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@node Tweaking output
@chapter Tweaking output

This chapter discusses how to modify output.  LilyPond is extremely
configurable; virtually every fragment of output may be changed.


@menu
* Tweaking basics::             
* The Internals Reference manual::  
* Appearance of objects::       
* Placement of objects::        
* Collisions of objects::       
* Further tweaking::            
@end menu

@node Tweaking basics
@section Tweaking basics

@menu
* Introduction to tweaks::      
* Objects and interfaces::      
* Naming conventions of objects and properties::  
* Tweaking methods::            
@end menu

@node Introduction to tweaks
@subsection Introduction to tweaks

@q{Tweaking} is a LilyPond term for the various methods available
to the user for modifying the actions taken during interpretation
of the input file and modifying the appearance of the printed 
output.  Some tweaks are very easy to use; others are more 
complex.  But taken together the methods available for tweaking 
permit almost any desired appearance of the printed music to be 
achieved.

In this section we cover the basic concepts required to understand
tweaking.  Later we give a variety of ready-made commands which can
simply be copied to obtain the same effect in your own scores, and
at the same time we show how these commands may be constructed so 
that you may learn how to develop your own tweaks.

Before starting on this Chapter you may wish to review the section
@ref{Contexts and engravers}, as Contexts, Engravers, and the
Properties contained within them are fundamental to understanding
and constructing Tweaks.

@node Objects and interfaces
@subsection Objects and interfaces

@cindex Objects
@cindex Grobs
@cindex Spanners
@cindex Interfaces

Tweaking involves modifying the internal operation and structures
of the LilyPond program, so we must first introduce some terms
which are used to describe those internal operations and 
structures.

The term @q{Object} is a generic term used to refer to the
multitude of internal structures built by LilyPond during the
processing of an input file.  So when a command like @code{\new
Staff} is encountered a new object of type @code{Staff} is
constructed.  That @code{Staff} object then holds all the
properties associated with that particular staff, for example, its
name and its key signature, together with details of the engravers
which have been assigned to operate within that staff's context.
Similarly, there are objects to hold the properties of all other
contexts, such as @code{Voice} objects, @code{Score} objects,
@code{Lyrics} objects, as well as objects to represent all
notational elements such as bar lines,
note heads, ties, dynamics, etc.  Every object has its own set of
property values.

Some types of object are given special names.  Objects which
represent items of notation on the printed output such as
 note heads, stems, slurs, ties, fingering, clefs, etc are called
@q{Layout objects}, often known as @q{Graphical Objects}, or
@q{Grobs} for short.  These are still objects in the generic sense
above, and so they too all have properties associated with them,
such as their position, size, color, etc.

Some layout objects are still more specialised.  Phrasing slurs,
crescendo hairpins, ottavo marks, and many other grobs are not
localised in a single place -- they have a starting point, an
ending point, and maybe other properties concerned with their
shape.  Objects with an extended shape like these are called
@q{Spanners}.

It remains to explain what @q{Interfaces} are.  Many objects,
even though they are quite different, share common features 
which need to be processed in the same way.
For example, all grobs have a color, a size, a position, etc,
and all these properties are processed in the same way during
LilyPond's
interpretation of the input file.  To simplify these internal
operations these common actions and properties are grouped 
together in an object called a @code{grob-interface}.  There
are many other groupings of common properties like this, each 
one given a name ending in @code{-interface}.  In total there
are over 100 such interfaces.  We shall see later why this is
of interest and use to the user.

These, then, are the main terms relating to objects which we 
shall use in this chapter.

@node Naming conventions of objects and properties
@subsection Naming conventions of objects and properties

We met some object naming conventions previously, in 
@ref{Contexts and engravers}.  Here for reference is a list
of the most common object and property types together with 
the conventions for naming them and a couple of examples of 
some real names.  We have used A to stand for any capitalised
alphabetic character and aaa to stand for any number of
lower-case alphabetic characters.  Other characters are used
verbatim. 

@multitable @columnfractions .33 .33 .33
@headitem Object/property type           
  @tab Naming convention
  @tab Example
@item Contexts
  @tab Aaaa or AaaaAaaaAaaa
  @tab Staff, GrandStaff
@item Layout Objects
  @tab Aaaa or AaaaAaaaAaaa
  @tab Slur, NoteHead
@item Engravers
  @tab Aaaa_aaa_engraver
  @tab Clef_engraver, Note_heads_engraver
@item Interfaces
  @tab aaa-aaa-interface
  @tab grob-interface, break-aligned-interface 
@item Context Properties
  @tab aaa or aaaAaaaAaaa
  @tab alignAboveContext, skipBars
@item Layout Object Properties
  @tab aaa or aaa-aaa-aaa
  @tab direction, beam-thickness
@end multitable

As we shall see shortly, the properties of different types of 
object are modified by different commands, so it is useful to
be able to recognise the type of object from the names of its
properties.


@node Tweaking methods
@subsection Tweaking methods

We have already met the commands @code{\set} and @code{\with},
used to change the properties of @strong{contexts} and to remove
and add @strong{engravers}, in
@ref{Modifying context properties} and @ref{Adding
and removing engravers}.  We now must meet one more command.

The command to change the properties of @strong{layout objects} is
@code{\override}.  Because this command has to modify
internal properties deep within LilyPond its syntax is not
as simple as the commands you have met so far.  It needs to
know precisely which property of which object in which context
has to be modified, and what its new value is to be.  Let's see
how this is done.

The general syntax of this command is:

@example
\override @emph{context}.@emph{layout_object} #'@emph{layout_property} = #@emph{value}
@end example

@noindent
This will set the property with the name @emph{layout_property}
of the layout object with the name 
@emph{layout_object}, which is a member of the @emph{context}
context, to the value @emph{value}.

The @emph{context} can be omitted (and usually is) when the
required context is unambiguously implied and is one of lowest
level contexts, i.e., @code{Voice}, @code{ChordNames} or
@code{Lyrics}, and we shall omit it in many of the following
examples.  We shall see later when it must be specified.

For now, don't worry about the @code{#'}, which must precede the
layout property, and the @code{#}, which must precede the value.
These must always be present in exactly this form.  This is the
most common command used in tweaking, and most of the rest of
this chapter will be directed to presenting examples of how it is
used.

Once overridden, the property retains its new value until it is
overridden again or a @code{\revert} command is encountered.
The @code{\revert} command has the following syntax and causes
the value of the property to revert to its original default
value; note, not its previous value if several @code{\override}
commands have been issued.

@example
\revert @emph{context}.@emph{layout_object} #'@emph{layout_property}
@end example

Again, just like @emph{context} in the @code{\override} command,
@emph{context} is often not needed.  It will be omitted
in many of the following examples.

There is another form of the override command, 
@code{\overrideProperty}, which is occasionally required.
We mention it here for completeness, but for details see 
@ruser{Difficult tweaks}.
@c Maybe explain in a later iteration  -td

The final tweaking command which is available is @code{\tweak}.
This should be used to change the properties of objects which
occur at the same musical moment, such as the notes within a
chord.  Using @code{\override} would affect all the notes
within a chord, whereas @code{\tweak} affects just the following
item in the input stream.  You may find the details in
@ruser{Objects connected to the input}.

@node The Internals Reference manual
@section The Internals Reference manual

@cindex Internals Reference

@menu
* Properties of layout objects::  
* Properties found in interfaces::  
* Types of properties::         
@end menu

@node Properties of layout objects
@subsection Properties of layout objects

@cindex properties of layout objects
@cindex properties of grobs
@cindex grobs, properties of
@cindex layout objects, properties of

Suppose you have a slur in a score which, to your mind,
appears too thin and you'd like to draw it a little heavier.
How do you go about doing this?  You know from the statements
earlier about the flexibility of LilyPond that such a thing
should be possible, and you would probably guess that an
@code{\override} command would be needed.  But is there a
heaviness property for a slur, and if there is, how might it 
be modified?  This is where the Internals Reference manual
comes in.  It contains all the information you might need to
construct this and all other @code{\override} commands.

Before we look at the Internals Reference a word of warning.
This is a @strong{reference} document, which means there is
little or no explanation contained within it: its purpose is
to present information precisely and concisely.  This
means it might look daunting at first sight.  Don't worry!
The guidance and explanation presented here will enable you
to extract the information from the Internals Reference for
yourself with just a little practice.

@cindex override example
@cindex Internals Reference, example of using

Let's use a concrete example with a simple fragment of real
music:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 6/8
  {
    r4 b8 b[( g]) g | 
    g[( e]) e d[( f]) a | 
    a g
  }
  \addlyrics {
    The man who feels love's sweet e -- mo -- tion 
  }
}
@end lilypond 

Suppose now that we decide we would like the slurs to be a
little heavier.  Is this possible?  The slur is certainly a
layout object, so the question is, @q{Is there a property
belonging to a slur which controls the heaviness?}  To answer
this we must look in the Internals Reference, or IR for short.

The IR for the version of LilyPond you are using may be found
on the LilyPond website at @uref{http://lilypond.org}.  Go to the
documentation page and click on the Internals Reference link.
For learning purposes you should use the standard html version,
not the @q{one big page} or the PDF.  For the next few
paragraphs to make sense you will need to actually do this
as you read.

Under the heading @strong{Top} you will see five links.  Select
the link to the @emph{Backend}, which is where information about
layout objects is to be found.  There, under the heading 
@strong{Backend}, select the link to @emph{All layout objects}.
The page that appears lists all the layout objects used in your
version of LilyPond, in alphabetic order.  Select the link to
Slur, and the properties of Slurs are listed.

(An alternative way of finding this page is from the Notation
Reference.  On one of the pages that deals with slurs you may
find a link to the Internals Reference.  This link will
take you directly to this page, but often it is easier to go
straight to the IR and search there.)

This Slur page in the IR tells us first that Slur objects are 
created by the 
Slur_engraver.  Then it lists the standard settings.  Note
these are @strong{not} in alphabetic order.  Browse down
them looking for a property that might control the heaviness
of slurs, and you should find

@example
@code{thickness} (number)
     @code{1.2}
     Line thickness, generally measured in @code{line-thickness}
@end example

This looks a good bet to change the heaviness. It tells us that
the value of @code{thickness} is a simple @emph{number},
that the default value is 1.2, and that the units are
in another property called @code{line-thickness}.

As we said earlier, there are few to no explanations in the IR,
but we already have enough information to try changing the
slur thickness.  We see that the name of the layout object 
is @code{Slur}, that the name of the property to change is
@code{thickness} and that the new value should be a number
somewhat larger than 1.2 if we are to make slurs thicker.

We can now construct the @code{\override} command by simply
substituting the values we have found for the names, omitting
the context.  Let's use a very large value for the thickness 
at first, so we can be sure the command is working.  We get:

@example
  \override Slur #'thickness = #5.0
@end example 

Don't forget the @code{#'} preceding the
property name and and @code{#} preceding the new value!

