More goodness from Trevor D.

[lilypond.git] / Documentation / user / tweaks.itely

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

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

@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.

NB: This chapter is still under development and this version is
somewhat experimental; nothing is fixed.  Don't translate yet!



@menu
* Tweaking basics::             
* The Internals Reference manual::  
* Appearance of objects::       
* Placement of objects::        
* Collisions of objects::       
* Page layout::                 
* Common tweaks::               
* TODO other name::             
@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 potted 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 @code{Voice} objects, @code{Score} objects,
@code{Lyric} objects, and objects to represent barlines,
noteheads, ties, dynamics, etc, each with their own set of
properties.

Some types of object are given special names.  Objects which
represent items of notation on the printed output such as
noteheads, 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, Layout Objects     
  @tab Aaaa or AaaaAaaaAaaa
  @tab Staff, GrandStaff, 
@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}.
It is not really a new command but a shorthand for an 
@code{\override} command which must be used in a few special 
circumstances.  Again, we shall not discuss or use it here.  
You may find the details in 
@ruser{Objects connected to the input}.
@c Maybe explain in a later iteration  -td

@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::             
@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 barlines 
in a piece of music.  But the barlines are normally inserted
automatically.  How do we prevent them printing?

As before, we go to the IR to find the layout object which prints
barlines.  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 barlines (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 barlines 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 barlines 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 barlines 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 barlines 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 barlines 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 barlines.

@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 barlines.  
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 barlines 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 barlines 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 barlines 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 noteheads, 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 barlines, 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 noteheads 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.

@cindex magstep
@cindex size, changing
@cindex stem length, changing
@cindex staff line spacing, changing

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 stems 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 of all
other objects may be modified in analogous ways.

@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
noteheads, 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 

@c TODO Mention somewhere, probably not here, how to avoid staves
@c interleaving too much by specifying skyline-horizontal-padding
@c Presumably this moves the skyline boundary further out, making
@c objects appear wider?  Not yet tested.  -td

@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.

@c TODO put @code{} round all these

@multitable @columnfractions .2 .2 .2 .4
@headitem Down/Left           
  @tab Up/Right
  @tab Revert
  @tab Effect 
@item \arpeggioDown
  @tab \arpeggioUp
  @tab \arpeggioNeutral
  @tab Arrow is at bottom, at top, or no arrow
@item \dotsDown
  @tab \dotsUp
  @tab \dotsNeutral
  @tab Direction of movement to avoid staff lines
@item \dynamicDown
  @tab \dynamicUp
  @tab \dynamicNeutral
  @tab
@item \phrasingSlurDown
  @tab \phrasingSlurUp
  @tab \phrasingSlurNeutral
  @tab Note: distinct from slur commands
@item \slurDown     
  @tab \slurUp
  @tab \slurNeutral
  @tab
@item \stemDown
  @tab \stemUp
  @tab \stemNeutral
  @tab
@item \textSpannerDown
  @tab \textSpannerUp
  @tab \textSpannerNeutral
  @tab Text entered as spanner is below/above staff
@item \tieDown
  @tab \tieUp
  @tab \tieNeutral
  @tab
@item \tupletDown
  @tab \tupletUp
  @tab \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 @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.
It is set to @code{#f} (the default) for all within-staff objects,
and is set explicitly to the default numerical value appropriate 
to the each object as it is created.  The following table shows
the default numerical values for some of the commonest 
outside-staff objects.  Others may be found in the object 
properties listed in the IR.

@multitable @columnfractions .3 .3 
@headitem Layout Object           @tab Priority
@item @code{BarNumber}            @tab @code{ 100}
@item @code{DynamicLineSpanner}   @tab @code{ 250}
@item @code{DynamicText}          @tab @code{ 250}
@item @code{MetronomeMark}        @tab @code{1000}
@item @code{OttavaBracket}        @tab @code{ 400}
@item @code{RehearsalMark}        @tab @code{1500}
@item @code{TextScript}           @tab @code{ 450}
@end multitable

TODO Add example showing iteraction of all these

If this ordering does not give you the placing you want, the
priority of the objects may be overridden.  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 table 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.  Can we instead position them all
at the same distance from 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{fatText}
command.

@subheading \fatText

@funindex \fatText
@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{\fatText} 
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]
\fatText  % 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{\emptyText}.  Remember @code{\once} only works with
@code{\override}, @code{\set}, @code{\revert} or @code{unset},
so cannot be used with @code{\fatText}.

