release: 1.3.126

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

@node Internals
@chapter Internals

@menu
* Conversion stages::              Lilypond is a multi-pass program.
* Moment:: 
* Grobs::                          Graphical object  
* Engraver::
* Music_iterator::
* Music::
* Molecules::                       Molecules are stand-alone descriptions of output
* Font metrics::                    Font metrics
@end menu

@node Conversion stages
@section Conversion stages

When translating the input to notation, there are number of distinct
phases.  We list them here:

@table @code

@item Parsing:

The .ly file is read, and converted to a list of @code{Scores}, which
each contain @code{Music} and paper/midi-definitions.

@item Interpreting music
@cindex interpreting music

All music events are "read" in the same order as they would be played
(or read from paper). At every step of the interpretation, musical
events are delivered to
interpretation contexts,
@cindex engraver
which use them to build grobs (or MIDI objects, for MIDI output).

@item Prebreaking

@cindex prebreaking

At places where line breaks may occur, clefs and bars are prepared for
a possible line break. 

@item Preprocessing

@cindex preprocessing

In this stage, all information that is needed to determine line breaking
is computed. 

@item Break calculation:

The lines and horizontal positions of the columns are determined.

@item Breaking

Relations between all grobs are modified to reflect line breaks. See
also @ref{Pointer substitution}.

@item Outputting:

All vertical dimensions and spanning objects are computed, and all grobs
are output, line by line.

@end table

@node Moment
@section Moment

Moment is a rational number. Since GUILE doesn't support them natively,
so we created our own rational data type.

@defun moment?
@end defun

@defun make-moment num den
create the rational number @var{num}/@var{den}. 
@end defun

@node Grobs
@section Grobs

This section is about Grobs (short for Graphical Objects), which are
formatting objects used to create the final output. This material is
normally the domain of LilyPond gurus, but occasionally, a normal user
also has to deal with grobs.

The most simple interaction with Grobs are when you use
@code{\override}:

@example
        \property Voice.Stem \override #'direction = #1
@end example

This piece of lily input causes all stem objects to be stem-up
henceforth.  In effect, you are telling lilypond to extend the defintion
of the "Stem" grob with the setting @code{direction := 1}.  Of course
there are many more ways of customizing Lily output, and since most of
them involve Grobs in some form, this section explains some details of
how grobs work.

@menu
* What is a grob?::
* Callbacks::
* Setting grob properties::
* Items and Spanners::
* Pointer substitution::
* Grob Scheme functions::
@end menu

@node What is a grob?
@subsection What is a grob?

In music notation, lots of symbols are related in some way.  You can
think of music notation as a graph where nodes are formed by the
symbols, and the arcs by their relations. A grob is node in that
graph. A grob stores references to other grobs, the directed edges in
the graph.

The objective of this big graph of grobs, is to specify the notation
problem. The solution of this problem is a description of the printout
that is in closed form, i.e. but a list of values.  These values are
Molecules. (see @ref{Molecules})

All grobs have an X and Y-position on the page.  These X and Y positions
are stored in a relative format, so they can easily be combined by
stacking them, hanging one grob to the side of another, and coupling
them into a grouping-grob.

Each grob has a reference point, or parent: the position of a grob is
stored relative to that reference point. For example the X-reference
point of a staccato dot usually is the note head that it applies
to. Whenever the note head is moved, the staccato dot moves along
automatically.

If you keep following offset reference points, you will always end up at
the root-object. This root object is called @rgrob{Line_of_score}, and it
represents a system (ie. a line of music).

All grobs carry a set of grob-properties.  In the Stem example above,
the property @code{direction} is set to value @code{1}.  The function
that draws the symbol (@code{Stem::brew_molecule}) uses the value of
@code{direction} to determine how to print the stem and the flag.  The
appearance of a grob is determined solely by the values of its
properties.

Often, a grob also is associated with a symbol. On the other hand,
Some grobs do not print any symbols, but take care of grouping
objects. For example, there is a separate grob that stacks staffs
vertically, so they are not printed in overstrike. The
@rgrob{NoteCollision} is another example of an abstract grob.  It only
moves around chords, but doesn't print anything.

A complete list of grob types is found in
@ref{(lilypond-internals)LilyPond backend}

Grobs are created in the "Interpreting music" phase, by things in
LilyPond called engravers.  In this phase of the translation, a load of
grobs are created, and they are linked into a giant network of objects.
This network of grobs forms the "specification" of the print
problem. This problem is then solved: configurations, directions,
dimensions, line breaks, etc.  are calculated. Finally,   the printing
description in the form of Molecules (@ref{Molecules})  is extracted from
the network. These are then dumped into the output file

@node Callbacks
@subsection Callbacks

Offsets of grobs are relative to a parent reference point. Most
positions are not known when an object is created, so these are
calculated as needed. This is done by adding a callback for a specific
direction.