The final question is, @q{Where should this command be
placed?}  While you are unsure and learning, the best 
answer is, @q{Within the music, before the first slur and
close to it.}  Let's do that:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 6/8
  {
    % Increase thickness of all following slurs from 1.2 to 5.0
    \override Slur #'thickness = #5.0
    r4 b8 b[( g]) g |
    g[( e]) e d[( f]) a |
    a g
  }
  \addlyrics {
    The man who feels love's sweet e -- mo -- tion 
  }
}
@end lilypond 

@noindent
and we see that the slur is indeed heavier.

So this is the basic way of constructing @code{\override}
commands.  There are a few more complications that we
shall meet in later sections, but you now know all the
essentials required to make up your own -- but you will
still need some practice.  This is provided in the examples
which follow.

@subheading Finding the context
@cindex context, finding

But first, what if we had needed to specify the Context?  
What should it be?  We could guess that slurs are in
the Voice context, as they are clearly closely associated
with individual lines of music, but can we be sure?  To
find out, go back to the top of the IR page describing the
Slur, where it says @q{Slur objects are created by: Slur
engraver}.  So slurs will be created in whichever context
the @code{Slur_engraver} is in.  Follow the link to the
@code{Slur_engraver} page.  At the very bottom it tells
us that @code{Slur_engraver} is part of five Voice contexts,
including the standard voice context, @code{Voice}, so our
guess was correct.  And because @code{Voice} is one of the
lowest level contexts which is implied unambiguously by 
the fact that we are entering notes, we can omit it in this 
location.

@subheading Overriding once only

@cindex overriding once only
@cindex once override
@funindex \once

As you can see, @emph{all} the slurs are thicker in the
final example above.  But what if we
wanted just the first slur to be thicker?  This is achieved
with the @code{\once} command.  Placed immediately before
the @code{\override} command it causes it to change only the
slur which begins on the @strong{immediately following} note.
If the
immediately following note does not begin a slur the command
has no effect at all -- it is not remembered until a slur
is encountered, it is simply discarded.  So the command with
@code{\once} must be
repositioned as follows:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 6/8
  {
    r4 b8
    % Increase thickness of immediately following slur only
    \once \override Slur #'thickness = #5.0
    b[( g]) g | 
    g[( e]) e d[( f]) a | 
    a g 
  }
  \addlyrics {
    The man who feels love's sweet e -- mo -- tion 
  }
}
@end lilypond 

@noindent
Now only the first slur is made heavier.

The @code{\once} command can also be used before @code{\set}
and @code{\unset}, and before the command to be introduced
in the following section -- @code{revert}.
 
@subheading Reverting

@cindex revert
@cindex default properties, reverting
@funindex \revert

Finally, what if we wanted just the first two slurs to be
heavier?  Well, we could use two commands, each preceded by
@code{\once} placed immediately before each of the notes where
the slurs begin:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 6/8
  {
    r4 b8
    % Increase thickness of immediately following slur only
    \once \override Slur #'thickness = #5.0
    b[( g]) g |
    % Increase thickness of immediately following slur only
    \once \override Slur #'thickness = #5.0
    g[( e]) e d[( f]) a | 
    a g
  }
  \addlyrics {
    The man who feels love's sweet e -- mo -- tion 
  }
}
@end lilypond 

@noindent
or we could omit the @code{\once} command and use the
@code{\revert} command
to return the @code{thickness} property to its default value
after the second slur:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 6/8
  {
    r4 b8
    % Increase thickness of all following slurs from 1.2 to 5.0
    \override Slur #'thickness = #5.0
    b[( g]) g |
    g[( e]) 
    % Revert thickness of all following slurs to default of 1.2
    \revert Slur #'thickness
    e d[( f]) a | 
    a g
  }
  \addlyrics {
    The man who feels love's sweet e -- mo -- tion 
  }
}
@end lilypond

@noindent
The @code{\revert} command can be used to return any property
changed with @code{\override} back to its default value.
You may use whichever method best suits what you want to do.

That concludes our introduction to the IR, and the basic
method of tweaking.  Several examples follow in the later
sections of this Chapter, partly to introduce you to some of the
additional features of the IR, and partly to give you more 
practice in extracting information from it.  These examples will
contain progressively fewer words of guidance and explanation. 

@node Properties found in interfaces
@subsection Properties found in interfaces

@cindex interface properties
@cindex properties in interfaces

Suppose now that we wish to print the lyrics in italics.  What
form of @code{\override} command do we need to do this?
We first look in the IR page listing @q{All layout objects}, as
before, and look for an object that might control lyrics.  We
find @code{LyricText}, which looks right.  Clicking on this shows
the settable properties for lyric text.  These include the 
@code{font-series} and @code{font-size}, but nothing that might
give an italic shape.   
This is because the shape property is one that is common to all
font objects, so, rather than including it in every layout
object, it is grouped together with other similar common
properties and placed in an @strong{Interface}, the
@code{font-interface}.

So now we need to learn how to find the properties of interfaces,
and to discover what objects use these interface properties.

Look again at the IR page which describes LyricText.  At the
bottom of the page is a list of clickable (in the html versions
of the IR) interfaces which LyricText supports.  The list has 
seven items, including @code{font-interface}.
Clicking on this brings up the properties associated
with this interface, which are also properties of all the objects
which support it, including LyricText.

Now we see all the user-settable properties which control fonts,
including @code{font-shape(symbol)}, where @code{symbol} can be
set to @code{upright}, @code{italics} or @code{caps}.

You will notice that that @code{font-series} and @code{font-size}
are also listed there.
This immediately raises the question: Why are the common font
properties @code{font-series} and @code{font-size} listed under
@code{LyricText} as well as under the interface
@code{font-interface} but @code{font-shape} is not?  The answer
is that @code{font-series} and @code{font-size} are changed
from their global default values when a @code{LyricText} object
is created, but @code{font-shape} is not.  The entries in
@code{LyricText} then tell you the values for those two
properties which apply to @code{LyricText}.  Other objects
which support @code{font-interface} will set these
properties diferently when they are created.

Let's see if we can now construct the @code{\override} command
to change the lyrics to italics.  The object is @code{LyricText},
the property is @code{font-shape} and the value is
@code{italic}.  As before, we'll omit the context.

As an aside, although it is an important one, note that because 
the values of
@code{font-shape} are symbols they must be introduced with a
single apostrophe, @code{'}.  That is why apostrophes
are needed before @code{thickness} in the earlier example
and @code{font-shape}.  These are both symbols too.
Symbols are special names which are known internally to 
LilyPond.  Some of them are the names of properties,
like @code{thickness} or @code{font-shape}, others are in
effect special values that can be given to properties, like
@code{italic}.  Note the distinction from arbitary
text strings, which would appear as @code{"a text string"}.

Ok, so the @code{\override} command we need to print the lyrics
in italics should be

@example
  \override LyricText #'font-shape = #'italic
@end example

@noindent
and this should be placed just in front of and close to the
lyrics which it should affect, like this:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 6/8
  {
    r4 b8 b[( g]) g |
    g[( e]) e d[( f]) a |
    a g
  }
  \addlyrics {
    \override LyricText #'font-shape = #'italic
    The man who feels love's sweet e -- mo -- tion 
  }
}
@end lilypond

@noindent
and the lyrics are all printed in italics.

@subheading Specifying context in lyric mode
@cindex context, specifying in lyric mode

In the case of lyrics, if you try specifying the context in the 
format given earlier the command will fail.  A syllable
entered in lyricmode is terminated by either a space,
a newline or a digit.  All other characters are included 
as part of the syllable.  For this reason a space or newline
must appear before the terminating @code{@}} to prevent it being
included as part of the final syllable.  Similarly,
spaces must be inserted before and after the
period or dot, @q{.}, separating the context name from the
object name, as otherwise the two names are run together and
the interpreter cannot recognise them.  So the command should be:

@example
  \override Lyrics . LyricText #'font-shape = #'italic
@end example

@warning{In lyrics always leave whitespace between the final
syllable and the terminating brace.}

@warning{In overrides in lyrics always place spaces around
the dot between the context name and the object name.}

@node Types of properties
@subsection Types of properties

@cindex Property types

So far we have seen two types of property: @code{number} and
@code{symbol}.  To be valid, the value given to a property
must be of the correct type and obey the rules for that type.
The type of property is always shown in brackets after the
property name in the IR.  Here is a list of the types you may 
need, together with the rules for that type, and some examples.
You must always add a hash symbol, @code{#}, of course,
to the front of these values when they are entered in the 
@code{\override} command.

@multitable @columnfractions .2 .45 .35
@headitem Property type           
  @tab Rules
  @tab Examples
@item Boolean
  @tab Either True or False, represented by #t or #f
  @tab @code{#t}, @code{#f}
@item Dimension (in staff space)
  @tab A positive decimal number (in units of staff space)
  @tab @code{2.5}, @code{0.34}
@item Direction
  @tab A valid direction constant or its numerical equivalent
  @tab @code{#LEFT}, @code{#CENTER}, @code{#UP},
       @code{1}, @code{-1}
@item Integer
  @tab A positive whole number
  @tab @code{3}, @code{1}
@item List
  @tab A bracketed set of items separated by spaces,
preceded by an apostrophe
  @tab @code{'(left-edge staff-bar)}, @code{'(1)},
       @code{'(1.0 0.25 0.5)}
@item Markup
  @tab Any valid markup
  @tab @code{\markup @{ \italic "cresc." @}}
@item Moment
  @tab A fraction of a whole note constructed with the 
make-moment function
  @tab @code{(ly:make-moment 1 4)},
       @code{(ly:make-moment 3 8)}
@item Number
  @tab Any positive or negative decimal value
  @tab @code{3.5}, @code{-2.45}
@item Pair (of numbers)
  @tab Two numbers separated by a @q{space . space} and enclosed
in brackets preceded by an apostrophe
  @tab @code{'(2 . 3.5)}, @code{'(0.1 . -3.2)}
@item Symbol
  @tab Any of the set of permitted symbols for that property,
preceded by an apostrophe
  @tab @code{'italic}, @code{'inside}
@item Unknown
  @tab A procedure or @code{#f} (to cause no action)
  @tab @code{bend::print}, @code{ly:text-interface::print},
       @code{#f}
@item Vector
  @tab A list of three items enclosed in brackets and preceded
by a hash sign, @code{#}.
  @tab @code{#(#t #t #f)}
@end multitable

@node Appearance of objects
@section Appearance of objects

Let us now put what we have learnt into practice with a few
examples which show how tweaks may be used to change the 
appearance of the printed music.

@menu
* Visibility and color of objects::  
* Size of objects::             
* Length and thickness of objects::  
@end menu

@node Visibility and color of objects
@subsection Visibility and color of objects

In the educational use of music we might wish to print a score
with certain elements omitted as an exercise for the student,
who is required to supply them.  As a simple example,
let us suppose the exercise is to supply the missing bar lines 
in a piece of music.  But the bar lines are normally inserted
automatically.  How do we prevent them printing?

