmer's reference is available as an HTML document.  It is
highly recommended that you read it in HTML form, either online or
by downloading the HTML documentation.  This section will be much more
difficult to understand if you are using the
PDF manual.
@end ifnothtml

Follow the link to @rinternals{Fingering}.  At the top of the
page, you will see

@quotation
Fingering objects are created by: @rinternals{Fingering_engraver} and
@rinternals{New_fingering_engraver}.
@end quotation

By following related links inside the program reference, we can follow the
flow of information within the program:

@itemize

@item @rinternals{Fingering}:
@rinternals{Fingering} objects are created by:
@rinternals{Fingering_engraver}

@item @rinternals{Fingering_engraver}:
Music types accepted: @rinternals{fingering-event}

@item @rinternals{fingering-event}:
Music event type @code{fingering-event} is in Music expressions named
@rinternals{FingeringEvent}
@end itemize

This path goes against the flow of information in the program: it
starts from the output, and ends at the input event.  You could
also start at an input event, and read with the flow of
information, eventually ending up at the output object(s).

The program reference can also be browsed like a normal document.  It
contains chapters on
@ifhtml
@rinternals{Music definitions},
@end ifhtml
@ifnothtml
@code{Music definitions}
@end ifnothtml
on @rinternals{Translation}, and the @rinternals{Backend}.  Every
chapter lists all the definitions used and all properties that may be
tuned.


@node Layout interfaces
@subsection Layout interfaces

@cindex interface, layout
@cindex layout interface
@cindex grob

The HTML page that we found in the previous section describes the
layout object called @rinternals{Fingering}.  Such an object is a
symbol within the score.  It has properties that store numbers (like
thicknesses and directions), but also pointers to related objects.  A
layout object is also called a @emph{Grob}, which is short for Graphical
Object.  For more details about Grobs, see @rinternals{grob-interface}.

The page for @code{Fingering} lists the definitions for the
@code{Fingering} object.  For example, the page says

@quotation
@code{padding} (dimension, in staff space):

@code{0.5}
@end quotation

@noindent
which means that the number will be kept at a distance of at least 0.5
of the note head.


Each layout object may have several functions as a notational or
typographical element.  For example, the Fingering object
has the following aspects

@itemize
@item
Its size is independent of the horizontal spacing, unlike slurs or beams.

@item
It is a piece of text.  Granted, it is usually a very short text.

@item
That piece of text is typeset with a font, unlike slurs or beams.

@item
Horizontally, the center of the symbol should be aligned to the
center of the note head.

@item
Vertically, the symbol is placed next to the note and the staff.

@item
The vertical position is also coordinated with other superscript
and subscript symbols.
@end itemize

Each of these aspects is captured in so-called @emph{interface}s,
which are listed on the @rinternals{Fingering} page at the bottom

@quotation
This object supports the following interfaces:
@rinternals{item-interface},
@rinternals{self-alignment-interface},
@rinternals{side-position-interface}, @rinternals{text-interface},
@rinternals{text-script-interface}, @rinternals{font-interface},
@rinternals{finger-interface}, and @rinternals{grob-interface}.
@end quotation

Clicking any of the links will take you to the page of the respective
object interface.  Each interface has a number of properties.  Some of
them are not user-serviceable (@q{Internal properties}), but others
can be modified.

We have been talking of @emph{the} @code{Fingering} object, but actually it
does not amount to much.  The initialization file (see
@rlearning{Other sources of information})
@file{scm/@/define@/-grobs@/.scm} shows the soul of the @q{object},

@example
(Fingering
  . ((padding . 0.5)
     (avoid-slur . around)
     (slur-padding . 0.2)
     (staff-padding . 0.5)
     (self-alignment-X . 0)
     (self-alignment-Y . 0)
     (script-priority . 100)
     (stencil . ,ly:text-interface::print)
     (direction . ,ly:script-interface::calc-direction)
     (font-encoding . fetaNumber)
     (font-size . -5)           ; don't overlap when next to heads.
     (meta . ((class . Item)
     (interfaces . (finger-interface
                    font-interface
                    text-script-interface
                    text-interface
                    side-position-interface
                    self-alignment-interface
                    item-interface))))))
@end example

@noindent
As you can see, the @code{Fingering} object is nothing more than a
bunch of variable settings, and the webpage in the Internals Reference
is directly generated from this definition.


@node Determining the grob property
@subsection Determining the grob property

@c TODO remove this (it's in the LM)
@c Replace with more factual directions

Recall that we wanted to change the position of the @b{2} in

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

Since the @b{2} is vertically positioned next to its note, we have to
meddle with the interface associated with this positioning.  This is
done using @code{side-position-interface}.  The page for this interface
says

@quotation
@code{side-position-interface}

Position a victim object (this one) next to other objects (the
support).  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.
@end table
@end quotation

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

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

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


In this case, the context for this tweak is @code{Voice}.  This
fact can also be deduced from the program reference, for the page for
the @rinternals{Fingering_engraver} plug-in says

@quotation
Fingering_engraver is part of contexts: @dots{} @rinternals{Voice}
@end quotation


@node Naming conventions
@subsection Naming conventions

Another thing that is needed, is an overview of the various naming
conventions:

    scheme functions: lowercase-with-hyphens (incl. one-word
names)
    scheme functions: ly:plus-scheme-style
    music events, music classes and music properties:
as-scheme-functions
    Grob interfaces: scheme-style
    backend properties: scheme-style (but X and Y!)
    contexts (and MusicExpressions and grobs): Capitalized or
CamelCase
    context properties: lowercaseFollowedByCamelCase
    engravers:
Capitalized_followed_by_lowercase_and_with_underscores

Which of these are conventions and which are rules?
Which are rules of the underlying language, and which are
LP-specific?


@node Modifying properties
@section Modifying properties

@c TODO change the menu and subsection node names to use
@c backslash once the new macro to handle the refs
@c is available.  Need to find and change all refs at
@c the same time. -td

@menu
* Overview of modifying properties::
* The set command::
* The override command::
* The tweak command::
* set versus override::
@end menu


@node Overview of modifying properties
@subsection Overview of modifying properties

Each context is responsible for creating certain types of graphical
objects.  The settings used for printing these objects are also stored by
context.  By changing these settings, the appearance of objects can be
altered.

The syntax for this is

@example
\override @var{context}.@var{name} #'@var{property} = #@var{value}
@end example

Here @var{name} is the name of a graphical object, like
@code{Stem} or @code{NoteHead}, and @var{property} is an internal
variable of the formatting system (@q{grob property} or @q{layout
property}).  The latter is a symbol, so it must be quoted.  The
subsection @ref{Modifying properties}, explains what to fill in
for @var{name}, @var{property}, and @var{value}.  Here we only
discuss the functionality of this command.

The command

@verbatim
\override Staff.Stem #'thickness = #4.0
@end verbatim

@noindent
makes stems thicker (the default is 1.3, with staff line thickness as a
unit).  Since the command specifies @code{Staff} as context, it only
applies to the current staff.  Other staves will keep their normal
appearance.  Here we see the command in action:

@lilypond[quote,verbatim,relative=2,fragment]
c4
\override Staff.Stem #'thickness = #4.0
c4
c4
c4
@end lilypond

The @code{\override} command changes the definition of the @code{Stem}
within the current @code{Staff}.  After the command is interpreted
all stems are thickened.