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
\fatText  % and turn on fatText
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{\fatText} 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 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.  There is a property to control this, but
it is not so easy to find in the IR.  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 our example: 

@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



@node Collisions of objects
@section Collisions of objects

@menu
* Moving objects::              
* Fixing overlapping notation::  
@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
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} taken into account by LilyPond
when positioning the objects may be modified.  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 change can apply to all instances of
the same type of object.  Such properties include:

@itemize @bullet
@item @code{direction}

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

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

The @code{padding} property specifies the gap that must be left 
between
the top and bottom edges of objects which are positioned
either above or below another previously placed object, or
between the sides of objects which are positioned to the left or
right of previously placed objects.
Instead of @code{padding} some objects have instead a
@code{left-padding} or @code{right-padding} property.
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.

The @code{staff-padding} property is closely related.  @code{padding}
controls the minimum amount of space between an object and the nearest
other object (generally the note or the staff lines);
@code{staff-padding} controls the minimum amount of space between an
object and the staff.

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 property might not be located in the
obvious object.  For example, the placement of accidentals is
controlled by @code{left-padding} and @code{right-padding},
but these are to be found in the @code{AccidentalPlacement}
object.

@item @code{self-alignment-X}

This property aligns the object to the left, to the right, or
centers 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}.  The movement is limited by the length of the
object.

@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 repositioned manually
by changing their separation distance from other objects, or by
repositioning them relative to the staff center line.  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 the user is responsible for any collisions that
might follow.  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
repositioning are:

@itemize @bullet
@item @code{extra-offset}
@cindex extra-offset property

This property applies to any layout object
supporting the @code{grob-interface}.  It takes a pair of
numbers giving 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 @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 noteheads overlapping.  These are called
note columns, and an object called @code{NoteColumn} is created
to lay out the notes in that column.  There are separate columns
for each voice.  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 is preferable to the
@code{extra-offset} property for this purpose as there is no need
to work out the distance in staff-spaces.

@item @code{positions}
@cindex positions property

This is most useful for manually adjusting the slope and height
of beams, ties, 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.

@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.

@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.

@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 .33 .66
@headitem Object type           @tab Object name
@item Articulations             @tab @code{Script}
@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}
@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 padding
@cindex padding

TODO Find a better example

Let's try the effect of @code{padding} on the rather artificial
example above.  This is a property of
@code{DynamicLineSpanner}, not @code{DynamicText}.

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
\dynamicUp
% Increase vertical padding by 5 units
\override DynamicLineSpanner #'padding = #'5
a4\f b\mf c\mp b\p
@end lilypond

Dynamics are positioned vertically with respect to the note to
which they are attached.  We see that this has moved all the
dynamics up by five staff spaces, but has had no effect on the
horizontal separation, nor on the placement of the dynamics
relative to each other.

The @code{padding} property can be set to increase
(or decrease) the distance between symbols that are printed
above or below notes.  This applies to all objects with
@code{side-position-interface}.

@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}.


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

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 extra-offset
@cindex extra-offset property

Another solution gives us complete control over placing the object -- we
can move it horizontally or vertically.  This is done with the
@code{extra-offset} property.  It is slightly more complicated and can
cause other problems.  When we move objects with @code{extra-offset},
the movement is done after LilyPond has placed all other objects.  This
means that the result can overlap with other objects.

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
  % temporary code to break this example:
  \override TextScript #'outside-staff-priority = ##f
\once \override TextScript #'extra-offset = #'( 1.0 . -1.0 )
e4^\markup{ \italic ritenuto } g b e
@end lilypond

With @code{extra-offset}, the first number controls the horizontal
movement (left is negative); the second number controls the vertical
movement (up is positive).  After a bit of experimenting, we decided
that these values look good

@lilypond[quote,fragment,ragged-right,verbatim,relative=2]
  % temporary code to break this example:
  \override TextScript #'outside-staff-priority = ##f
\once \override TextScript #'extra-offset = #'( -1.6 . 1.0 )
e4^\markup{ \italic ritenuto } g b e
@end lilypond

@noindent
Again, these numbers are simply the result of a few experiments and
looking at the output.  You might prefer the text to be slightly higher,
or to the left, or whatever.  Try it and look at the result!

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
@cindex force-hshift property

We introduce just one here, the @code{force-hshift} property of
@code{NoteColumn}.  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} 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 notehead'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


TODO To be continued ....