Before we tackle this, let us remember that object properties
are grouped in what are called @emph{interfaces} -- see
@ref{Properties found in interfaces}.  This is simply to
group together those properties that are commonly required
together -- if one of them is required for an object, so are
the others.  Some objects then need the properties in some
interfaces, others need them from other interfaces.  The
interfaces which contain the properties required by a 
particular grob are listed in the IR at the bottom of the
page describing that grob, and those properties may be
viewed by looking at those interfaces.

We explained how to find information about grobs in 
@ref{Properties of layout objects}.  Using the same approach,
we go to the IR to find the layout object which prints
bar lines.  Going via @emph{Backend} and @emph{All layout objects}
we find there
is a layout object called @code{BarLine}.  Its properties include
two that control its visibility: @code{break-visibility} and
@code{stencil}.  Barline also supports a number of interfaces,
including the @code{grob-interface}, where we find the
@code{transparent} and the @code{color} properties.  All
of these can affect the visibility of bar lines (and, of course,
by extension, many other layout objects too.)  Let's consider
each of these in turn.

@subheading stencil
@cindex stencil property

This property controls the appearance of the bar lines by specifying
the symbol (glyph) which should be printed.  In common
with many other properties, it can be set to print nothing by 
setting its value to @code{#f}.  Let's try it, as before, omitting
the implied Context, @code{Voice}:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override BarLine #'stencil = ##f
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

The bar lines are still printed.  What is wrong?  Go back to the
IR and look again at the page giving the properties of BarLine.
At the top of the page it says @qq{Barline objects are created
by: Bar_engraver}.  Go to the @code{Bar_engraver} page.
At the bottom
it gives a list of Contexts in which the bar engraver operates.
All of them are of the type @code{Staff}, so the reason the
@code{\override} command failed to work as expected is because
@code{Barline} is not in the default @code{Voice} context.
If the context
is specified wrongly, the command simply does not work.  No
error message is produced, and nothing is logged in the log
file.  Let's try correcting it by adding the correct context:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.BarLine #'stencil = ##f
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

Now the bar lines have vanished.

@subheading break-visibility

@cindex break-visibility property

We see from the @code{BarLine} properties in the IR that the
@code{break-visibility} property requires a vector of three
booleans.
These control respectively whether bar lines are printed at
the end of a line, in the middle of lines, and at the beginning
of lines.  For our example we want all bar lines to be suppressed,
so the value we need is @code{#(#f #f #f)}.
Let's try that, remembering
to include the @code{Staff} context.  Note also that in writing
this value we have two hash signs before the opening bracket.
One is required as part of the value to introduce a vector, 
and one is required, as always, to precede the value itself in 
the @code{\override} command.

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.BarLine #'break-visibility = ##(#f #f #f)
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

And we see this too removes all the bar lines.

@subheading transparent
@cindex transparent property

We see from the properties specified in the @code{grob-interface}
page in the IR that the @code{transparent} property is a boolean.
This
should be set to @code{#t} to make the grob transparent.
In this next example let us make the time signature invisible 
rather than the bar lines.  
To do this we need to find the grob name for the time signature.
Back to
the @q{All layout objects} page in the IR to find the properties
of the @code{TimeSignature} layout object.  This is produced by
the @code{Time_signature_engraver} which you can check also lives
in the @code{Staff} context and also supports the
@code{grob-interface}.  So the command to make the time signature
transparent is:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.TimeSignature #'transparent = ##t
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

@noindent
The time signature is gone, but this command leaves a gap where 
the time signature should be.  Maybe this is what is wanted for
an exercise for the student to fill it in, but in other 
circumstances a gap might be undesirable.  To remove it, the 
stencil for the time signature should be set to @code{#f}
instead:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.TimeSignature #'stencil = ##f
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

@noindent
and the difference is obvious: setting the stencil to @code{#f}
removes the object entirely; making the object @code{transparent}
leaves it where it is, but makes it invisible.

@subheading color
@cindex color property

Finally we could make the bar lines invisible by coloring
them white.  The @code{grob-interface} specifies that the
color property value is a list, but there is no
explanation of what that list should be.  The list it
requires is actually a list of values in internal units,
but, to avoid having to know what these are, several ways
are provided to specify colors.  The first way is to use one
of the @q{normal} colours listed in the first table in
@ruser{List of colors}.  To set the bar lines to white
we write:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.BarLine #'color = #white
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

@noindent
and again, we see the bar lines are not visible.  Note that
@emph{white} is not preceded by an apostrophe -- it is not
a symbol, but a @emph{function}.  When called, it provides
the list of internal values required to set the color to
white.  The other colors in the normal list are functions
too.  To convince yourself this is working you might like 
to change the color to one of the other functions in the
list.

@cindex color, X11
@cindex X11 colors

The second way of changing the color is to use the list of
X11 color names in the second list in @ruser{List of colors}.
However, these must be preceded by another function, which
converts X11 color names into the list of internal values,
@code{x11-color}, like this:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.BarLine #'color = #(x11-color 'white)
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

@noindent
Note that in this case the function @code{x11-color} takes
a symbol as an argument, so the symbol must be preceded by
an apostrophe and the two enclosed in brackets.

@cindex rgb colors
@cindex color, rgb

There is yet a third function, one which converts RGB values into
internal colors -- the @code{rgb-color} function.  This takes
three arguments giving the intensities of the red, green and 
blue colors.  These take values in the range 0 to 1.  So to
set the color to red the value should be @code{(rgb-color 1 0 0)}
and to white it should be @code{(rgb-color 1 1 1)}:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.BarLine #'color = #(rgb-color 1 1 1)
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

Finally, there is also a grey scale available as part of the
X11 set of colors.  These range from black, @code{'grey0'},
to white, @code{'grey100}, in steps of 1.  Let's illustrate
this by setting all the layout objects in our example to
various shades of grey:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
{
  \time 12/16
  \override Staff.StaffSymbol   #'color = #(x11-color 'grey30)
  \override Staff.TimeSignature #'color = #(x11-color 'grey60)
  \override Staff.Clef          #'color = #(x11-color 'grey60)
  \override Voice.NoteHead      #'color = #(x11-color 'grey85)
  \override Voice.Stem          #'color = #(x11-color 'grey85)
  \override Staff.BarLine       #'color = #(x11-color 'grey10)
  c4 b8 c d16 c d8 |
  g, a16 b8 c d4 e16 |
  e8
}
@end lilypond

@noindent
Note the contexts associated with each of the layout objects.
It is important to get these right, or the commands will not
work!  Remember, the context is the one in which the appropriate
engraver is placed.  The default context for engravers can be
found by starting from the layout object, going from there to
the engraver which produces it, and on the engraver page in the
IR it tells you in which context the engraver will normally be 
found.  


@node Size of objects
@subsection Size of objects

Let us begin by looking again at the earlier example 
see @ref{Nesting music expressions}) which showed
how to introduce a new temporary staff, as in an @rglos{ossia}.

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\new Staff ="main" {
       \relative g' {
         r4 g8 g c4 c8 d |
         e4 r8
         <<
           { f c c }
           \new Staff \with {
             alignAboveContext = "main" }
           { f8 f c }
         >>
         r4 |
       }
     }
@end lilypond

Ossia are normally written without clef and time signature, and
are usually printed slightly smaller than the main staff.  We
already know now how to remove the clef and time signature --
we simply set the stencil of each to @code{#f}, as follows:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\new Staff ="main" {
  \relative g' {
    r4 g8 g c4 c8 d |
    e4 r8
    <<
      { f c c }
      \new Staff \with {
        alignAboveContext = "main" 
      }
      {
        \override Staff.Clef #'stencil = ##f
        \override Staff.TimeSignature #'stencil = ##f
        { f8 f c }
      }
    >>
    r4 |
  }
}
@end lilypond

@noindent
where the extra pair of braces after the @code{\with} clause are
required to ensure the enclosed overrrides and music are applied
to the ossia staff.

But what is the difference between modifying the staff context by 
using @code{\with} and modifying the stencils of the clef and the
time signature with \override?  The main difference is that 
changes made in a @code{\with} clause are made at the time the
context is created, and remain in force as the @strong{default}
values for the duration of that context, whereas
@code{\set} or @code{\override} commands embedded in the
music are dynamic -- they make changes synchronised with
a particular point in the music.  If changes are unset or
reverted using @code{\unset} or @code{\revert} they return to
their default values, which will be the ones set in the 
@code{\with} clause, or if none have been set there, the normal
default values.

Some context properties
can be modified only in @code{\with} clauses.  These are those
properties which cannot sensibly be changed after the context 
has been created.  @code{alignAboveContext} and its partner,
@code{alignBelowContext}, are two such properties -- once the
staff has been created its alignment is decided and it would
make no sense to try to change it later.

The default values of layout object properties can also be set
in @code{\with} clauses.  Simply use the normal @code{\override}
command leaving out the context name, since this is unambiguously
defined as the context which the @code{\with} clause is modifying.
If fact, an error will be generated if a context is specified
in this location.

So we could replace the example above with

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\new Staff ="main" {
  \relative g' {
    r4 g8 g c4 c8 d |
    e4 r8
    <<
      { f c c }
      \new Staff \with {
        alignAboveContext = "main" 
        % Don't print clefs in this staff
        \override Clef #'stencil = ##f
        % Don't print time signatures in this staff
        \override TimeSignature #'stencil = ##f
      }
        { f8 f c }
    >>
    r4 |
  }
}
@end lilypond

Finally we come to changing the size of layout objects.

Some layout objects are created as glyphs selected from
a typeface font.  These include note heads, accidentals, markup, 
clefs, time signatures, dynamics and lyrics.  
Their size is changed by modifying the
@code{font-size} property, as we shall shortly see.  Other
layout objects such as slurs and ties -- in general, spanner
objects -- are drawn individually, so there is no 
@code{font-size}
associated with them.  These objects generally derive their
size from the objects to which they are attached, so usually
there is no need to change their size manually.  Still other 
properties such as the length of stems and bar lines, thickness 
of beams and other lines, and the separation of staff lines all 
need to be modified in special ways. 

Returning to the ossia example, let us first change the font-size.
We can do this in two ways.  We can either change the size of the
fonts of each object type, like @code{NoteHead}s with commands
like

@example
\override NoteHead #'font-size = #-2
@end example

or we can change the size of all fonts by setting a special
property, @code{fontSize}, using @code{\set}, or by including
it in a @code{\with} clause (but without the @code{\set}).

@example
\set fontSize = #-2
@end example

Both of these statements would cause the font size to be reduced
by 2 steps from its previous value, where each
step reduces or increases the size by approximately 12%.

Let's try it in our ossia example:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\new Staff ="main" {
  \relative g' {
    r4 g8 g c4 c8 d |
    e4 r8
    <<
      { f c c }
      \new Staff \with {
        alignAboveContext = "main" 
        \override Clef #'stencil = ##f
        \override TimeSignature #'stencil = ##f
        % Reduce all font sizes by ~24%
        fontSize = #-2
      }
        { f8 f c }
    >>
    r4 |
  }
}
@end lilypond

This is still not quite right.  The note heads and flags are 
smaller, but the stems are too long in proportion and the
staff lines are spaced too widely apart.  These need to be
scaled down in proportion to the font reduction.  The next
sub-section discusses how this is done.