Analogous to @code{\set}, the @var{context} argument may be left out,
causing the default context @code{Voice} to be used.  Adding
@code{\once} applies the change during one timestep only.

@lilypond[quote,fragment,verbatim,relative=2]
c4
\once \override Stem #'thickness = #4.0
c4
c4
@end lilypond

The @code{\override} must be done before the object is
started.  Therefore, when altering @emph{Spanner} objects such as slurs
or beams, the @code{\override} command must be executed at the moment
when the object is created.  In this example,

@lilypond[quote,fragment,verbatim,relative=2]
\override Slur #'thickness = #3.0
c8[( c
\override Beam #'thickness = #0.6
c8 c])
@end lilypond

@noindent
the slur is fatter but the beam is not.  This is because the command for
@code{Beam} comes after the Beam is started, so it has no effect.

Analogous to @code{\unset}, the @code{\revert} command for a context
undoes an @code{\override} command; like with @code{\unset}, it only
affects settings that were made in the same context.  In other words, the
@code{\revert} in the next example does not do anything.

@example
\override Voice.Stem #'thickness = #4.0
\revert Staff.Stem #'thickness
@end example

Some tweakable options are called @q{subproperties} and reside inside
properties.  To tweak those, use commands of the form

@c leave this as a long long
@example
\override @var{context}.@var{name} #'@var{property} #'@var{subproperty} = #@var{value}
@end example

@noindent
such as

@example
\override Stem #'details #'beamed-lengths = #'(4 4 3)
@end example


@seealso

Internals: @rinternals{OverrideProperty}, @rinternals{RevertProperty},
@rinternals{PropertySet}, @rinternals{Backend}, and
@rinternals{All layout objects}.


@knownissues

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



@node The set command
@subsection The @code{\set} command

@cindex properties
@funindex \set
@cindex changing properties

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,

@example
\set @var{context}.@var{prop} = #@var{value}
@end example

For example,
@lilypond[quote,verbatim,relative=2,fragment]
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 @code{ChordNames}, @code{Voice}, or
@code{Lyrics}) is used.  In this example,

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

@noindent
the @var{context} argument to @code{\set} is left out, so automatic
beaming is switched off in the current @rinternals{Voice}.  Note that
the bottom-most context does not always contain the property that you
wish to change -- for example, attempting to set the @code{skipBars}
property (of the bottom-most context, in this case @code{Voice}) will
have no effect.

@lilypond[quote,verbatim,relative=2,fragment]
R1*2
\set skipBars = ##t
R1*2
@end lilypond

Contexts are hierarchical, so if a bigger context was specified, for
example @code{Staff}, then the change would also apply to all
@code{Voice}s in the current stave.  The change is applied
@q{on-the-fly}, during the music, so that the setting only affects the
second group of eighth notes.

@funindex \unset

There is also an @code{\unset} command,
@example
\unset @var{context}.@var{prop}
@end example

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

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

@noindent
introduces a property setting at @code{Staff} level.  The setting also
applies to the current @code{Voice}.  However,

@example
\unset Voice.autoBeaming
@end example

@noindent
does not have any effect.  To cancel this setting, the @code{\unset}
must be specified on the same level as the original @code{\set}.  In
other words, undoing the effect of @code{Staff.autoBeaming = ##f}
requires
@example
\unset Staff.autoBeaming
@end example

Like @code{\set}, the @var{context} argument does not have to be
specified for a bottom context, so the two statements

@example
\set Voice.autoBeaming = ##t
\set autoBeaming = ##t
@end example

@noindent
are equivalent.


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

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

the property @code{fontSize} is unset automatically after the second
note.

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



@node The override command
@subsection The @code{\override} command

Commands which change output generally look like

@example
\override Voice.Stem #'thickness = #3.0
@end example

@noindent
To construct this tweak we must determine these bits of information:

@itemize
@item the context: here @code{Voice}.
@item the layout object: here @code{Stem}.
@item the layout property: here @code{thickness}.
@item a sensible value: here @code{3.0}.
@end itemize

Some tweakable options are called @q{subproperties} and reside inside
properties.  To tweak those, use commands in the form

@example
\override Stem #'details #'beamed-lengths = #'(4 4 3)
@end example

@cindex internal documentation
@cindex finding graphical objects
@cindex graphical object descriptions
@cindex tweaking
@funindex \override
@cindex internal documentation

For many properties, regardless of the data type of the property, setting the
property to false ( @code{##f} ) will result in turning it off, causing
LilyPond to ignore that property entirely.  This is particularly useful for
turning off grob properties which may otherwise be causing problems.

We demonstrate how to glean this information from the notation manual
and the program reference.


@node The tweak command
@subsection The @code{\tweak} command

@funindex \tweak
@cindex tweaking

In some cases, it is possible to take a short-cut for tuning
graphical objects.  For objects that are created directly from
an item in the input file, you can use the @code{\tweak} command.
For example:

@lilypond[relative=2,verbatim]
< c
  \tweak #'color #red
  d
  g
  \tweak #'duration-log #1
  a
> 4
-\tweak #'padding #8
-^
@end lilypond

@cindex chord, modifying one note in

But the main use of the @code{\tweak} command is to modify just
one of a number of notation elements which start at the same musical
moment, like the notes of a chord, or tuplet brackets which start
at the same time.

For an introduction to the syntax and uses of the tweak command
see @rlearning{Tweaking methods}.

The @code{\tweak} command sets a property in the following object
directly, without requiring the grob name or context to be
specified.  For this to work, it is necessary for the @code{\tweak}
command to remain immediately adjacent to the object to which it is
to apply after the input file has been converted to a music stream.
This is often not the case, as many additional elements are inserted
into the music stream implicitly.  For example, when a note which is
not part of a chord is processed, Lilypond implicitly inserts a
@code{ChordEvent} event before the note, so separating the tweak
from the note.  However, if chord symbols are placed round the
tweak and the note, the @code{\tweak} command comes after the
@code{ChordEvent} in the music stream, so remaining adjacent to the
note, and able to modify it.

So, this works:

@lilypond[relative=2,verbatim,quote]
<\tweak #'color #red c>4
@end lilypond

@noindent
but this does not:

@lilypond[relative=2,verbatim,quote]
\tweak #'color #red c4
@end lilypond

When several similar items are placed at the same musical moment,
the @code{\override} command cannot be used to modify just one of
them -- this is where the @code{\tweak} command must be used.
Items which may appear more than once at the same musical moment
include the following:

@c TODO expand to include any further uses of \tweak
@itemize
@item note heads of notes inside a chord
@item articulation signs on a single note
@item ties between notes in a chord
@item tuplet brackets starting at the same time
@end itemize

@c TODO add examples of these

@noindent
and @code{\tweak} may be used to modify any single occurrence of
these items.

Notably the @code{\tweak} command cannot be used to modify stems,
beams or accidentals, since these are generated later by note heads,
rather than by music elements in the input stream.  Nor can a
@code{\tweak} command be used to modify clefs or time signatures,
since these become separated from any preceding @code{\tweak}
command in the input stream by the automatic insertion of extra
elements required to specify the context.