Suppose you have the following code in a .ly file.
@example
        #(define (my-callback gr axis)
                (*  2.0 (get-gr-property grob 'direction))
        )

....

        \property Voice.Stem \override #'Y-offset-callbacks = #(list
                        my-callback)
@end example

When the Y-offset of a Stem object is needed, LilyPond will
automatically execute all callbacks for that object. In this case, it
will find @code{my-callback}, and execute that. The result is that the
stem is translated by two staff spaces in its direction.

(note: Y-offset-callbacks is also a property) 


Offset callbacks can be stacked, ie.

@example
        \property .... \override #'Y-offset-callbacks = #(list
                callback1 callback2 callback3)

@end example

The callbacks will be executed in the order callback3 callback2
callback1. This is used for quantized positioning: the staccato dot is
above or below a note head, and it must not be on a staff-line.

To achieve this, for the staccato there are two callbacks: one callback
that positions the grob above or below the note head, and one callback
that rounds the Y-position of the grob to the nearest open space.

Similarly, the size of a grob are determined through callbacks, settable
with grob properties @code{X-extent-callback} and @code{Y-extent-callback}.
There can be only one extent-callback for each axis. No callback (value #f)
means: "empty in this direction". If you fill in a pair, that pair
hard-codes the extent in that coordinate.


@node Setting grob properties
@subsection Setting grob properties

Grob properties are stored as GUILE association lists, with symbols as
keys.   From C++, element properties can be accessed using the functions

@example
  SCM  get_grob_property (SCM) const;
  void set_grob_property (const char * , SCM val);
  void set_immutable_grob_property (const char * , SCM val);
  void set_immutable_grob_property (SCM key, SCM val);  
  void set_grob_property (SCM , SCM val);  
  void set_grob_pointer (const char*, SCM val);
  SCM  remove_grob_property (const char* nm);
@end example

In GUILE, LilyPond provides

@example
        ly-get-grob-property GROB SYMBOL
        ly-set-grob-property GROB SYMBOL VALUE
@end example

All lookup functions identify undefined properties with 
end-of-list (ie. @code{'()} in Scheme or @code{SCM_EOL} in C)

Properties are stored in two ways:
@itemize @bullet
@item mutable properties:
element properties that change from object to object. The storage of
these are private to a grob. Typically this is used to store lists of
pointers to other grobs

@item immutable properties:
element properties that are shared across different grobs of the same
type. The storage is shared, and hence it is read-only. Typically, this
is used to store function callbacks, and values for shared element
properties are read from @file{scm/element-description.scm}.
@end itemize

There are two ways to manually set grob properties.

You can change immutable grob properties. This is done with the
\override syntax:

@example
        \property Voice.Stem \override #'direction = #1
@end example

This will push the entry @code{'(direction . 1)} on the immutable
property list for stems, in effect overriding the setting from
@file{scm/element-description.scm}. This can be undone by 

@example
        \property Voice.stem \revert #'direction
@end example

If you use this a lot, this gets old quickly. So we also have a
shorthand,

@example
        \property Context.GrobType \set #'prop = #VAL
@end example

this does a @code{\revert} followed by a @code{\override}

The second way is \outputproperty. This construct looks like

@example
        \context ContextName \outputproperty @var{pred} #@var{sym} = #@var{val}
@end example

In this case, in every grob that satisfies @var{pred}, the property
assignment @var{sym} = @var{val} is done.  For example

@example
        \outputproperty
                #(lambda (gr) (string? (ly-get-grob-property gr
                        'text)))
                #'extra-offset = #'(-1.0 . 0.0)
@end example

This shifts all elements that have a @code{text} property one staff
space to the left. This mechanism is rather clumsy to use, but it allows
you tweak any setting of any grob.

@node Items and Spanners
@unnumberedsubsec Items and Spanners

Grobs can also be distinguished in their role in the horizontal spacing.
A lot of grobs define constraints on the spacing by their sizes. For
example, note heads, clefs, stems, and all other symbols with a fixed
shape.  These grobs form a subtype called @code{Item}.

Other grobs have a shape that depends on the horizontal spacing. For
example, slur, beam, tie, etc. These grobs form a subtype called
@code{Spanner}. All spanners have two span-points (these must be
@code{Item}s), one on the left and one on the right. The left bound is
also the X-reference point.

Some items need special treatment for line breaking. For example, a
clef is normally only printed at the start of a line (ie. after a line
break).  To model this, `breakable' items (clef, key signature, bar lines,
etc.) are copied twice. Then we have three versions of each breakable
item: one version if there is no line break, one version that is printed
before the line break (at the end of a system), one version that is
printed after the line break.