@node Length and thickness of objects
@subsection Length and thickness of objects

@cindex Distances
@cindex Thickness
@cindex Length
@cindex magstep
@cindex size, changing
@cindex stem length, changing
@cindex staff line spacing, changing

Distances and lengths in LilyPond are generally measured in
staff-spaces, the distance between adjacent lines in the staff,
(or occasionally half staff spaces) while most @code{thickness}
properties are measured in units of an internal property called
@code{line-thickness.}  For example, by default, the lines of
hairpins are given a thickness of 1 unit of @code{line-thickness},
while the @code{thickness} of a note stem is 1.3.  Note, though,
that some thickness properties are different; for example, the
thickness of beams is measured in staff-spaces.

So how are lengths to be scaled in proportion to the font size?
This can be done with the help of a special function called
@code{magstep} provided for exactly this purpose.  It takes
one argument, the change in font size (#-2 in the example above)
and returns a scaling factor suitable for reducing other
objects in proportion.  It is used like this:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\new Staff ="main" {
  \relative g' {
    r4 g8 g c4 c8 d |
    e4 r8
    <<
      { f c c }
      \new Staff \with {
        alignAboveContext = "main" 
        \override Clef #'stencil = ##f
        \override TimeSignature #'stencil = ##f
        fontSize = #-2
        % Reduce stem length and line spacing to match
        \override StaffSymbol #'staff-space = #(magstep -2)
      }
        { f8 f c }
    >>
    r4 |
  }
}
@end lilypond

@noindent
Since the length of stems and many other length-related properties
are always calculated relative to the
value of the @code{staff-space} property these are automatically
scaled down in length too.  Note that this affects only the
vertical scale of the ossia -- the horizontal scale is determined
by the layout of the main music in order to remain synchronised 
with it, so it is not affected by any of these changes in size.
Of course, if the scale of all the main music were changed in this
way then the horizontal spacing would be affected.  This is 
discussed later in the layout section.

This, then, completes the creation of an ossia.  The sizes and
lengths of all other objects may be modified in analogous ways.

For small changes in scale, as in the example above, the
thickness of the various drawn lines such as bar lines,
beams, hairpins, slurs, etc does not usually require global 
adjustment.  If the thickness of any particular layout object
needs to be adjusted this can be best achieved by overriding its
@code{thickness} property.  An example of changing the thickness
of slurs was shown above in @ref{Properties of layout objects}.
The thickness of all drawn objects (i.e., those not produced
from a font) may be changed in the same way. 


@node Placement of objects
@section Placement of objects

@menu
* Automatic behaviour::         
* Within-staff objects::        
* Outside staff objects::       
@end menu


@node Automatic behaviour
@subsection Automatic behaviour

There are some objects in musical notation that belong to
the staff and there are other objects that should be 
placed outside the staff.  These are called within-staff
objects and outside-staff objects respectively.

Within-staff objects are those that are located on the staff
-- note heads, stems, accidentals, etc.  The positions of
these are usually fixed by the music itself -- they are
vertically positioned on specific lines of the staff or are 
tied to other objects that are so positioned.  Collisions of
note heads, stems and accidentals in closely set chords are
normally avoided automatically.  There are commands and
overrides which can modify this automatic behaviour, as we
shall shortly see.

Objects belonging outside the staff include things such as
rehearsal marks, text and dynamic markings.  LilyPond's rule for
the vertical placement of outside-staff objects is to place them
as close to the staff as possible but not so close that they
collide with any other object.  LilyPond uses the
@code{outside-staff-priority} property to determine the order in
which the objects should be placed, as follows.

First, LilyPond places all the within-staff objects.
Then it sorts the outside-staff objects according to their 
@code{outside-staff-priority}.  The outside-staff objects are
taken one by one, beginning with the object with the lowest
@code{outside-staff-priority}, and placed so that they do not
collide with any objects that have already been placed. That is, 
if two outside-staff grobs are competing for the same space, the 
one with the lower @code{outside-staff-priority} will be placed
closer to the staff.  If two objects have the same
@code{outside-staff-priority} the one encountered first will be
placed closer to the staff.

In the following example all the markup texts have the same 
priority (since it is not explicity set).  Note that @q{Text3}
is automatically positioned close to the staff again, nestling
under @q{Text2}.

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
c2^"Text1"
c^"Text2"
c^"Text3"
c^"Text4"
@end lilypond

Staves are also positioned, by default, as closely together as
possible (subject to a minimum separation).  If notes project
a long way towards an adjacent staff they will force the
staves further apart only if an overlap of the notation
would otherwise occur.  The following example demonstrates
this @q{nestling} of the notes on adjacent staves:

@lilypond[quote,ragged-right,verbatim]
<<
  \new Staff {
    \relative c' { c a, }
  }
  \new Staff {
    \relative c'''' { c a, }
  }
>>
@end lilypond 


@node Within-staff objects
@subsection Within-staff objects

We have already seen how the commands @code{\voiceXXX} affect
the direction of slurs, ties, fingering and 
everything else which depends on the direction of the stems.
These commands are essential when writing polyphonic music to
permit interweaving melodic lines to be distinguished.
But occasionally it may be necessary to override this automatic
behaviour.  This can be done for whole sections of music or even
for an individual note.  The property which controls this
behaviour is the @code{direction} property of each layout object.
We first explain what this does, and then introduce a number of
ready-made commands which avoid your having to code explicit
overrides for the more common modifications.

Some layout objects like slurs and ties curve, bend or point 
either up or down; others like stems and flags also move to
right or left when they point up or down.  This is controlled
automatically when @code{direction} is set.

The following example shows in bar 1 the default behaviour of 
stems, 
with those on high notes pointing down and those on low notes
pointing up, followed by four notes with all stems forced down, 
four notes with all stems forced up, and finally four notes
reverted back to the default behaviour.

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
a4 g c a
\override Stem #'direction = #DOWN
a g c a
\override Stem #'direction = #UP
a g c a
\revert Stem #'direction
a g c a
@end lilypond

Here we use the constants @code{DOWN} and @code{UP}.
These have the values @code{-1} and @code{+1} respectively, and
these numerical values may be used instead.  The value @code{0}
may also be used in some cases.  It is simply treated as meaning
@code{UP} for stems, but for some objects it means @q{center}.
There is a constant, @code{CENTER} which has the value @code{0}.

However, these explicit overrides are not usually used, as there
are simpler equivalent predefined commands available.  
Here is a table of the commonest.  The meaning of each is stated
where it is not obvious.

@multitable @columnfractions .2 .2 .2 .4
@headitem Down/Left           
  @tab Up/Right
  @tab Revert
  @tab Effect
@item @code{\arpeggioDown}
  @tab @code{\arpeggioUp}
  @tab @code{\arpeggioNeutral}
  @tab Arrow is at bottom, at top, or no arrow
@item @code{\dotsDown}
  @tab @code{\dotsUp}
  @tab @code{\dotsNeutral}
  @tab Direction of movement to avoid staff lines
@item @code{\dynamicDown}
  @tab @code{\dynamicUp}
  @tab @code{\dynamicNeutral}
  @tab
@item @code{\phrasingSlurDown}
  @tab @code{\phrasingSlurUp}
  @tab @code{\phrasingSlurNeutral}
  @tab Note: distinct from slur commands
@item @code{\slurDown}
  @tab @code{\slurUp}
  @tab @code{\slurNeutral}
  @tab
@item @code{\stemDown}
  @tab @code{\stemUp}
  @tab @code{\stemNeutral}
  @tab
@item @code{\textSpannerDown}
  @tab @code{\textSpannerUp}
  @tab @code{\textSpannerNeutral}
  @tab Text entered as spanner is below/above staff
@item @code{\tieDown}
  @tab @code{\tieUp}
  @tab @code{\tieNeutral}
  @tab
@item @code{\tupletDown}
  @tab @code{\tupletUp}
  @tab @code{\tupletNeutral}
  @tab Tuplets are below/above notes
@end multitable

Note that these predefined commands may @strong{not} be
preceded by @code{\once}.  If you wish to limit the
effect to a single note you must either use the equivalent
@code{\once \override} command or use the predefined command
followed after the affected note by the corresponding 
@code{\xxxNeutral} command.

@subheading Fingering
@cindex fingering, placement

The placement of fingering is also affected by the value
of its @code{direction} property, but there are special
commands which allow the fingering of individual notes
of chords to be controlled, with the fingering being placed
above, below, to the left or to the right of each note.

First, here's the effect of @code{direction} on fingering,
the first bar shows the default, then the effect of specifying
@code{DOWN} and @code{UP}:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
c-5 a-3 f-1 c'-5
\override Fingering #'direction = #DOWN
c-5 a-3 f-1 c'-5
\override Fingering #'direction = #UP
c-5 a-3 f-1 c'-5
@end lilypond

This is how to control fingering on single notes, but the 
@code{direction}
property is ignored for chords.  Instead, by default, the
fingering is automatically placed both above and below the 
notes of a chord, as shown:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
<c-5 g-3>
<c-5 g-3 e-2>
<c-5 g-3 e-2 c-1>
@end lilypond

Greater control over the placement of fingering of the 
individual notes in a chord is possible by using
the @code{\set fingeringOrientations} command.  The format of
this command is

@example
@code{\set fingeringOrientations = #'([up] [left/right] [down])}
@end example

@noindent
@code{\set} is used because @code{fingeringOrientations} is a
property of the @code{Voice} context, created and used by the
@code{New_fingering_engraver}.

The property may be set to a list of one to three values.
It controls whether fingerings may be placed above (if 
@code{up} appears in the list), below (if @code{down} appears),
to the left (if @code{left} appears, or to the right
(if @code{right} appears).  Conversely, if a location is not
listed, no fingering is placed there.  LilyPond takes these 
contraints and works out the best placement for the fingering
of the notes of the following chords.  Note that @code{left} and
@code{right} are mutually exclusive -- fingering may be placed
only on one side or the other, not both.

To control the placement of the fingering of a single note 
using this command it is necessary to write it as a single 
note chord by placing angle brackets round it.
  
Here are a few examples:

@lilypond[quote,fragment,ragged-right,verbatim,relative=1]
\set fingeringOrientations = #'(left)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(left)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(up left down)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(up left)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(right)
<f-2>
< c-1  e-2 g-3 b-5 > 4
@end lilypond 

@noindent
If the fingering seems a little crowded the @code{font-size}
could be reduced.  The default value can be seen from the
@code{Fingering} object in the IR to be @code{-5}, so let's
try @code{-7}:

@lilypond[quote,fragment,ragged-right,verbatim,relative=1]
\override Fingering #'font-size = #-7
\set fingeringOrientations = #'(left)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(left)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(up left down)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(up left)
<f-2>
< c-1  e-2 g-3 b-5 > 4
\set fingeringOrientations = #'(right)
<f-2>
< c-1  e-2 g-3 b-5 > 4
@end lilypond

@node Outside staff objects
@subsection Outside staff objects

Outside-staff objects are automatically placed to avoid collisions.
Objects with the lower value of the @code{outside-staff-priority}
property are placed nearer to the staff, and other outside-staff
objects are then raised as far as necessary to avoid collisions.
The @code{outside-staff-priority} is defined in the
@code{grob-interface} and so is a property of all layout objects.
By default it is set to @code{#f} for all within-staff objects,
and to a numerical value appropriate to each outside-staff object
when the object is created.  The following table shows
the default numerical values for some of the commonest
outside-staff objects which are, by default, placed in the
@code{Staff} or @code{Voice} contexts.