But the @code{\tweak} command can be used as an alternative to
the @code{\override} command to modify those notational elements
that do not cause any additional implicit elements to be added
before them in the music stream.  For example, slurs may be
modified in this way:

@lilypond[verbatim,quote,relative=1]
c-\tweak #'thickness #5 ( d e f)
@end lilypond

Also several @code{\tweak} commands may be placed before a
notational element -- all affect it:

@lilypond[verbatim,quote,relative=1]
c
-\tweak #'style #'dashed-line
-\tweak #'dash-fraction #0.2
-\tweak #'thickness #3
-\tweak #'color #red
 \glissando
f'
@end lilypond

The music stream which is generated from a section of an input file,
including any automatically inserted elements, may be examined,
see @ref{Displaying music expressions}.  This may be helpful in
determining what may be modified by a @code{\tweak} command.

@seealso

Learning Manual:
@rlearning{Tweaking methods}.

Notation Reference:
@ref{Displaying music expressions}.

@knownissues

@cindex tweaks in a variable
The @code{\tweak} command cannot be used inside a variable.

@cindex tweaks in lyrics
The @code{\tweak} commands cannot be used in @code{\lyricmode}.

@cindex tweaking control points
@cindex control points, tweaking

The @code{\tweak} command cannot be used to modify the control
points of just one of several ties in a chord, other than the first
one encountered in the input file.

@node set versus override
@subsection @code{\set} vs. @code{\override}

We have seen two methods of changing properties: @code{\set} and
@code{\override}.  There are actually two different kinds of
properties.

Contexts can have properties, which are usually named in
@code{studlyCaps}.  They mostly control the translation from
music to notation, eg. @code{localKeySignature} (for determining
whether to print accidentals), @code{measurePosition} (for
determining when to print a bar line).  Context properties can
change value over time while interpreting a piece of music;
@code{measurePosition} is an obvious example of
this.  Context properties are modified with @code{\set}.

There is a special type of context property: the element
description.  These properties are named in @code{StudlyCaps}
(starting with capital letters).  They contain the
@q{default settings} for said graphical object as an
association list.  See @file{scm/@/define@/-grobs@/.scm}
to see what kind of settings there are.  Element descriptions
may be modified with @code{\override}.

@code{\override} is actually a shorthand;

@example
\override @var{context}.@var{name} #'@var{property} = #@var{value}
@end example

@noindent
is more or less equivalent to

@c  leave this long line -gp
@example
\set @var{context}.@var{name} #'@var{property} = #(cons (cons '@var{property} @var{value}) <previous value of @var{context})
@end example

The value of @code{context} (the alist) is used to initialize
the properties of individual grobs.  Grobs also have
properties, named in Scheme style, with
@code{dashed-words}.  The values of grob properties change
during the formatting process: formatting basically amounts
to computing properties using callback functions.

@code{fontSize} is a special property: it is equivalent to
entering @code{\override ... #'font-size} for all pertinent
objects.  Since this is a common change, the special
property (modified with @code{\set}) was created.


@node Useful concepts and properties
@section Useful concepts and properties


@menu
* Input modes::
* Direction and placement::
* Distances and measurements::
* Spanners::
* Visibility of objects::
* Line styles::
* Rotating objects::
@end menu

@node Input modes
@subsection Input modes

The way in which the notation contained within an input file is
interpreted is determined by the current input mode.

@strong{Chord mode}

This is activated with the @code{\chordmode} command, and causes
input to be interpreted with the syntax of chord notation, see
@ref{Chord notation}.  Chords are rendered as notes on a staff.

Chord mode is also activated with the @code{\chords} command.
This also creates a new @code{ChordNames} context and
causes the following input to be interpreted with the syntax of
chord notation and rendered as chord names in the @code{ChordNames}
context, see @ref{Printing chord names}.

@strong{Drum mode}

This is activated with the @code{\drummode} command, and causes
input to be interpreted with the syntax of drum notation, see
@ref{Basic percussion notation}.

Drum mode is also activated with the @code{\drums} command.
This also creates a new @code{DrumStaff} context and causes the
following input to be interpreted with the syntax of drum notation
and rendered as drum symbols on a drum staff, see @ref{Basic
percussion notation}.

@strong{Figure mode}

This is activated with the @code{\figuremode} command, and causes
input to be interpreted with the syntax of figured bass, see
@ref{Entering figured bass}.

Figure mode is also activated with the @code{\figures} command.
This also creates a new @code{FiguredBass} context and causes the
following input to be interpreted with the figured bass syntax
and rendered as figured bass symbols in the @code{FiguredBass}
context, see @ref{Introduction to figured bass}.

@strong{Fret and tab modes}

There are no special input modes for entering fret and tab symbols.

To create tab diagrams, enter notes or chords in note mode and
render them in a @code{TabStaff} context, see
@ref{Default tablatures}.

To create fret diagrams above a staff, you have two choices.
You can either use the @code{FretBoards} context (see
@ref{Automatic fret diagrams} or you can enter them as a markup
above the notes using the @code{\fret-diagram} command (see
@ref{Fret diagram markups}).

@strong{Lyrics mode}

This is activated with the @code{\lyricmode} command, and causes
input to be interpreted as lyric syllables with optional durations
and associated lyric modifiers, see @ref{Vocal music}.

Lyric mode is also activated with the @code{\addlyrics} command.
This also creates a new @code{Lyrics} context and an implicit
@code{\lyricsto} command which associates the following lyrics
with the preceding music.

@strong{Markup mode}

This is activated with the @code{\markup} command, and causes
input to be interpreted with the syntax of markup, see
@ref{Text markup commands}.

@c silly work-around for texinfo broken-ness
@c (@strong{Note...} causes a spurious cross-reference in Info)
@b{Note mode}

This is the default mode or it may be activated with the
@code{\notemode} command.  Input is interpreted as pitches,
durations, markup, etc and typeset as musical notation on a staff.

It is not normally necessary to specify note mode explicitly, but
it may be useful to do so in certain situations, for example if you
are in lyric mode, chord mode or any other mode and want to insert
something that only can be done with note mode syntax.

For example, to indicate dynamic markings for the verses of a
choral pieces it is necessary to enter note mode to interpret
the markings:

@lilypond[verbatim,relative=2,quote]
{ c4 c4 c4 c4 }
\addlyrics {
  \notemode{\set stanza = \markup{ \dynamic f 1. } }
  To be sung loudly
}
\addlyrics {
  \notemode{\set stanza = \markup{ \dynamic p 2. } }
  To be sung quietly
}
@end lilypond



@node Direction and placement
@subsection Direction and placement

In typesetting music the direction and placement of many items is
a matter of choice.  For example, the stems of notes can
be directed up or down; lyrics, dynamics, and other expressive
marks may be placed above or below the staff; text may be aligned
left, right or center; etc.  Most of these choices may be left to
be determined automatically by LilyPond, but in some cases it may
be desirable to force a particular direction or placement.

@strong{Default actions}

By default some directions are always up or always down (e.g.
dynamics or fermata), while other things can alternate between
up or down based on the stem direction (like slurs or accents).

@c TODO Add table showing these

@strong{Context layout}

Contexts are positioned in a system from top to bottom in the
order in which they are encountered.  Note, however, that a
context will be created implicitly if a command is encountered
when there is no suitable context available to contain it.

@c TODO Add example ?

