[lilypond.git] / Documentation / lilygut.pod

=head1 NAME

LilyGuts - doco to the internals of GNU LilyPond

=head1 DESCRIPTION

This page documents some aspects of the internals of GNU LilyPond. Some of
this stuff comes from e-mail I wrote, some from e-mail others wrote,
some are large comments taken away from the headers. This is why this
page may be a little incoherent.

You should use doc++ to take a peek at the sources.

This should become a Hacking-HOWTO. If you find any confusing stuff
here, let me know.  I am a hacker, and don't spend enough time doccing
what I write. (Most stuff here which refers to the code is slightly outdated)

If you finally have mastered some internal piece of lily, your
explanation could be added here.

=head1 OVERVIEW

GNU LilyPond is a "multi-pass" system. The different passes have been
created so that they do not depend on each other. In a later stage
some parts may be moved into libraries, or seperate programs, or they
might be integrated in larger systems.

=over 4

=item Parsing:

No difficult algorithms. The .ly file is read, and converted to a list
of C<Scores>, which each contain C<Music> and paper/midi-definitions.

=item Creating elements

The music is walked column by column. The iterators which do the
walking report the Request to Translators which use this information
to create elements, either MIDI or "visual" elements. The translators
form a hierarchy; the ones for paper output are Engravers, for MIDI
Performers).

The translators swallow requests, create elements, broadcast them to
other translators on higher or same level  in the hierarchy:

The stem of a voice A is broadcast to the staff which contains A, but
not to the noteheads of A, and not to the stems, beams and noteheads
of a different voice (say B) or a different staff. The stem and
noteheads of A are coupled, because the the Notehead_engraver
broadcasts its heads, and the Stem catches these.

The engraver which agrees to handle a request decides whether to to
honor the request, ignore it, or merge it with other requests. Merging
of requests is preferably done with other requests done by members of
the same voicegroups (beams, brackets, stems). In this way you can put
the voices of 2 instruments in a conductor's score so they make chords
(the Stem_reqs of both instruments will be merged).

=item Prebreaking

Breakable stuff (eg. clefs and bars) are copied into pre and postbreaks. 

=item Preprocessing

Some dependencies are resolved, such as the direction of stems, beams,
and "horizontal" placement issues (the order of clefs,  keys etc,
placement of chords in multi-voice music), 

=item Break calculation:

The lines and horizontal positions of the columns are determined.

=item Breaking

Through some magical interactions with Line_of_score and Super_elem
(check out the source) the "lines" are produced. 

All other spanners can figure across which lines they are spread. If
applicable, they break themselves into pieces. After this, each piece
(or, if there are no pieces, the original spanner itself) throws out
any dependencies which are in the wrong line.

=item Postprocesing:

Some items and all spanners need computation after the PCol positions
are determined. Examples: slurs, vertical positions of staffs.

=item Output paper

=back

=head1 INTERNALS

This chapter deals with the internals of Mudela. 

=head2 Requests

As you can see, most information is stored in the form of a request. 
In music typesetting, the user might want to cram a lot more symbols
on the paper than actually fits. To reflect this idea (the user asks
more than we can do), the container for this data is called Request.

In a lot of other formats this would be called an 'Event'

=over 4

=item C<Barcheck_req>

Checks during music processing if start of this voice element
coincides with the start of a measure. Handy to check if you left out
some voice elts.

=item C<Note_req>

LilyPond has to decide if the ball should be hanging left or
right. This influences the horizontal dimensions of a column, and this
is why request processing should be done before horizontal spacing.

Other voices' frivolities may cause the need for accidentals, so this
is also for the to decide. The engraver can decide on positioning based on
ottava commands and the appropriate clef.

=item C<Rest_req>

Typeset a rest.

=item C<Span_req>

This type of request typically results in the creation of a C<Spanner>

=item C<Beam_req>

Start/stop a beam.

Engraver has to combine this request with the stem_request, since the
number of flags that a stem wants to carry will determine the
number of beams.

=item  C<Dynamic>

Each dynamic is bound to one note (a crescendo spanning multiple
notes is thought to be made of two "dynamics": a start and a stop).
Dynamic changes can occur in a smaller time than the length of its
note, therefore fore each  C<Dynamic> request carries a time, measured
from the start of its note.

=head2 Voice groups

Voice group is a (time-dependent) collection of voices which share
some characteristics (slurs, stems) at some time.

=head1 Request_engraver

In the previous section the idea of Request has been explained, but
this only solves one half of the problem. The other half is
deciding which requests should be honored, which should merged with
other requests, and which should be ignored. Consider this (pseudo)input

        < % chord
                \music { [c() c] }
                \music { [e() e] }
        >

Both the c and e are part of a chord (they are in the same
Voice_group), so they should share the beams, and the two [ ] pairs
should be merged. The slurs OTOH are specific for each voice, so they
should not be shared.