@multitable @columnfractions .3 .3 .3
@headitem Layout Object           
  @tab Priority     
  @tab Controls position of:
@item @code{MultiMeasureRestText}
  @tab @code{450}
  @tab Text over full-bar rests
@item @code{TextScript}
  @tab @code{450}
  @tab Markup text
@item @code{OttavaBracket}
  @tab @code{400}
  @tab Ottava brackets
@item @code{TextSpanner}
  @tab @code{350}
  @tab Text spanners
@item @code{DynamicLineSpanner}
  @tab @code{250}
  @tab All dynamic markings
@item @code{VoltaBracketSpanner}
  @tab @code{100}
  @tab Volta brackets
@item @code{TrillSpanner}
  @tab @code{50}
  @tab Spanning trills
@end multitable

Here is an example showing the default placement of some of
these.

@cindex text spanner
@funindex \startTextSpan
@funindex \stopTextSpan
@cindex ottava bracket

@lilypond[quote,fragment,ragged-right,verbatim,relative=1]
% Set details for later Text Spanner
\override TextSpanner #'bound-details #'left #'text
    = \markup { \small \bold Slower }
% Place dynamics above staff
\dynamicUp
% Start Ottava Bracket
#(set-octavation 1)
c' \startTextSpan
% Add Dynamic Text
c\pp
% Add Dynamic Line Spanner
c\<
% Add Text Script
c^Text
c c
% Add Dynamic Text
c\ff c \stopTextSpan
% Stop Ottava Bracket
#(set-octavation 0)
c, c c c
@end lilypond

This example also shows how to create Text Spanners --
text with extender lines above a section of music.  The
spanner extends from the @code{\startTextSpan} command to
the @code{\stopTextSpan} command, and the format of the
text is defined by the @code{\override TextSpanner} command.
For more details see @ruser{Text spanners}.

It also shows how ottava brackets are created.

Note that bar numbers, metronome marks and rehearsal marks
are not shown. By default these are created in the
@code{Score} context and their @code{outside-staff-priority}
is ignored relative to the layout objects which are created
in the @code{Staff} context.
If you wish to place bar numbers, metronome marks or rehearsal
marks in accordance with the value of their
@code{outside-staff-priority} the @code{Bar_number_engraver},
@code{Metronome_mark_engraver} or @code{Mark_engraver} respectively
should be removed from the @code{Score} context and placed in the
top @code{Staff} context.  If this is done, these marks will be
given the following default @code{outside-staff-priority} values:

@multitable @columnfractions .3 .3
@headitem Layout Object           @tab Priority
@item @code{RehearsalMark}        @tab @code{1500}
@item @code{MetronomeMark}        @tab @code{1000}
@item @code{BarNumber}            @tab @code{ 100}
@end multitable

If the default values of @code{outside-staff-priority} does not
give you the placing you want, the priority of any of the objects
may be overridden.  Suppose we would
like the ottava bracket to be placed below the text spanner in the
example above.  All we need to do is to look up the priority of
@code{OttavaBracket} in the IR or in the tables above, and reduce
it to a value lower than that of a @code{TextSpanner}, remembering
that @code{OttavaBracket} is created in the @code{Staff} context:

@lilypond[quote,fragment,ragged-right,verbatim,relative=1]
% Set details for later Text Spanner
\override TextSpanner #'bound-details #'left #'text
    = \markup { \small \bold Slower }
% Place dynamics above staff
\dynamicUp
%Place following Ottava Bracket below Text Spanners
\once \override Staff.OttavaBracket #'outside-staff-priority = #340
% Start Ottava Bracket
#(set-octavation 1)
c' \startTextSpan
% Add Dynamic Text
c\pp
% Add Dynamic Line Spanner
c\<
% Add Text Script
c^Text
c c
% Add Dynamic Text
c\ff c \stopTextSpan
% Stop Ottava Bracket
#(set-octavation 0)
c, c c c
@end lilypond

Changing the @code{outside-staff-priority} can also be used to
control the vertical placement of individual objects, although
the results may not always be desirable.  Suppose we would
like @qq{Text3} to be placed above @qq{Text4} in the example
under Automatic behaviour, above (see @ref{Automatic behaviour}).
All we need to do is to look up the priority of @code{TextScript}
in the IR or in the tables above, and increase the priority of
@qq{Text3} to a higher value:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
c2^"Text1"
c^"Text2"
\once \override TextScript #'outside-staff-priority = #500
c^"Text3"
c^"Text4"
@end lilypond

This certainly lifts @qq{Text3} above @qq{Text4} but it also
lifts it above @qq{Text2}, and @qq{Text4} now drops down.
Perhaps this is not so good.  What we would really like to do
is to position all the annotation at the same distance above
the staff?  To do this, we clearly
will need to space the notes out horizontally to make more
room for the text.  This is done using the @code{textLengthOn}
command.

@subheading \textLengthOn

@funindex \textLengthOn
@cindex notes, spreading out with text

By default, text produced by markup takes up no horizontal space
as far as laying out the music is concerned.  The @code{\textLengthOn}
command reverses this behaviour, causing the notes to be spaced
out as far as is necessary to accommodate the text:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\textLengthOn  % Cause notes to space out to accommodate text
c2^"Text1"
c^"Text2"
c^"Text3"
c^"Text4"
@end lilypond  

The command to revert to the default behaviour is 
@code{\textLengthOff}.  Remember @code{\once} only works with
@code{\override}, @code{\set}, @code{\revert} or @code{unset},
so cannot be used with @code{\textLengthOn}.

Markup text will also avoid notes which project above the staff.
If this is not desired, the automatic displacement upwards may
be turned off by setting the priority to @code{#f}.  Here's an
example to show how markup text interacts with such notes.

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
% This markup is short enough to fit without collision
c2^"Tex"
c''2
R1
% This is too long to fit, so it is displaced upwards
c,,2^"Text"
c''2
R1
% Turn off collision avoidance
\once \override TextScript #'outside-staff-priority = ##f
c,,2^"Long Text   "
c''2
R1
% Turn off collision avoidance
\once \override TextScript #'outside-staff-priority = ##f
\textLengthOn  % and turn on textLengthOn
c,,2^"Long Text   "  % Spaces at end are honoured
c''2
@end lilypond


@subheading Dynamics

Dynamic markings will normally be positioned beneath the
staff, but may be positioned above with the @code{dynamicUp}
command.  They will be positioned vertically relative to the
note to which they are attached, and will float below (or above)
all within-staff objects such as phrasing slurs and bar numbers.
This can give quite acceptable results, as this example
shows:

@lilypond[quote,fragment,ragged-right,verbatim,relative=1]
\clef "bass"
\key aes \major
\time 9/8
\dynamicUp
bes4.~\f\< \( bes4 bes8 des4\ff\> c16 bes\! |
ees,2.~\)\mf ees4 r8 |
@end lilypond

However, if the notes and attached dynamics are close
together the automatic placement will avoid collisions
by displacing later dynamic markings further away, but this may 
not be the optimum placement, as this rather artificial example
shows:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\dynamicUp
a4\f b\mf c\mp b\p
@end lilypond

@noindent
Should a similar situation arise in @q{real} music, it may
be preferable to space out the notes
a little further, so the dynamic markings can all fit at the
same vertical distance from the staff.  We were able to do this
for markup text by using the @code{\textLengthOn} command, but there
is no equivalent command for dynamic marks.  So we shall have to
work out how to do this using @code{\override} commands.

@subheading Grob sizing

@cindex grob sizing
@cindex sizing grobs
@cindex @code{X-offset}
@cindex @code{Y-offset}
@cindex @code{X-extent}
@cindex @code{Y-extent}

First we must learn how grobs are sized.  All grobs have a
reference point defined within them which is used to position 
them relative to their parent object.  This point in the grob
is then positioned at a horizontal distance, @code{X-offset},
and at a vertical distance, @code{Y-offset}, from its parent.
The horizontal extent of the object is given by a pair of
numbers, @code{X-extent}, which say where the left and right
edges are relative to the reference point.  The vertical extent
is similarly defined by a pair of numbers, @code{Y-extent}.
These are properties of all grobs which support the 
@code{grob-interface}.

@cindex @code{extra-spacing-width}

By default, outside-staff objects are given a width of zero so
that they may overlap in the horizontal direction.  This is done
by the trick of adding infinity to the leftmost extent and 
minus infinity to the rightmost extent by setting the 
@code{extra-spacing-width} to @code{'(+inf.0 . -inf.0)}.  So
to ensure they do not overlap in the horizontal direction we
must override this value of @code{extra-spacing-width} to
@code{'(0 . 0)} so the true width shines through.  This is
the command to do this for dynamic text:

@example
\override DynamicText #'extra-spacing-width = #'(0 . 0)
@end example

@noindent
Let's see if this works in our previous example:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\dynamicUp
\override DynamicText #'extra-spacing-width = #'(0 . 0)
a4\f b\mf c\mp b\p
@end lilypond

@noindent
Well, it has certainly stopped the dynamic marks being
displaced, but two problems remain.  The marks should be
spaced a little further apart and it would be better
if they were all the same distance from the staff.
We can solve the first problem easily.  Instead of making
the @code{extra-spacing-width} zero we could add a little
more to it.  The units are the space between two staff
lines, so moving the left edge half a unit to the left and the
right edge half a unit to the right should do it:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\dynamicUp
% Extend width by 1 staff space
\override DynamicText #'extra-spacing-width = #'(-0.5 . 0.5)
a4\f b\mf c\mp b\p
@end lilypond

@noindent
This looks better, but maybe we would prefer the dynamic marks
to be aligned along the same baseline rather than going up and
down with the notes.  The property to do this is
@code{staff-padding} which is covered in the following section.


@node Collisions of objects
@section Collisions of objects

@menu
* Moving objects::              
* Fixing overlapping notation::  
* Real music example::          
@end menu

@node Moving objects
@subsection Moving objects

This may come as a surprise, but LilyPond is not perfect.  Some
notation elements can overlap.  This is unfortunate, but in fact
rather rare.  Usually the need to move objects is for clarity or
aesthetic reasons -- they would look better with a little more
or a little less space around them.