The default order in which contexts are laid out can be changed,
see @ref{Aligning contexts}

@strong{Articulation direction indicators}

When adding articulations to notes the direction indicator,
@code{^} (meaning @qq{up}), @code{_} (meaning @qq{down}) or
@code{-} (meaning @qq{use default direction}), can usually be
omitted, in which case @code{-} is assumed.  But a direction
indicator is @strong{always} required before

@itemize
@item @code{\tweak} commands
@item @code{\markup} commands
@item @code{\tag} commands
@item string markups, e.g. -"string"
@item fingering instructions, e.g. @code{-1}
@item articulation shortcuts, e.g. @code{-.}, @code{->}, @code{--}
@end itemize

@strong{The direction property}

The position or direction of many layout objects is controlled
by the @code{direction} property.

The value of the @code{direction} property may be
set to @code{1}, meaning @qq{up} or @qq{above}, or to @code{-1},
meaning @qq{down} or @qq{below}.  The symbols @code{UP} and
@code{DOWN} may be used instead of @code{1} and @code{-1}
respectively.  The default direction may be specified by setting
@code{direction} to @code{0} or @code{CENTER}.  Alternatively,
in many cases predefined commands
exist to specify the direction.  These are all of the form

@noindent
@code{\xxxUp}, @code{xxxDown}, @code{xxxNeutral}

@noindent
where @code{xxxNeutral} means @qq{use the default direction}.
See @rlearning{Within-staff objects}.

In a few cases, arpeggio being the only common example, the value
of the @code{direction} property specifies whether the object
is to be placed to the right or left of the parent object.  In
this case @code{-1} or @code{LEFT} means @qq{to the left} and
@code{1} or @code{RIGHT} means @qq{to the right}.  @code{0}
or @code{CENTER} means @qq{use the default} direction, as before.

@ignore
These all have side-axis set to #X
AmbitusAccidental - direction has no effect
Arpeggio - works
StanzaNumber - not tried
TrillPitchAccidental - not tried
TrillPitchGroup - not tried
@end ignore



@node Distances and measurements
@subsection Distances and measurements

To be written

@c FIXME DISCUSS after working on other sections.

@c TODO: staff spaces.  Maybe move into tweaks?


@node Spanners
@subsection Spanners

Many objects of musical notation extend over several notes or even
several bars.  Examples are crescendi, trills, tuplet brackets, and
volta repeat brackets.  Such objects are called @qq{spanners}, and
have special properties to control their appearance and behaviour.
Some of these properties are common to all spanners; others are
restricted to a sub-set of the spanners.

The information that determines the end-points of spanners is computed
on-the-fly, but it is possible to override these:

@c TODO change and extend example

@lilypond[relative=2,ragged-right,verbatim,fragment]
e2 \glissando f
\once \override Glissando #'bound-details #'right #'Y = #-2
e2 \glissando f
@end lilypond


The following properties can be used for the left and right
@code{bound-details}:

@table @code
@item Y
This sets the Y-coordinate of the end point, in staff space.  By
default, it is the center of the bound object, so for a glissando
it points to the vertical center of the note head.

For horizontal spanners, such as text spanner and trill spanners,
it is hardcoded to 0.

@item attach-dir
This determines where the line starts and ends in X-direction,
relative to the bound object.  So, a value of @code{-1} (or
@code{LEFT}) makes the line start/end at the left side of the note
head it is attached to.

@item X
This is the absolute coordinate of the end point.  It is usually
computed on the fly, and there is little use in overriding it.

@item stencil
Line spanners may have symbols at the beginning or end, which is
contained in this sub-property.  This is for internal use, it is
recommended to use @code{text}.

@item text
This is a markup that is evaluated to yield stencil.  It is used
to put @i{cresc.} and @i{tr} on horizontal spanners.

@lilypond[quote,ragged-right,fragment,relative=2,verbatim]
\override TextSpanner #'bound-details #'left #'text
   = \markup { \small \bold Slower }
c2\startTextSpan b c a\stopTextSpan
@end lilypond

@item stencil-align-dir-y
@item stencil-offset
Without setting this, the stencil is simply put there at the
end-point, as defined by the @code{X} and @code{Y} sub properties.
Setting either @code{stencil-align-dir-y} or @code{stencil-offset}
will move the symbol at the edge relative to the end point of the
line

@lilypond[relative=1,fragment,verbatim]
\override TextSpanner #'bound-details
  #'left #'stencil-align-dir-y = #DOWN
\override TextSpanner #'bound-details
  #'right #'stencil-align-dir-y = #UP

\override TextSpanner #'bound-details
  #'left #'text = #"gggg"
\override TextSpanner #'bound-details
  #'right #'text = #"hhhh"
c4^\startTextSpan c c c \stopTextSpan
@end lilypond

@item arrow
Setting this sub property to @code{#t} produce an arrowhead at the
end of the line.

@item padding
This sub property controls the space between the specified
end-point of the line and the actual end.  Without padding, a
glissando would start and end in the center of each note head.

@end table

The music function \endSpanners terminates spanners and hairpins
after exactly one note.

@lilypond[verbatim,quote,ragged-right,relative=2,fragment]
\endSpanners
c2 \startTextSpan c2
c2 \< c2
@end lilypond

When using \endSpanners it is not necessary to close
\startTextSpan with \stopTextSpan, nor is it necessary to close
hairpins with \!.



@seealso

Internals Reference: @rinternals{TextSpanner},
@rinternals{Glissando}, @rinternals{VoiceFollower},
@rinternals{TrillSpanner},
@rinternals{line-spanner-interface}.


@node Visibility of objects
@subsection Visibility of objects

@cindex objects, visibility of
@cindex grobs, visibility of
@cindex visibility of objects

There are four main ways in which the visibility of layout objects
can be controlled: their stencil can be removed, they can be made
transparent, they can be colored white, or their
@code{break-visibility} property can be overridden.  The first
three apply to all layout objects; the last to just a few -- the
@emph{breakable} objects.  The Learning Manual introduces these
four techniques, see @rlearning{Visibility and color of objects}.

There are also a few other techniques which are specific to
certain layout objects.  These are covered under Special
considerations.

@menu
* Removing the stencil::        
* Making objects transparent::  
* Painting objects white::      
* Using break-visibility::      
* Special considerations::      
@end menu


@node Removing the stencil
@unnumberedsubsubsec Removing the stencil

@cindex stencil, removing

Every layout object has a stencil property.  By default this is set
to the specific function which draws that object.  If this property
is overridden to @code{#f} no function will be called and the object
will not be drawn.  The default action can be recovered with
@code{\revert}.

@lilypond[quote,verbatim,relative=1]
a1 a
\override Score.BarLine #'stencil = ##f
a a
\revert Score.BarLine #'stencil
a a a
@end lilypond

@node Making objects transparent
@unnumberedsubsubsec Making objects transparent

@cindex transparent, making objects

Every layout object has a transparent property which by default is
set to @code{#f}.  If set to @code{#t} the object still occupies
space but is made invisible.