Whether these versions are visible and take up space, is determined by
the outcome of the visibility-lambda. This is a function taking a
direction (-1, 0 or 1) and returns a cons of booleans, signifying wether
this grob should be transparent and have no extent.

@node Pointer substitution
@unnumberedsubsec Pointer substitution


Symbols that cross line-breaks (such as slurs) cause some more
complications. When a  spanner crosses a line-break, then the spanner is
"broken into pieces", for every line that the spanner is in, a copy of
the grob is made. A substitution process redirects all grob-reference
so that spanner grob will only reference other grobs in the same line.

@node Grob Scheme functions
@unnumberedsubsec Grob Scheme functions


@defun ly-get-grob-property g sym
  Get the value of a value in grob @var{g} of property @var{sym}. It
will return @code{'()} (end-of-list) if @var{g} doesn't have @var{sym} set.
@end  defun

@defun ly-set-grob-property g sym val
@end defun

@defun ly-get-spanner-bound spanner dir
@end defun

@defun ly-grob? g
@end defun

@node Duration
@section Duration

@defun make-duration length dotcount

@var{length} is the negative logarithm (base 2) of the duration:
1 is a half note, 2 is a quarter note, 3 is an eighth
note, etc.  The number of dots after the note is given by
@var{dotcount}.

@end defun


@node Pitch data type
@section Pitch data type



@defun make-pitch octave note shift

@var{octave} is specified by an integer, zero for the octave
containing middle C.  @var{note} is a number from 0 to 7, with 0
corresponding to C and 7 corresponding to B.  The shift is zero for a
natural, negative to add flats, or positive to add sharps.
@end defun

@defun pitch-octave
@end defun

@defun pitch-notename
@end defun

@defun pitch-alteration
@end defun

@defun pitch-semitones
@end defun

@defun Pitch::transpose
@end defun

@node Engraver
@section Engraver

@defun ly-get-trans-property
@end defun

@defun ly-set-trans-property
@end defun

@node Music_iterator
@section Music_iterator

@defun c++-function?
@end defun

@node Music
@section Music

@defun ly-get-mus-property
@end defun

@defun ly-set-mus-property
@end defun


@node Molecules
@section Molecules

@cindex Molecule
@cindex Atom
@cindex Output description

The objective of any typesetting system is to put ink on paper in the
right places. For LilyPond, this final stage is left to the TeX and the
printer subsystem. For lily, the last stage in processing a score is
outputting a description of what to put where.  This description roughly
looks like

@example
        PUT glyph AT (x,y)
        PUT glyph AT (x,y)
        PUT glyph AT (x,y) 
@end example

you merely have to look at the tex output of lily to see this.
Internally these instructions are encoded in Molecules:@footnote{At some
point LilyPond also contained Atom-objects, but they have been replaced
by Scheme expressions, making the name outdated.}.  A molecule is an
object that combines dimension information (how large is this glyph ?)
with what-to-print-where.

Conceptually, Molecules can be constructed from Scheme code, by
translating a Molecule and by combining two molecules. In BNF
notation:

@example
 Molecule = COMBINE Molecule Molecule
           | TRANSLATE Offset Molecule
           | GLYPH-DESCRIPTION
           ;
@end example

If you are interested in seeing how this information is stored, you
can run with the @code{-f scm} option. The scheme expressions are then
dumped onto the output file.

(refer to the C++ code for more details). All visible,
ie. non-transparant, grobs have a callback to create a Molecule. The
name of the property is @code{molecule-callback}, and its value should
be a Scheme function taking one argument (the grob) and returning a
Molecule.

@defun molecule? m
@end defun

@defun ly-combine-molecule-at-edge  mol1 axis dir mol2 padding
@end defun

@defun ly-get-molecule-extent! mol axis
@end defun

@defun ly-set-molecule-extent! mol axis extent
@end defun

@node Font metrics
@section Font metrics

The font object represents the metric information of a font. Every font
that is loaded into LilyPond can be accessed via Scheme. 

LilyPond only needs to know the dimension of glyph to be able to process
them. This information is present in font-metric files. LilyPond can
read two types of font-metrics: @TeX{} Font Metric files (tfm files) and
Adobe Font Metric files (afm files).  AFM files are more versatile, and
LilyPond needs those features to typeset musical symbols. So LilyPond
will always try to load afm files first. 


@defun ly-get-default-font gr
This returns the default font for grob @var{gr}.
@end defun

@defun ly-find-glyph-by-name font name
This function retrieves a Molecule for the glyph named @var{name} in
@var{font}.  The font must be available as a afm file.
@cindex afm file

@end defun

@node Miscellaneous Scheme functions
@section Miscellaneous Scheme functions

@defun ly-input-location?
@end defun

@defun ly-warn
@end defun

@defun ly-version
@end defun

@defun ly-gulp-file
@end defun

@defun dir?
@end defun

@defun ly-number->string
@end defun