There are three main main approaches to resolving overlapping
notation.  They should be considered in the following order:

@enumerate
@item
The @strong{direction} of one of the overlapping objects may
be changed using the predefined commands listed above for
within-staff objects (see @ref{Within-staff objects}).
Stems, slurs, beams, ties, dynamics, text and tuplets may be
repositioned easily in this way.  The limitation is that you
have a choice of only two positions, and neither may be
suitable.

@item
The @strong{object properties}, which LilyPond uses
when positioning layout objects, may be modified using
@code{\override}.  The advantages
of making changes to this type of property are (a) that some
other objects will be moved automatically if necessary to make
room and (b) the single override can apply to all instances of
the same type of object.  Such properties include:
@itemize

@item
@code{direction}

This has already been covered in some detail -- see
@ref{Within-staff objects}.

@item
@code{padding}, @code{left-padding},
@code{right-padding}, @code{staff-padding}

@cindex left-padding property
@cindex padding property
@cindex right-padding property
@cindex staff-padding property
As an object is being positioned the value of its @code{padding}
property specifies the gap that must be left between itself and
the nearest edge of the object against which it is being
positioned.  Note that it is the @code{padding} value of the object
@strong{being placed} that is used;
the @code{padding} value of the object which is already placed is
ignored.  Gaps specified by @code{padding} can be applied
to all objects which support the @code{side-position-interface}.

Instead of @code{padding}, the placement of groups of accidentals
is controlled by @code{left-padding} and @code{right-padding}.
These properties are to be found in the @code{AccidentalPlacement}
object which, note, lives in the @strong{staff} context.  Because
accidentals are always positioned after and to the left of
note heads only the @code{right-padding} property has any effect.

The @code{staff-padding} property is closely related to the
@code{padding} property:  @code{padding}
controls the minimum amount of space between any object which
supports the @code{side-position-interface} and the nearest
other object (generally the note or the staff lines);
@code{staff-padding} applies only to those objects which are always
set outside the staff -- it controls the minimum amount of space
that should be inserted between that object and the staff.  Note
that @code{staff-padding} has no effect on objects which are
positioned relative to the note rather than the staff, even though
it may be overridden without error for such objects -- it is simply
ignored.

To discover which padding property is required for the object
you wish to reposition, you
need to return to the IR and look up the object's properties.
Be aware that the padding properties might not be located in the
obvious object, so look in objects that appear to be related.

All padding values are measured in staff spaces.  For most
objects, this value is set by default to be around 1.0 or less
(it varies with each object).  It may be overridden if a larger
(or smaller) gap is required.

@item
@code{self-alignment-X}

@cindex self-alignment-X property
This property can be used to align the object to the left, to 
the right, or to center it with respect to the parent object's 
reference point.  It may be used with all objects which support
the @code{self-alignment-interface}.  In general these are objects
that contain text.  The values are @code{LEFT}, @code{RIGHT}
or @code{CENTER}.  Alternatively, a numerical value between 
@code{-1} and @code{+1} may be specified, where @code{-1} is 
left-aligned, @code{+1} is right-aligned, and numbers in between 
move the text progressively from left-aligned to right-aligned.
Numerical values greater than @code{1} may be specified to move
the text even further to the left, or less than @code{-1} to
move the text even further to the right.  A change of @code{1}
in the value corresponds to a movement of half the text's length.

@item
@code{extra-spacing-width}

@cindex extra-spacing-width property
This property is available for all objects which support the
@code{item-interface}.  It takes two numbers, the first is added
to the leftmost extent and the second is added to the rightmost
extent.  Negative numbers move the edge to the left, positive to
the right, so to widen an object the first number must be negative,
the second positive.  Note that not all objects honour both
numbers.  For example, the @code{Accidental} object only takes
notice of the first (left edge) number.

@item
@code{staff-position}

@cindex staff-position property
@code{staff-position} is a property of the
@code{staff-symbol-referencer-interface}, which is supported by
objects which are positioned relative to the staff.  It specifies
the vertical position of the object relative to the center line
of the staff in half staff-spaces.  It is useful in resolving
collisions between layout objects like multi-measure rests, ties
and notes in different voices.

@item
@code{force-hshift}

@cindex force-hshift property

Closely spaced notes in a chord, or notes occuring at the same
time in different voices, are arranged in two, occasionally more,
columns to prevent the note heads overlapping.  These are called
note columns, and an object called @code{NoteColumn} is created
to lay out the notes in that column.

The @code{force-hshift}
property is a property of a @code{NoteColumn} (actually of the
@code{note-column-interface}).  Changing it permits a note column
to be moved in units appropriate to a note column, viz. the note
head width of the first voice note.  It should be used in
complex situations where the normal @code{\shiftOn} commands (see
@ref{Explicitly instantiating voices}) do
not resolve the note conflict.  It is preferable to the
@code{extra-offset} property for this purpose as there is no need
to work out the distance in staff-spaces, and moving the notes
into or out of a @code{NoteColumn} affects other actions such as
merging note heads.

@end itemize

Objects do not all have all of these properties in general.
It is necessary to go to the IR to look up which properties
are available for the object in question.

@item
Finally, when all else fails, objects may be manually repositioned
relative to the staff center line verically, or by
displacing them by any distance to a new position.  The
disadvantages are that the correct values for the repositioning
have to be worked out, often by trial and error, for every object
individually, and, because the movement is done after LilyPond has
placed all other objects, the user is responsible for avoiding any
collisions that might ensue.  But the main difficulty with this
approach is that the repositioning values may need to be reworked
if the music is later modified.  The properties that can be used
for this type of manual repositioning are:

@table @code
@item extra-offset
@cindex extra-offset property
This property applies to any layout object
supporting the @code{grob-interface}.  It takes a pair of
numbers which specify the extra displacement in the horizontal and
vertical directions.  Negative numbers move the object to
the left or down.  The units are staff-spaces.  The extra
displacement is made after the typesetting of objects is
finished, so an object may be repositioned anywhere without
affecting anything else.

@item positions
@cindex positions property
This is most useful for manually adjusting the slope and height
of beams, slurs, and tuplets.   It takes a pair of numbers
giving the position of the left and right ends of the beam, slur,
etc. relative to the center line of the staff.  Units are
staff-spaces.  Note, though, that slurs and phrasing slurs cannot
be repositioned by arbitrarily large amounts.  LilyPond first
generates a list of possible positions for the slur and by default
finds the slur that @qq{looks best}.  If the @code{positions}
property has been overridden the slur that is closest to the
requested positions is selected from the list.
@end table

Objects do not all have all of these properties in general.
It is necessary to go to the IR to look up which properties
are available for the object in question.

@end enumerate

Here is a list of the objects which are most likely to be
involved in collisions, together with the name of the object which
should be looked up in the IR in order to discover which properties
should be used to move them.

@multitable @columnfractions .5 .5
@headitem Object type           @tab Object name
@item Articulations             @tab @code{Script}
@item Beams                     @tab @code{Beam}
@item Dynamics (vertically)     @tab @code{DynamicLineSpanner}
@item Dynamics (horizontally)   @tab @code{DynamicText}
@item Fingerings                @tab @code{Fingering}
@item Rehearsal / Text marks    @tab @code{RehearsalMark}
@item Slurs                     @tab @code{Slur}
@item Text e.g. @code{^"text"}  @tab @code{TextScript}
@item Ties                      @tab @code{Tie}
@item Tuplets                   @tab @code{TupletBracket}
@end multitable


@node Fixing overlapping notation
@subsection Fixing overlapping notation

Let's now see how the properties in the previous section can
help to resolve overlapping notation.

@subheading @code{padding} property
@cindex padding property

The @code{padding} property can be set to increase
(or decrease) the distance between symbols that are printed
above or below notes.

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

@lilypond[quote,fragment,relative=1,verbatim]
% This will not work, see below:
\override MetronomeMark #'padding = #3
\tempo 4=120
c1
% This works:
\override Score.MetronomeMark #'padding = #3
\tempo 4=80
d1
@end lilypond

Note in the second example how important it is to figure out what
context handles a certain object.  Since the @code{MetronomeMark}
object
is handled in the @code{Score} context, property changes in the
@code{Voice} context will not be noticed.  For more details, see
@ruser{Constructing a tweak}.

If the @code{padding} property of an object is increased when that
object is in a stack of objects being positioned according to
their @code{outside-staff-priority}, then that object and all 
objects outside it are moved.
 

@subheading left-padding and right-padding properties
@cindex left-padding property
@cindex right-padding property

TODO Example showing the utility of these properties  -td

@subheading staff-padding property
@cindex staff-padding property

@code{staff-padding} can be used to align objects such as dynamics
along a baseline at a fixed height above the staff, rather than
at a height dependent on the position of the note to which they
are attached.  It is not a property of
@code{DynamicText} but of @code{DynamicLineSpanner}.
This is because the baseline should apply equally to @strong{all}
dynamics, including those created as extended spanners.
So this is the way to align the dynamic marks in the example
taken from the previous section:

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\dynamicUp
% Extend width by 1 unit
\override DynamicText #'extra-spacing-width = #'(-0.5 . 0.5)
% Align dynamics to a base line 2 units above staff
\override DynamicLineSpanner #'staff-padding = #2
a4\f b\mf c\mp b\p
@end lilypond


@subheading self-alignment-X property
@cindex self-alignment-X property

The following example shows how this can resolve the collision
of a string fingering object with a note's stem by aligning the
right edge with the reference point of the parent note:

@lilypond[quote,fragment,ragged-right,verbatim,relative=3]
\voiceOne
< a \2 >
\once \override StringNumber #'self-alignment-X = #RIGHT
< a \2 >
@end lilypond

@subheading staff-position property
@cindex staff-position property

Multimeasure rests in one voice can collide with notes in another.
Since these rests are typeset centered between the bar lines, it
would require significant effort for LilyPond to figure out which
other notes might collide with it, since all the current collision
handling between notes and between notes and rests is done only
for notes and rests that occur at the same time.  Here's an
example of a collision of this type:

@lilypond[quote,verbatim,fragment,ragged-right, relative=1]
<< {c c c c} \\ {R1} >>
@end lilypond

The best solution here is to move the multimeasure rest down,
since the rest is in voice two.
The default in @code{\voiceTwo} (i.e. in the second voice of a
@code{<<@{...@} \\ @{...@}>>} construct)
is that @code{staff-position} is set to -4 for MultiMeasureRest,
so we need to move it, say, four half-staff spaces down to
@code{-8}.

@lilypond[quote,verbatim,fragment,ragged-right, relative=1]
<<
  {c c c c}
\\
  \override MultiMeasureRest #'staff-position = #-8
  {R1}
>>
@end lilypond

This is better than using, for example, @code{extra-offset},
because the ledger line above the rest is inserted automatically.

@subheading extra-offset property
@cindex extra-offset property

The @code{extra-offset} property provides complete control over the
positioning of an object both horizontally and vertically.