@lilypond[quote,verbatim,relative=2]
a4 a
\once \override NoteHead #'transparent = ##t
a a
@end lilypond

@node Painting objects white
@unnumberedsubsubsec Painting objects white

@cindex objects, coloring
@cindex coloring objects
@cindex layers
@cindex printing order
@cindex overwriting objects
@cindex objects, overwriting
@cindex grobs, overwriting

Every layout object has a color property which by default is set
to @code{black}.  If this is overridden to @code{white} the object
will be indistinguishable from the white background.  However,
if the object crosses other objects the color of the crossing
points will be determined by the order in which they are drawn,
and this may leave a ghostly image of the white object, as shown
here:

@lilypond[quote,verbatim,relative=2]
\override Staff.Clef #'color = #white
a1
@end lilypond

This may be avoided by changing the order of printing the objects.
All layout objects have a @code{layer} property which should be set
to an integer.  Objects with the lowest value of @code{layer} are
drawn first, then objects with progressively higher values are drawn,
so objects with higher values overwrite objects with lower values.
By default most objects are assigned a @code{layer} value of
@code{1}, although a few objects, including @code{StaffSymbol} and
@code{BarLine}, are assigned a value of @code{0}.  The order of
printing objects with the same value of @code{layer} is indeterminate.

In the example above the white clef, with a default @code{layer}
value of @code{1}, is drawn after the staff lines (default
@code{layer} value @code{0}), so overwriting them.  To change this,
the @code{Clef} object must be given in a lower value of
@code{layer}, say @code{-1}, so that it is drawn earlier:

@lilypond[quote,verbatim,relative=2]
\override Staff.Clef #'color = #white
\override Staff.Clef #'layer = #-1
a1
@end lilypond

@node Using break-visibility
@unnumberedsubsubsec Using break-visibility

@c TODO Add making other objects breakable

@cindex break-visibility

Most layout objects are printed only once, but some like
bar lines, clefs, time signatures and key signatures, may need
to be printed twice when a line break occurs -- once at the end
of the line and again at the start of the next line.  Such
objects are called @emph{breakable}, and have a property, the
@code{break-visibility} property to control their visibility
at the three positions in which they may appear -- at the
start of a line, within a line if they are changed, and at the
end of a line if a change takes place there.

For example, the time signature
by default will be printed at the start of the first line, but
nowhere else unless it changes, when it will be printed at the
point at which the change occurs.  If this change occurs at the
end of a line the new time signature will be printed at the start
of the next line and a cautionary time signature will be printed
at the end of the previous line as well.

This behaviour is controlled by the @code{break-visibility}
property, which is explained in
@c Leave this ref on a newline - formats incorrectly otherwise -td
@rlearning{Visibility and color of objects}.  This property takes
a vector of three booleans which, in order, determine whether the
object is printed at the end of, within the body of, or at the
beginning of a line.  Or to be more precise, before a line break,
where there is no line break, or after a line break.

Alternatively, seven of the eight combinations may be specified
by pre-defined functions, defined in @file{scm/output-lib.scm},
where the last three columns indicate whether the layout objects
will be visible in the positions shown at the head of the columns:

@multitable @columnfractions .40 .15 .1 .1 .1
@c TODO check these more carefully
@headitem Function                   @tab Vector                  @tab Before @tab At no    @tab After
@headitem form                       @tab form                    @tab break  @tab break    @tab break

@item @code{all-invisible}           @tab @code{'#(#f #f #f)} @ @ @tab no     @tab no       @tab no
@item @code{begin-of-line-visible}   @tab @code{'#(#f #f #t)}     @tab no     @tab no       @tab yes
@item @code{end-of-line-visible}     @tab @code{'#(#t #f #f)}     @tab yes    @tab no       @tab no
@item @code{all-visible}             @tab @code{'#(#t #t #t)}     @tab yes    @tab yes      @tab yes
@c The center-visible function is not defined
@c @item @code{center-visible}       @tab @code{'#(#f #t #f)}     @tab no     @tab yes      @tab no
@item @code{begin-of-line-invisible} @tab @code{'#(#t #t #f)}     @tab yes    @tab yes      @tab no
@item @code{end-of-line-invisible}   @tab @code{'#(#f #t #t)}     @tab no     @tab yes      @tab yes
@item @code{center-invisible}        @tab @code{'#(#t #f #t)}     @tab yes    @tab no       @tab yes
@end multitable

The @code{center-visible} function is not pre-defined.

The default settings of @code{break-visibility} depend on the
layout object.  The following table shows all the layout objects
of interest which are affected by @code{break-visibility} and the
default setting of this property:

@multitable @columnfractions .3 .3 .4

@headitem Layout object   @tab Usual context  @tab Default setting

@c omit Ambitus as it appears not to be affected by break-visibility -td
@c @item @code{Ambitus}          @tab as specified   @tab @code{begin-of-line-visible}
@item @code{BarLine}             @tab @code{Score}          @tab calculated
@item @code{BarNumber}           @tab @code{Score}          @tab @code{begin-of-line-visible}
@c omit the following item until it can be explained -td
@c @item @code{BreakAlignGroup}  @tab @code{Score}          @tab calculated
@item @code{BreathingSign}       @tab @code{Voice}          @tab @code{begin-of-line-invisible}
@item @code{Clef}                @tab @code{Staff}          @tab @code{begin-of-line-visible}
@item @code{Custos}              @tab @code{Staff}          @tab @code{end-of-line-visible}
@item @code{DoublePercentRepeat} @tab @code{Voice}          @tab @code{begin-of-line-invisible}
@c omit KeyCancellation until it can be explained -td
@c @item @code{KeyCancellation}  @tab ??             @tab @code{begin-of-line-invisible}
@item @code{KeySignature}        @tab @code{Staff}          @tab @code{begin-of-line-visible}
@c omit LeftEdge until it can be explained -td
@c @item @code{LeftEdge}         @tab @code{Score}          @tab @code{center-invisible}
@item @code{OctavateEight}       @tab @code{Staff}          @tab @code{begin-of-line-visible}
@item @code{RehearsalMark}       @tab @code{Score}          @tab @code{end-of-line-invisible}
@item @code{TimeSignature}       @tab @code{Staff}          @tab @code{all-visible}

@end multitable

The example below shows the use of the vector form to control the
visibility of barlines:

@lilypond[quote,verbatim,relative=1,ragged-right]
f4 g a b
f4 g a b
% Remove bar line at the end of the current line
\once \override Score.BarLine #'break-visibility = #'#(#f #t #t)
\break
f4 g a b
f4 g a b
@end lilypond

Although all three components of the vector used to override
@code{break-visibility} must be present, not all of them are
effective with every layout object, and some combinations may
even give errors.  The following limitations apply:

@itemize @bullet
@item Bar lines cannot be printed at start of line.
@item A bar number cannot be printed at the start of the first
line unless it is set to be different from 1.
@item Clef -- see below
@item Double percent repeats are either all printed or all
suppressed.  Use begin-of line-invisible to print and
all-invisible to suppress.
@item Key signature -- see below
@item OctavateEight -- see below
@end itemize

@node Special considerations
@unnumberedsubsubsec Special considerations

@strong{@emph{Visibility following explicit changes}}

@cindex key signature, visibility following explicit change
@cindex explicitKeySignatureVisibility
@cindex clef, visibility following explicit change
@cindex explicitClefVisibility

The @code{break-visibility} property controls the visibility of
key signatures and changes of clef only at the start of lines,
i.e. after a break.  It has no effect on the visibility of the
key signature or clef following an explicit key change or an
explicit clef change within or at the end of a line.  In the
following example the key signature following the explicit change
to B-flat major is still visible, even though @code{all-invisible}
is set.