TODO Old stuff follows



@node Page layout
@section Page layout

@menu
* Fitting music onto fewer pages::  
@end menu

@node Fitting music onto fewer pages
@subsection Fitting music onto fewer pages

Sometimes you can end up with one or two staves on a second
(or third, or fourth...) page.  This is annoying, especially
if you look at previous pages and it looks like there is plenty
of room left on those.

When investigating layout issues, @code{annotate-spacing} is
an invaluable tool.  This command prints the values of various
layout spacing commands; see @ruser{Displaying spacing}, for more
details.  From the output of @code{annotate-spacing}, we can
see which margins we may wish to alter.

Other than margins, there are a few other options to save space:

@itemize
@item
You may tell LilyPond to place systems as close together as
possible (to fit as many systems as possible onto a page), but
then to space those systems out so that there is no blank
space at the bottom of the page.

@example
\paper @{
  between-system-padding = #0.1
  between-system-space = #0.1
  ragged-last-bottom = ##f
  ragged-bottom = ##f
@}
@end example

@item
You may force the number of systems (i.e., if LilyPond wants
to typeset some music with 11 systems, you could force it to
use 10).

@example
\paper @{
  system-count = #10
@}
@end example

@item
Avoid (or reduce) objects which increase the vertical size of
a system.  For example, volta repeats (or alternate repeats)
require extra space.  If these repeats are spread over two
systems, they will take up more space than one system with
the volta repeats and another system without.

Another example is moving dynamics which @q{stick out} of
a system.

@lilypond[verbatim,quote,fragment]
\relative c' {
  e4 c g\f c
  \override DynamicLineSpanner #'padding = #-1.8
  \override DynamicText #'extra-offset = #'( -2.1 . 0)
  e4 c g\f c
}
@end lilypond

@item
Alter the horizontal spacing via @code{SpacingSpanner}.  See
@ruser{Changing horizontal spacing}, for more details.

@lilypond[verbatim,quote]
\score {
  \relative c'' {
    g4 e e2 | f4 d d2 | c4 d e f | g4 g g2 |
    g4 e e2 | f4 d d2 | c4 e g g | c,1 |
    d4 d d d | d4 e f2 | e4 e e e | e4 f g2 |
    g4 e e2 | f4 d d2 | c4 e g g | c,1 |
  }
  \layout {
    \context {
      \Score
      \override SpacingSpanner
                #'base-shortest-duration = #(ly:make-moment 1 4)
    }
  }
}
@end lilypond

@end itemize





@c Old stuff follows  -td

@node Common tweaks
@section Common tweaks

blah blah

@menu
* Other common tweaks::         
@end menu



@node Other common tweaks
@subsection Other common tweaks

Some overrides are so common that predefined commands are provided as
short-cuts, such as @code{\slurUp} and @code{\stemDown}.  These
commands are described in the Notation Reference under the appropriate
sections.

The complete list of modifications available for each type of
object (like slurs or beams) are documented in the Program
Reference.  However, many layout objects share properties which can be
used to apply generic tweaks.

@itemize


@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.

@cindex transparent objects
@cindex removing objects
@cindex hiding objects
@cindex invisible objects
The following example demonstrates how to connect different voices
using ties.  Normally, ties only connect two notes in the same
voice.  By introducing a tie in a different voice,

@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

@end itemize

@cindex Tweaks, distances
@cindex Distances

Distances in LilyPond are measured in staff-spaces, while most
thickness properties are measured in line-thickness.  Some
properties are different; for example, the thickness of beams
are measured in staff-spaces.  For more information, see the
relevant portion of the program reference.



@node TODO other name
@section TODO other name

@menu
* Predefined tweaks::           
* Advanced tweaks with Scheme::  
* Avoiding tweaks with slower processing::  
* The three methods of tweaking::  
@end menu

@node Predefined tweaks
@subsection Predefined tweaks

TODO: Some tweaks are really common, blah blah.

for example \slurUp, \fatText.

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


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.

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,

@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,

@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


@node The three methods of tweaking
@subsection The three methods of tweaking

FIXME write this.

@verbatim
\override
@end verbatim

@verbatim
\with {

}
@end verbatim

@verbatim
\layout{ \context

}}
@end verbatim



FIXME discuss \tweak





FIXME:
- There is a section in the manual on \set vs \override (3.3.7),
which is incomplete.  First it doesn't mention \overrideProperty,
nor does it mention properties which are not capitalized at all.
And it should explain that \override should be used to set
properties with hyphenated names, like auto-knee-gap.  (I had to
experiment to find out how to do this.)