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

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


@subheading force-hshift property
@cindex force-hshift property
@c FIXME: formatting stuff  (ie not important right now IMO)
@c @a nchor Chopin finally corrected TODOgp

We can now see how to apply the final corrections to the Chopin
example introduced at the end of @ref{I'm hearing Voices}, which
was left looking like this:

@lilypond[quote,verbatim,fragment,ragged-right]
\new Staff \relative c'' {
  \key aes \major
  <<
    { c2 aes4. bes8 } \\
    { aes2 f4 fes   } \\
    { \voiceFour
      <ees c>2
      des2
    }
  >> |
  <c ees aes c>1 |
}
@end lilypond

@noindent
The lower two notes of the first chord (i.e,
those in the third voice) should not be shifted away from the
note column of the higher two notes.  To correct this we set
@code{force-hshift}, which is a property of
@code{NoteColumn}, of these notes to zero.
The lower note of the second chord is best placed just to the
right of the higher notes.  We achieve this by setting
@code{force-hshift} of this note to 0.5, ie half a note head's
width to the right of the note column of the higher notes.

Here's the final result:

@lilypond[quote,verbatim,fragment,ragged-right]
\new Staff \relative c'' {
  \key aes \major
  <<
    { c2 aes4. bes8 } \\
    { aes2 f4 fes   } \\
    { \voiceFour
      \once \override NoteColumn #'force-hshift = #0 <ees c>2
      \once \override NoteColumn #'force-hshift = #0.5 des2
    }
  >> |
  <c ees aes c>1 |
}
@end lilypond


@subheading positions property
@cindex positions property

The @code{positions} property allows the position and slope of
 tuplets, slurs, phrasing slurs and beams to be controlled
manually.  Here's an example which has an ugly phrasing slur
due to its trying to avoid the slur on the acciaccatura.

@lilypond[quote,verbatim,fragment,ragged-right,relative=1]
r4  \acciaccatura e8\( d8 c ~c d c d\)
@end lilypond

@noindent
We could simply move the phrasing slur above the notes, and this
would be the preferred solution:

@lilypond[quote,verbatim,fragment,ragged-right,relative=1]
r4
\phrasingSlurUp
\acciaccatura e8\( d8 c ~c d c d\)
@end lilypond

@noindent
but if there were some reason why this could not be done the
other alternative would be to move the left end of the phrasing
slur down a little using the @code{positions} property.  This
also resolves the rather nasty shape.

@lilypond[quote,verbatim,fragment,ragged-right,relative=1]
r4
\once \override PhrasingSlur #'positions = #'(-4 . -3)
\acciaccatura
e8\( d8 c ~c d c d\)
@end lilypond

Here's a further example taken from the opening of the left-hand
staff of Chopin's Prelude Op 28 No. 2.  We see that the beam
collides with the upper notes:

@lilypond[quote,verbatim,fragment,ragged-right]
{
\clef "bass"
<< {b,8 ais, b, g,} \\ {e, g e, g} >>
<< {b,8 ais, b, g,} \\ {e, g e, g} >>
}
@end lilypond

@noindent
This can only be resolved by manually moving both ends of the beam
up from their position at 2 staff-spaces above the center line to,
say, 3:

@lilypond[quote,verbatim,fragment,ragged-right]
{
  \clef "bass"
  <<
    \override Beam #'positions = #'(3 . 3)
    {b,8 ais, b, g,}
  \\
    {e, g e, g}
  >>
  << {b,8 ais, b, g,} \\ {e, g e, g} >>
}
@end lilypond

@noindent
Note that the override continues to apply in the first voice of
the second block of quavers, but not to any of the beams in the
second voice.

@node Real music example
@subsection Real music example

We end this section on Tweaks by showing the steps to be taken to 
deal with a tricky example which needs several tweaks to produce 
the desired output.  The example has been deliberately chosen to 
illustrate the use of the Notation Reference to resolve unusual 
problems with notation.  It is not representative of more usual 
engraving process, so please do not let these difficulties put 
you off!  Fortunately, difficulties like these are not very common!

The example is from Chopin's Première Ballade, Op. 23, bars 6 to
9, the transition from the opening Lento to Moderato.
Here, first, is what we want the output to look like, but to avoid
over-complicating the example too much we have left out the
dynamics, fingering and pedalling.

@c The following should appear as music without code
@lilypond[quote,ragged-right]
rhMusic = \relative c'' {
  r2
  c4.\( g8 |
  \once \override Tie #'staff-position = #3.5
  bes1~ |
  \bar "||"
  \time 6/4
  \override Staff.NoteCollision #'merge-differently-headed = ##t
  \override Staff.NoteCollision #'merge-differently-dotted = ##t
  bes2.^\markup {\bold "Moderato"} r8
  <<
    {c,8[ d fis bes a] | }
  \\
    % Reposition the c2 to the right of the merged note
    {c,8~ \once \override NoteColumn #'force-hshift = #1.0
    % Move the c2 out of the main note column so the merge will work
    \shiftOnn c2}
  \\
    % Stem on the d2 must be down to permit merging
    {s8 \stemDown \once \override Stem #'transparent = ##t d2}
  \\
    {s4 fis4.}
  >>
  \revert Staff.NoteCollision #'merge-differently-headed
  \revert Staff.NoteCollision #'merge-differently-dotted
  g2.\)
}

lhMusic = \relative c' {
  r2 <c g ees>2( |
  <d g, d>1)\arpeggio |
  r2. d,,4 r4 r |
  r4
}

\score {
  \new PianoStaff <<
    \new Staff = "RH"  <<
      \key g \minor
      \rhMusic
    >>
    \new Staff = "LH" <<
      \key g \minor
      \clef "bass"
      \lhMusic
    >>
  >>
}
@end lilypond

We note first that the right hand part in the third bar
requires four voices.  These are the five beamed eighth notes,
the tied C, the half-note D which is merged with the eighth note
D, and the dotted quarter note F-sharp, which is also merged with
the eighth note at the same pitch.  Everything else is in a single
voice, so the easiest way is to introduce these four voices
temporarily at the time they are needed.  If you have forgotten
how to do this, look at @ref{I'm hearing Voices}.  Let us begin
by entering the notes as two variables and setting up the staff
structure in a score block, and see what LilyPond produces by
default:

@lilypond[quote,verbatim,ragged-right]
rhMusic = \relative c'' {
  r2 c4. g8 |
  bes1~ |
  \time 6/4
  bes2. r8
  % Start polyphonic section of four voices
  <<
    {c,8 d fis bes a | }
  \\
    {c,8~ c2 | }
  \\
    {s8 d2 | }
  \\
    {s4 fis4. | }
  >>
  g2.
}

lhMusic = \relative c' {
  r2 <c g ees>2 |
  <d g, d>1 |
  r2. d,,4 r4 r |
  r4
}

\score {
  \new PianoStaff <<
    \new Staff = "RH"  <<
      \key g \minor
      \rhMusic
    >>
    \new Staff = "LH" <<
      \key g \minor
      \clef "bass"
      \lhMusic
    >>
  >>
}
@end lilypond

All the notes are right, but the appearance is far from
satisfactory.  The tie clashes with the change in time signature,
the beaming in the third bar is wrong, the notes are not
merged together, and several notation elements are missing.  
Let's first deal with the easier things.
We can correct the beaming by inserting a beam
manually, and we can easily add the left hand slur and the right 
hand phrasing slur, since these were all covered in the Tutorial.
Doing this gives:

@lilypond[quote,verbatim,ragged-right]
rhMusic = \relative c'' {
  r2 c4.\( g8 |
  bes1~ |
  \time 6/4
  bes2. r8
  % Start polyphonic section of four voices
  <<
    {c,8[ d fis bes a] | }
  \\
    {c,8~ c2 | }
  \\
    {s8 d2 | }
  \\
    {s4 fis4. | }
  >>
  g2.\)
}

lhMusic = \relative c' {
  r2 <c g ees>2( |
  <d g, d>1) |
  r2. d,,4 r4 r |
  r4
}

\score {
  \new PianoStaff <<
    \new Staff = "RH"  <<
      \key g \minor
      \rhMusic
    >>
    \new Staff = "LH" <<
      \key g \minor
      \clef "bass"
      \lhMusic
    >>
  >>
}
@end lilypond

The first bar is now correct.  The second bar contains an arpeggio
and is terminated by a double bar line.  How do we do these, as they
have not been mentioned in this Learning Manual?  This is where
we need to turn to the Notation Reference.  Looking up @q{arpeggio}
and @q{bar line} in the
index quickly shows us that an arpeggio is produced by appending
@code{\arpeggio} to a chord, and a double bar line is produced by
the @code{\bar "||"} command.  That's easily done.  We next need
to correct the collision of the tie with the time signature.  This
is best done by moving the tie upwards.  Moving objects was covered
earlier in @ref{Moving objects}, which says that objects positioned
relative to the staff can be moved by overriding their
@code{staff-position} property, which is specified in half staff
spaces relative to the center line of the staff.  So the following
override placed just before the first tied note would move the tie
up to 3.5 half staff spaces above the center line:

@code{\once \override Tie #'staff-position = #3.5}

This completes bar two, giving:

@lilypond[quote,verbatim,ragged-right]
rhMusic = \relative c'' {
  r2 c4.\( g8 |
  \once \override Tie #'staff-position = #3.5
  bes1~ |
  \bar "||"
  \time 6/4
  bes2. r8
  % Start polyphonic section of four voices
  <<
    {c,8[ d fis bes a] | }
  \\
    {c,8~ c2 | }
  \\
    {s8 d2 | }
  \\
    {s4 fis4. | }
  >>
  g2.\)
}

lhMusic = \relative c' {
  r2 <c g ees>2( |
  <d g, d>1)\arpeggio |
  r2. d,,4 r4 r |
  r4
}

\score {
  \new PianoStaff <<
    \new Staff = "RH"  <<
      \key g \minor
      \rhMusic
    >>
    \new Staff = "LH" <<
      \key g \minor
      \clef "bass"
      \lhMusic
    >>
  >>
}
@end lilypond

On to bar three and the start of the Moderato section.  The
tutorial showed how to add embolded text with the
@code{\markup} command, so adding @q{Moderato} in bold is easy.
But how do we merge notes in different voices together?  The
index in the Notation Reference does not mention merging,
but a search of the text for @q{merge} quickly leads us to
the overrides for merging differently headed and differently
dotted notes in @ruser{Collision Resolution}.  In our
example we need to merge both types of note for the duration
of the polyphonic section in bar 3, so using the information
in the Notation Reference we add

@example
\override Staff.NoteCollision #'merge-differently-headed = ##t
\override Staff.NoteCollision #'merge-differently-dotted = ##t
@end example

@noindent
to the start of that section and

@example
\revert Staff.NoteCollision #'merge-differently-headed
\revert Staff.NoteCollision #'merge-differently-dotted
@end example

@noindent
to the end, giving:

@lilypond[quote,verbatim,ragged-right]
rhMusic = \relative c'' {
  r2 c4.\( g8 |
  \once \override Tie #'staff-position = #3.5
  bes1~ |
  \bar "||"
  \time 6/4
  bes2.^\markup {\bold "Moderato"} r8
  \override Staff.NoteCollision #'merge-differently-headed = ##t
  \override Staff.NoteCollision #'merge-differently-dotted = ##t
  % Start polyphonic section of four voices
  <<
    {c,8[ d fis bes a] | }
  \\
    {c,8~ c2 | }
  \\
    {s8 d2 | }
  \\
    {s4 fis4. | }
  >>
  \revert Staff.NoteCollision #'merge-differently-headed
  \revert Staff.NoteCollision #'merge-differently-dotted  
  g2.\)
}

lhMusic = \relative c' {
  r2 <c g ees>2( |
  <d g, d>1)\arpeggio |
  r2. d,,4 r4 r |
  r4
}

\score {
  \new PianoStaff <<
    \new Staff = "RH"  <<
      \key g \minor
      \rhMusic
    >>
    \new Staff = "LH" <<
      \key g \minor
      \clef "bass"
      \lhMusic
    >>
  >>
}
@end lilypond

These overrides have merged the two F-sharp notes, but not the two 
on D.  Why not?  The answer is there in the same section in the
Notation Reference -- notes being merged must have stems in 
opposite directions and two notes cannot be merged successfully if 
there is a third note in the same note column.  Here the two D's 
both have upward stems and there is a third note -- the C.  We know
how to change the stem direction using @code{\stemDown}, and
the Notation Reference also says how to move the C -- apply a shift
using one of the @code{\shift} commands.  But which one?
The C is in voice two which has shift off, and the two D's are in
voices one and three, which have shift off and shift on,
respectively.  So we have to shift the C a further level still
using @code{\shiftOnn} to avoid it interferring with the two D's.  
Applying these changes gives:

@lilypond[quote,verbatim,ragged-right]
rhMusic = \relative c'' {
  r2 c4.\( g8 |
  \once \override Tie #'staff-position = #3.5
  bes1~ |
  \bar "||"
  \time 6/4
  bes2.^\markup {\bold "Moderato"} r8
  \override Staff.NoteCollision #'merge-differently-headed = ##t
  \override Staff.NoteCollision #'merge-differently-dotted = ##t
  % Start polyphonic section of four voices
  <<
    {c,8[ d fis bes a] | }
  \\
    % Move the c2 out of the main note column so the merge will work
    {c,8~ \shiftOnn c2 | }
  \\
    % Stem on the d2 must be down to permit merging
    {s8 \stemDown d2 | }
  \\
    {s4 fis4. | }
  >>
  \revert Staff.NoteCollision #'merge-differently-headed
  \revert Staff.NoteCollision #'merge-differently-dotted
  g2.\)
}

lhMusic = \relative c' {
  r2 <c g ees>2( |
  <d g, d>1)\arpeggio |
  r2. d,,4 r4 r |
  r4
}

\score {
  \new PianoStaff <<
    \new Staff = "RH"  <<
      \key g \minor
      \rhMusic
    >>
    \new Staff = "LH" <<
      \key g \minor
      \clef "bass"
      \lhMusic
    >>
  >>
}
@end lilypond

Nearly there.  Only two problems remain: The downward stem on the
merged D should not be there, and the C would be better positioned
to the right of the D's.  We know how to do both of these from the
earlier tweaks: we make the stem transparent, and move the C with
the @code{force-hshift} property.  Here's the final result:

@lilypond[quote,verbatim,ragged-right]
rhMusic = \relative c'' {
  r2
  c4.\( g8 |
  \once \override Tie #'staff-position = #3.5
  bes1~ |
  \bar "||"
  \time 6/4
  bes2.^\markup {\bold "Moderato"} r8
  \override Staff.NoteCollision #'merge-differently-headed = ##t
  \override Staff.NoteCollision #'merge-differently-dotted = ##t
  <<
    {c,8[ d fis bes a] | }
  \\
    % Reposition the c2 to the right of the merged note
    {c,8~ \once \override NoteColumn #'force-hshift = #1.0
    % Move the c2 out of the main note column so the merge will work
    \shiftOnn c2}
  \\
    % Stem on the d2 must be down to permit merging
    {s8 \stemDown \once \override Stem #'transparent = ##t d2}
  \\
    {s4 fis4.}
  >>
  \revert Staff.NoteCollision #'merge-differently-headed
  \revert Staff.NoteCollision #'merge-differently-dotted
  g2.\)
}

lhMusic = \relative c' {
  r2 <c g ees>2( |
  <d g, d>1)\arpeggio |
  r2. d,,4 r4 r |
  r4
}

\score {
  \new PianoStaff <<
    \new Staff = "RH"  <<
      \key g \minor
      \rhMusic
    >>
    \new Staff = "LH" <<
      \key g \minor
      \clef "bass"
      \lhMusic
    >>
  >>
}
@end lilypond


@node Further tweaking
@section Further tweaking

@menu
* Other uses for tweaks::       
* Using variables for tweaks::  
* Other sources of information::  
* Advanced tweaks with Scheme::  
* Avoiding tweaks with slower processing::  
@end menu

@node Other uses for tweaks
@subsection Other uses for tweaks

@itemize

@cindex transparent property
@cindex objects, making invisible
@cindex removing objects
@cindex objects, removing
@cindex hiding objects
@cindex invisible objects

@item
Setting the @code{transparent} property will cause an object 
to be printed in @q{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, ties, and beams
can be attached to it.

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,

@lilypond[quote,fragment,relative=2]
<< {
  b8~ b8\noBeam
} \\ {
  b[ g8]
} >>
@end lilypond

@noindent
and blanking the first up-stem in that voice, the tie appears to cross
voices:

@lilypond[quote,fragment,relative=2,verbatim]
<< {
  \once \override Stem #'transparent = ##t
  b8~ b8\noBeam
} \\ {
  b[ g8]
} >>
@end lilypond

To make sure that the just-blanked stem doesn't squeeze the tie too much,
we also lengthen the stem, by setting the @code{length} to
@code{8},

@lilypond[quote,fragment,relative=2,verbatim]
<< {
  \once \override Stem #'transparent = ##t
  \once \override Stem #'length = #8
  b8~ b8\noBeam
} \\ {
  b[ g8]
} >>
@end lilypond

@cindex stencil property

@item
For outside-staff objects it is usually better to override the
object's @code{stencil} property rather than its @code{transparent}
property when you wish to remove it from the printed output.  
Setting the @code{stencil} property of an object to @code{#f} will
remove that object entirely from the printed output.  This means it
has no effect on the placement of other objects placed relative to
it.

For example, if we wished to change the metronome setting in order
to simulate a fermata in the MIDI output we would not want the
metronome marking to appear in the printed output, and we would
not want it to influence the spacing between the two systems or
the spacing of the notes on the staff.  So setting its 
@code{stencil} property to @code{#f} would be the best way.
We show here the effect of the two methods:

@lilypond[quote,verbatim]
\score {
  \relative c'' {
    % Visible tempo marking
    \tempo 4=120
    a4 a a
    \once \override Score.MetronomeMark #'transparent = ##t
    % Invisible tempo marking to lengthen fermata note in MIDI
    \tempo 4=80
    a\fermata
    \once \override Score.MetronomeMark #'stencil = ##f
    % Invisible tempo marking to restore tempo in MIDI
    \tempo 4=120
    a a a a
  }
  \midi { }
}
@end lilypond

@noindent
Both methods remove the metronome mark from the printed output,
and both affect the MIDI timing as required, but the first
(transparent) metronome mark still influences the note spacing
while the second (with no stencil) does not.

@end itemize

@node Using variables for tweaks
@subsection Using variables for tweaks

TODO Add section

@node Other sources of information
@subsection Other sources of information

The Internals Reference documentation contains a lot of information
about LilyPond, but even more information can be gathered from
looking at the internal LilyPond files.

TODO Show example, then explain where to find ly/propert-ly.

Some default settings (such as the definitions for
@code{\header@{@}}s) are stored as @code{.ly} files.  Other
settings (such as the definitions of markup commands) are
stored as @code{.scm} (Scheme) files.  Further explanation is
outside the scope of this manual; users should be warned that
a substantial amount of technical knowledge or time is required
to understand these files.

@itemize

@item Linux: @file{@var{installdir}/lilypond/usr/share/lilypond/current/}

@item OSX:
@file{@var{installdir}/LilyPond.app/Contents/Resources/share/lilypond/current/}.
To access this, either @code{cd} into this directory from the
Terminal, or control-click on the LilyPond application and select
@q{Show Package Contents}.

@item Windows: @file{@var{installdir}/LilyPond/usr/share/lilypond/current/}

@end itemize

The @file{ly/} and @file{scm/} directories will be of
particular interest.  Files such as @file{ly/property-init.ly} and
@file{ly/declarations-init.ly} define all the common tweaks.


@node Advanced tweaks with Scheme
@subsection Advanced tweaks with Scheme

We have seen how LilyPond output can be heavily modified using
commands like
@code{\override TextScript #'extra-offset = ( 1 . -1)}.  But
we have even more power if we use Scheme.  For a full explantion
of this, see the @ref{Scheme tutorial}, and
@ruser{Interfaces for programmers}.

We can use Scheme to simply @code{\override} commands,

TODO Check this is a valid example with skylining

@lilypond[quote,verbatim,ragged-right]
padText = #(define-music-function (parser location padding) (number?)
#{
  \once \override TextScript #'padding = #$padding
#})

\relative c''' {
  c4^"piu mosso" b a b
  \padText #1.8
  c4^"piu mosso" d e f
  \padText #2.6
  c4^"piu mosso" fis a g
}
@end lilypond

We can use it to create new commands,

TODO Check this is a valid example with skylining

@lilypond[quote,verbatim,ragged-right]
tempoMark = #(define-music-function (parser location padding marktext)
                                    (number? string?)
#{
  \once \override Score . RehearsalMark #'padding = $padding
  \once \override Score . RehearsalMark #'extra-spacing-width = #'(+inf.0 . -inf.0)
  \mark \markup { \bold $marktext }
#})

\relative c'' {
  c2 e
  \tempoMark #3.0 #"Allegro"
  g c
}
@end lilypond

Even music expressions can be passed in.

@lilypond[quote,verbatim,ragged-right]
pattern = #(define-music-function (parser location x y) (ly:music? ly:music?)
#{
  $x e8 a b $y b a e
#})

\relative c''{
  \pattern c8 c8\f
  \pattern {d16 dis} { ais16-> b\p }
}
@end lilypond


@node Avoiding tweaks with slower processing
@subsection Avoiding tweaks with slower processing

LilyPond can perform extra checks while it processes files.  These
commands will take extra time, but the result may require fewer
manual tweaks.

@example
%%  makes sure text scripts and lyrics are within the paper margins
\override Score.PaperColumn #'keep-inside-line = ##t 
\override Score.NonMusicalPaperColumn #'keep-inside-line = ##t
@end example

In some cases (see issue 246), this must be done before
@code{\override} commands can be processed.

@verbatim
\new Score \with {
    \override PaperColumn #'keep-inside-line = ##t 
    \override NonMusicalPaperColumn #'keep-inside-line = ##t
  } {
   ..
}
@end verbatim