@lilypond[quote,verbatim,relative=1,ragged-right]
\key g \major
f4 g a b
% Try to remove all key signatures
\override Staff.KeySignature #'break-visibility = #all-invisible
\key bes \major
f4 g a b
\break
f4 g a b
f4 g a b
@end lilypond

The visibility of such explicit key signature and clef changes is
controlled by the @code{explicitKeySignatureVisibility} and
@code{explicitClefVisibility} properties.  These are the equivalent
of the @code{break-visibility} property and both take a vector of
three booleans or the predefined functions listed above, exactly like
@code{break-visibility}.  Both are properties of the Staff context,
not the layout objects themselves, and so they are set using the
@code{\set} command.  Both are set by default to @code{all-visible}.
These properties control only the visibility of key signatures and
clefs resulting from explicit changes and do not affect key
signatures and clefs at the beginning of lines;
@code{break-visibility} must still be overridden in the appropriate
object to remove these.

@lilypond[quote,verbatim,relative=1,ragged-right]
\key g \major
f4 g a b
\set Staff.explicitKeySignatureVisibility = #all-invisible
\override Staff.KeySignature #'break-visibility = #all-invisible
\key bes \major
f4 g a b \break
f4 g a b
f4 g a b
@end lilypond

@strong{@emph{Visibility of cautionary accidentals}}

To remove the cautionary accidentals printed at an explicit key
change, set the Staff context property @code{printKeyCancellation}
to @code{#f}:

@lilypond[quote,verbatim,relative=1,ragged-right]
\key g \major
f4 g a b
\set Staff.explicitKeySignatureVisibility = #all-invisible
\set Staff.printKeyCancellation = ##f
\override Staff.KeySignature #'break-visibility = #all-invisible
\key bes \major
f4 g a b \break
f4 g a b
f4 g a b
@end lilypond

With these overrides only the accidentals before the notes remain
to indicate the change of key.

@c TODO Add visibility of cautionary accidentals before notes

@strong{@emph{Automatic bars}}

@cindex automaticBars
@cindex bar lines, suppressing

As a special case, the printing of bar lines can also be turned off
by setting the @code{automaticBars} property in the Score context.
If set to @code{#f}, bar lines will not be printed automatically;
they must be explicitly created with a @code{\bar} command.  Unlike
the @code{\cadenzaOn} predefined command, measures are still counted.
Bar generation will resume according to that count if this property
is later set to @code{#t}.  When set to @code{#f}, line breaks can
occur only at explicit @code{\bar} commands.

@c TODO Add example

@strong{@emph{Octavated clefs}}

@cindex octavated clefs, visibility of
@cindex visibility of octavated clefs
@cindex clefs, visibility of octavation

The small octavation symbol on octavated clefs is produced by the
@code{OctavateEight} layout object.  Its visibility is controlled
independently from that of the @code{Clef} object, so it is
necessary to apply any required @code{break-visibility} overrides
to both the @code{Clef} and the @code{OctavateEight} layout objects
to fully suppress such clef symbols at the start of each line.

For explicit clef changes, the @code{explicitClefVisibility}
property controls both the clef symbol and any octavation symbol
associated with it.


@seealso
Learning Manual:
@rlearning{Visibility and color of objects}


@node Line styles
@subsection Line styles

@c TODO: split the following explanations between expressive marks and
@c text-related stuff. Perhaps create a new subsection named
@c "Spanner limits", "Spanner boundaries"? -vv

Some performance indications, e.g., @i{rallentando} and
@i{accelerando} and @i{trills} are written as text and are
extended over many measures with lines, sometimes dotted or wavy.

These all use the same routines as the glissando for drawing the
texts and the lines, and tuning their behavior is therefore also
done in the same way.  It is done with a spanner, and the routine
responsible for drawing the spanners is
@code{ly:line-interface::print}.  This routine determines the
exact location of the two @i{span points} and draws a line in
between, in the style requested.

Here is an example of the different line styles available, and how
to tune them.

@lilypond[relative=2,ragged-right,verbatim,fragment]
d2 \glissando d'2
\once \override Glissando #'style = #'dashed-line
d,2 \glissando d'2
\override Glissando #'style = #'dotted-line
d,2 \glissando d'2
\override Glissando #'style = #'zigzag
d,2 \glissando d'2
\override Glissando #'style = #'trill
d,2 \glissando d'2
@end lilypond

The information that determines the end-points is computed
on-the-fly for every graphic object, but it is possible to
override these:

@c FIXME Complete
@lilypond[relative=2,ragged-right,verbatim,fragment]
e2 \glissando f
\once \override Glissando #'bound-details #'right #'Y = #-2
e2 \glissando f
@end lilypond

The @code{Glissando} object, like any other using the
@code{ly:line-interface::print} routine, carries a nested
association list.  In the above statement, the value for @code{Y}
is set to @code{-2} for the association list corresponding to the
right end point.  Of course, it is also possible to adjust the
left side with @code{left} instead of @code{right}.

If @code{Y} is not set, the value is computed from the vertical
position of right attachment point of the spanner.

In case of a line break, the values for the span-points are
extended with contents of the @code{left-broken} and
@code{right-broken} sublists, for example

@lilypond[relative=2,ragged-right,verbatim,fragment]
\override Glissando #'breakable = ##T
\override Glissando #'bound-details #'right-broken #'Y = #-3
c1 \glissando \break
f1
@end lilypond

For more details, see @ref{Spanners}.

@node Rotating objects
@subsection Rotating objects

Both layout objects and elements of markup text can be rotated by
any angle about any point, but the method of doing so differs.

@menu
* Rotating layout objects::
* Rotating markup::
@end menu

@node Rotating layout objects
@unnumberedsubsubsec Rotating layout objects

@cindex rotating objects
@cindex objects, rotating

All layout objects which support the @code{grob-interface} can be
rotated by setting their @code{rotation} property.  This takes a
list of three items: the angle of rotation counter-clockwise,
and the x and y coordinates of the point relative to the object's
reference point about which the rotation is to be performed.  The
angle of rotation is specified in degrees and the coordinates in
staff-spaces.

The angle of rotation and the coordinates of the rotation point must
be determined by trial and error.

@cindex hairpins, angled
@cindex angled hairpins

There are only a few situations where the rotation of layout
objects is useful; the following example shows one situation where
they may be:

@lilypond[quote,verbatim,relative=1]
g4\< e' d' f\!
\override Hairpin #'rotation = #'(20 -1 0)
g,,4\< e' d' f\!
@end lilypond

@node Rotating markup
@unnumberedsubsubsec Rotating markup

All markup text can be rotated to lie at any angle by prefixing it
with the @code{\rotate} command.  The command takes two arguments:
the angle of rotation in degrees counter-clockwise and the text to
be rotated.  The extents of the text are not rotated: they take
their values from the extremes of the x and y coordinates of the
rotated text.  In the following example the
@code{outside-staff-priority} property for text is set to @code{#f}
to disable the automatic collision avoidance, which would push some
of the text too high.

@lilypond[quote,verbatim,relative=1]
\override TextScript #'outside-staff-priority = ##f
g4^\markup { \rotate #30 "a G" }
b^\markup { \rotate #30 "a B" }
des^\markup { \rotate #30 "a D-Flat" }
fis^\markup { \rotate #30 "an F-Sharp" }
@end lilypond

@node Advanced tweaks
@section Advanced tweaks

@menu
* Aligning objects::
* Vertical grouping of grobs::
* Modifying stencils::
* Modifying shapes::
@end menu

@node Aligning objects
@subsection Aligning objects

Graphical objects which support the @code{self-alignment-interface} and/or
the @code{side-position-interface} can be
aligned to a previously placed object in a variety of ways.  For a list of these objects, see
@rinternals{self-alignment-interface} and @rinternals{side-position-interface}.

All graphical objects have a reference point, a horizontal extent and a
vertical extent.  The horizontal extent is a pair of numbers
giving the displacements from the reference point of the left and
right edges, displacements to the left being negative.  The
vertical extent is a pair of numbers giving the displacement from
the reference point to the bottom and top edges, displacements down
being negative.

An object's position on a staff is given by the values of the
@code{X-offset} and @code{Y-offset} properties.  The value of
@code{X-offset} gives the displacement from the x coordinate of
the reference point of the parent object, and the value of
@code{Y-offset} gives the displacement from the center line of the
staff.   The values of @code{X-offset} and
@code{Y-offset} may be set directly or may be set to be calculated
by procedures in order to achieve alignment with the parent object
in several ways.

@warning{Many objects have special positioning considerations which
cause any setting of @code{X-offset} or @code{Y-offset} to be
ignored or modified, even though the object supports the
@code{self-alignment-interface}.}

For example, an accidental can be repositioned
vertically by setting @code{Y-offset} but any changes to
@code{X-offset} have no effect.

Rehearsal marks may be aligned with
breakable objects such as bar lines, clef symbols, time signature
symbols and key signatures.  There are special properties to be
found in the @code{break-aligned-interface} for positioning rehearsal
marks on such objects.

@menu
* Setting @code{X-offset} and @code{Y-offset} directly::
* Using the @code{side-position-interface}::
* Using the @code{self-alignment-interface}::
* Using the @code{break-aligned-interface}::
@end menu

@node Setting @code{X-offset} and @code{Y-offset} directly
@unnumberedsubsubsec Setting @code{X-offset} and @code{Y-offset} directly

Numerical values may be given to the @code{X-offset} and @code{Y-offset}
properties of many objects.  The following example shows three
notes with the default fingering position and the positions with @code{X-offset}
and @code{Y-offset} modified.

@lilypond[verbatim,quote,relative=2]
a-3
a
-\tweak #'X-offset #0
-\tweak #'Y-offset #0
-3
a
-\tweak #'X-offset #-1
-\tweak #'Y-offset #1
-3
@end lilypond

@c TODO write more

@node Using the @code{side-position-interface}
@unnumberedsubsubsec Using the @code{side-position-interface}

An object which supports the @code{side-position-interface} can be
placed next to its parent object so that
the specified edges of the two objects touch.  The object may be
placed above, below, to the right or to the left of the parent.
The parent cannot be specified; it is determined by the order of
elements in the input stream.  Most objects have the associated
note head as their parent.

The values of the @code{side-axis} and @code{direction} properties
determine where the object is to be placed, as follows:

@c TODO add an example of each to the table

@multitable @columnfractions .3 .3 .3
@headitem @code{side-axis}  @tab @code{direction}  @tab
@headitem property          @tab property          @tab Placement

@item     @code{0}          @tab @code{-1}         @tab left
@item     @code{0}          @tab @code{1}          @tab right
@item     @code{1}          @tab @code{-1}         @tab below
@item     @code{1}          @tab @code{1}          @tab above

@end multitable

When @code{side-axis} is @code{0}, @code{X-offset} should be set to
the procedure @code{ly:side-position-interface::x-aligned-side}.
This procedure will return the correct value of @code{X-offset} to
place the object to the left or right side of the parent according
to value of @code{direction}.

When @code{side-axis} is @code{1}, @code{Y-offset} should be set to
the procedure @code{ly:side-position-interface::y-aligned-side}.
This procedure will return the correct value of @code{Y-offset} to
place the object to the top or bottom of the parent according
to value of @code{direction}.

@c TODO Add examples

@node Using the @code{self-alignment-interface}
@unnumberedsubsubsec Using the @code{self-alignment-interface}

@emph{Self-aligning objects horizontally}

The horizontal alignment of an object which supports the
@code{self-alignment-interface} is controlled by the value of
the @code{self-alignment-X} property, provided the object's
@code{X-offset} property is set to
@code{ly:self-alignment-interface::x-aligned-on-self}.
@code{self-alignment-X}  may be given any
real value, in units of half the total X extent of the
object.  Negative values move the object to the right, positive
to the left.  A value of @code{0} centers the object on the
reference point of its parent, a value of @code{-1} aligns the
left edge of the object on the reference point of its parent,
and a value of @code{1} aligns the right edge of the object on the
reference point of its parent.  The symbols @code{LEFT},
@code{CENTER} and @code{RIGHT} may be used instead of the values
@code{-1, 0, 1} respectively.

Normally the @code{\override} command would be used to modify the
value of @code{self-alignment-X}, but the @code{\tweak} command
can be used to separately align several annotations on a single
note:

@lilypond[quote,verbatim,relative=1]
a'
-\tweak #'self-alignment-X #-1
^"left-aligned"
-\tweak #'self-alignment-X #0
^"center-aligned"
-\tweak #'self-alignment-X #RIGHT
^"right-aligned"
-\tweak #'self-alignment-X #-2.5
^"aligned further to the right"
@end lilypond

@emph{Self-aligning objects vertically}

Objects may be aligned vertically in an analogous way to aligning
them horizontally if the @code{Y-offset} property is set to
@code{ly:self-alignment-interface::y-aligned-on-self}.  However,
other mechanisms are often involved in vertical alignment: the
value of @code{Y-offset} is just one variable taken into account.
This may make adjusting the value of some objects tricky.
The units are just half the vertical extent of the object, which
is usually quite small, so quite large numbers may be required.
A value of @code{-1} aligns the lower edge of the object with
the reference point of the parent object, a value of @code{0}
aligns the center of the object with the reference point of the
parent, and a value of @code{1} aligns the top edge of the object
with the reference point of the parent.  The symbols @code{DOWN},
@code{CENTER}, @code{UP} may be substituted for @code{-1, 0, 1}
respectively.

@emph{Self-aligning objects in both directions}

By setting both @code{X-offset} and @code{Y-offset}, an object may
be aligned in both directions simultaneously.

The following example shows how to adjust a fingering mark so
that it nestles close to the note head.

@lilypond[quote,verbatim,relative=2]
a
-\tweak #'self-alignment-X #0.5  % move horizontally left
-\tweak #'Y-offset #ly:self-alignment-interface::y-aligned-on-self
-\tweak #'self-alignment-Y #-1  % move vertically up
-3  % third finger
@end lilypond

@ignore
@unnumberedsubsubsec Using the @code{aligned-on-parent} procedures

@c Cannot document as they do not seem to operate consistently on all objects -td
@c TODO investigate further

The @code{aligned-on-parent} procedures are used in the same way
as the @code{aligned-on-self} procedures, they difference being
that they permit an object to be aligned with the @emph{edges} of
the parent rather than the parent's reference point.  The following
example shows the difference:

@c TODO Add example

@lilypond[verbatim,quote]
@end lilypond

@end ignore

@ignore
@unnumberedsubsubsec Using the @code{centered-on-parent} procedures

@c Cannot document as they do not seem to operate consistently on all objects -td
@c TODO investigate further

@end ignore

@c TODO The align-interface, BassFigureAlignment and VerticalAlignment

@node Using the @code{break-aligned-interface}
@unnumberedsubsubsec Using the @code{break-aligned-interface}

Rehearsal marks may be aligned with notation objects other
than bar lines.  These objects include @code{ambitus},
@code{breathing-sign}, @code{clef}, @code{custos}, @code{staff-bar},
@code{left-edge}, @code{key-cancellation}, @code{key-signature}, and
@code{time-signature}.

By default, rehearsal marks will be horizontally centered above the
object:

@lilypond[verbatim,quote,relative=1]
e1
% the RehearsalMark will be centered above the Clef
\override Score.RehearsalMark #'break-align-symbols = #'(clef)
\key a \major
\clef treble
\mark "↓"
e
% the RehearsalMark will be centered above the TimeSignature
\override Score.RehearsalMark #'break-align-symbols = #'(time-signature)
\key a \major
\clef treble
\time 3/4
\mark "↓"
e2.
@end lilypond

The alignment of the rehearsal mark relative to the notation object
can be changed, as shown in the following example.  In a score with
multiple staves, this setting should be done for all the staves.

@lilypond[verbatim,quote,relative=1]
% The RehearsalMark will be centered above the KeySignature
\override Score.RehearsalMark #'break-align-symbols = #'(key-signature)
\key a \major
\clef treble
\time 4/4
\mark "↓"
e1
% The RehearsalMark will be aligned with the left edge of the KeySignature
\once \override Score.KeySignature #'break-align-anchor-alignment = #LEFT
\mark "↓"
\key a \major
e
% The RehearsalMark will be aligned with the right edge of the KeySignature
\once \override Score.KeySignature #'break-align-anchor-alignment = #RIGHT
\key a \major
\mark "↓"
e
@end lilypond

The rehearsal mark can also be offset to the right or left of the left edge
by an arbitrary amount.  The units are staff-spaces:

@lilypond[verbatim,quote,relative=1]
% The RehearsalMark will be aligned with the left edge of the KeySignature
% and then shifted right by 3.5 staff-spaces
\override Score.RehearsalMark #'break-align-symbols = #'(key-signature)
\once \override Score.KeySignature #'break-align-anchor = #3.5
\key a \major
\mark "↓"
e
% The RehearsalMark will be aligned with the left edge of the KeySignature
% and then shifted left by 2 staff-spaces
\once \override Score.KeySignature #'break-align-anchor = #-2
\key a \major
\mark "↓"
e
@end lilypond



@node Vertical grouping of grobs
@subsection Vertical grouping of grobs

@c FIXME Expand this section

The VerticalAlignment and VerticalAxisGroup grobs work together.
VerticalAxisGroup groups together different grobs like Staff, Lyrics,
etc.  VerticalAlignment then vertically aligns the different grobs
grouped together by VerticalAxisGroup.  There is usually only one
VerticalAlignment per score but every Staff, Lyrics, etc. has its own
VerticalAxisGroup.


@node Modifying stencils
@subsection Modifying stencils

All layout objects have a @code{stencil} property which is part of
the @code{grob-interface}.  By default, this property is usually
set to a function specific to the object that is tailor-made to
render the symbol which represents it in the output.  For example,
the standard setting for the @code{stencil} property of the
@code{MultiMeasureRest} object is @code{ly:multi-measure-rest::print}.

The standard symbol for any object can be replaced by modifying the
@code{stencil} property to reference a different, specially-written,
procedure.  This requires a high level of knowledge of the internal
workings of LilyPond, but there is an easier way which can often
produce adequate results.

This is to set the @code{stencil} property to the procedure which
prints text -- @code{ly:text-interface::print} -- and to add a
@code{text} property to the object which is set to contain the
markup text which produces the required symbol.  Due to the
flexibility of markup, much can be achieved -- see in particular
@ref{Graphic notation inside markup}.

The following example demonstrates this by changing the note head
symbol to a cross within a circle.

@lilypond[verbatim,quote]
XinO = {
  \once \override NoteHead  #'stencil = #ly:text-interface::print
  \once \override NoteHead #'text = \markup {
    \combine
      \halign #-0.7 \draw-circle #0.85 #0.2 ##f
      \musicglyph #"noteheads.s2cross"
  }
}
\relative c'' {
  a a \XinO a a
}
@end lilypond

Any of the glyphs in the feta Font can be supplied to the
@code{\musicglyph} markup command -- see @ref{The Feta font}.

@c TODO Add inserting eps files or ref to later

@c TODO Add inserting Postscript or ref to later

@seealso

Notation Reference:
@ref{Graphic notation inside markup},
@ref{Formatting text},
@ref{Text markup commands},
@ref{The Feta font}.


@node Modifying shapes
@subsection Modifying shapes

@menu
* Modifying ties and slurs::
@end menu

@node Modifying ties and slurs
@unnumberedsubsubsec Modifying ties and slurs

Ties, slurs and phrasing slurs are drawn as third-order Bézier
curves.  If the shape of the tie or slur which is calculated
automatically is not optimum, the shape may be modified manually by
explicitly specifying the four control points required to define
a third-order Bézier curve.

Third-order or cubic Bézier curves are defined by four control
points.  The first and fourth control points are precisely the
starting and ending points of the curve.  The intermediate two
control points define the shape.  Animations showing how the curve
is drawn can be found on the web, but the following description
may be helpful.  The curve starts from the first control point
heading directly towards the second, gradually bending over to
head towards the third and continuing to bend over to head towards
the fourth, arriving there travelling directly from the third
control point.  The curve is entirely contained in the
quadrilateral defined by the four control points.

Here is an example of a case where the tie is not optimum, and
where @code{\tieDown} would not help.

@lilypond[verbatim,quote,relative=1]
<<
  { e1 ~ e }
\\
  { r4 <g c,> <g c,> <g c,> }
>>
@end lilypond

One way of improving this tie is to manually modify its control
points, as follows.

The coordinates of the Bézier control points are specified in units
of staff-spaces.  The X@tie{}coordinate is relative to the reference
point of the note to which the tie or slur is attached, and the
Y@tie{}coordinate is relative to the staff center line.  The
coordinates are entered as a list of four pairs of decimal numbers
(reals).  One approach is to estimate the coordinates of the two
end points, and then guess the two intermediate points.  The optimum
values are then found by trial and error.

It is useful to remember that a symmetric curve requires symmetric
control points, and that Bézier curves have the useful property that
transformations of the curve such as translation, rotation and
scaling can be achieved by applying the same transformation to the
curve's control points.

For the example above the following override gives a satisfactory
tie:

@lilypond[verbatim,quote,relative=1]
<<
  \once \override Tie
    #'control-points = #'((1 . -1) (3 . 0.6) (12.5 . 0.6) (14.5 . -1))
  { e1 ~ e1 }
\\
  { r4 <g c,> <g c,> <g c,>4  }
>>
@end lilypond

@knownissues

It is not possible to modify shapes of ties or slurs by changing
the @code{control-points} property if there are more than one at
the same musical moment, not even by using the @code{\tweak}
command.



@node Discussion of specific tweaks
@section Discussion of specific tweaks

@c TODO What should go in here?