The judge in this "allocation" problem a set of broker. It uses the
C<Request_engraver> to do most of the work.  For each request
C<Complex_staff> queries so-called C<Request_engraver>s if they want
to accept a request eg, the C<Notehead_engraver> will accept
C<Note_req>s, and turn down C<Slur_req>s. If the Music_iterator
cannot find a engraver that wants the request, it is junked (with a
warning message).

After all requests have been either assigned, or junked, the Engraver
will process the requests (which usually means creating an C<Item> or
C<Spanner>). If a C<Request_engraver> creates something, it tells
If all requests have been processed, then each Engraver is notified
of any created Score_element, via a broadcasting system.

=head2 example:

        c4

produces:

        note_request (duration 1/4)
        stem_request (duration 1/4)

note_request will be taken by a C<Notehead_engraver>, stem_request
will be taken by a C<Stem_beam_engraver>. C<Notehead_engraver> creates
a C<Notehead>, C<Stem_beam_engraver> creates a C<Stem>. Both announce
this to the Staff_engraver. Staff_engraver will tell
C<Stem_beam_engraver> about the C<Notehead>, which will add the
C<Notehead> to the C<Stem> it just created.

To decide on merging, several engravers have been grouped. Please
check F<init/engraver.ly>.


=head1 ITEMS and SPANNERS

The symbols that are printed, are generated by items and spanners
(staff-elements). An item has one horizontal position, whereas a
spanner spans several columns.

=head1 DEPENDENCIES

In music symbols depend on each other: the stems of a beam should
point in the same direction as the beam itself, so the stems of a beam
depend on the beam. In the same way do scripts depend on the direction
of the stem. To reflect this, LilyPond has the notion of dependency. 
It works in the same fashion that C<make> uses to build programs: before
a stem is calculated, its dependencies (the beam) should be
calculated. Before a slur is calculated, its dependencies (stems, noteheads)
should be calculated.

=head1 BREAKING

So what is this PREBREAK and POSTBREAK stuff?

Let's take text as an example. In German some compound
words change their spelling if they are broken: "backen" becomes
"bak-ken".  TeX has a mechanism to deal with this, you would define
the spelling of "backen" in TeX in this way

        \discretionary{bak-}{ken}{backen}

These 3 arguments are called "prebreak", "postbreak" and "nobreak"
text.

The same problem exists when typesetting music. If a line of music is
broken, the next line usually gets a clef. So in TeX terms, the clef
is a postbreak. The same thing happens with meter signs: Normally the
meter follows the bar. If a line is broken at that bar, the bar along
with the meter stays on the "last" line, but the next line also gets a
meter sign after the clef. Using the previous notation,

        \discretionary{bar meter}{clef meter}{ bar meter }

In GNU Lilypond, we have the same concepts (and the same
terminology). Each (nonrhythmic) symbol is typeset in  a nonrhythmic column
At a breakpoint, multiple symbols are printed; symbols to be printed
if the line is not broken, symbols to appear on the previous line, and
on the next line if it is broken.

=head1 SPACING


Some terminology: I call a vertical group of symbols (notes) which
start at the same time a "column".  Each line of a score has notes in
it, grouped in columns. The difference in starting time between those
columns makes it possible to determine ideal distances between those
columns.

Example:

        time ----->

        cols:   col1    col2    col3    col4


        voice1  1               1

        voice2  2       2       2       2


        (1 is a whole note, 2 a half note.)

        time_difference (col1 , col2) = 0.5 wholes,
        time_difference (col1 , col3) = 1 wholes,
        time_difference (col2 , col3) = 0.5 wholes,
        etc.

these differences are translated into ideal distances 

        distance (col1,col2) = 10 pt
        distance (col1,col3) = 14.1 pt
        distance (col2,col3) = 10 pt
        etc.

as you can see, these distance are conflicting. So instead of
satisfying all those ideals simultaneously, a compromise is sought.

This is Columbus' egg: GNU LilyPond attaches "springs" to each
column-pair.  each spring has an equilibrium-position which is equal to
the above mentioned distance, so

spring (col1, col2) and spring (col2,col3) try to push column 1
and 3 away (to a distance of 20pt) from each other, whereas the spring
between col 1 and col 3 tries to pull those two together (to a
distance of 14.1 pt). The net result of this pushing and pulling is an
equilibrium situation (the pushing cancels the pulling), which can be
calculated as the solution of Quadratic program: it is the solution
with minimum potential energy, for you physicists out there.

This algorithm for doing one line, gives a "badness" parameter for
each line (the potential energy). Now one can use TeX's algorithm for
making paragraphs (using this new version of "badness"): one should
try to minimise the overall badness of a paragraph. GNU LilyPond also
uses the concept of pre- and post-breaks.

(actually, it is a bit more complicated: each column also has a
minimum distance to other columns, to prevent symbols from running
into symbols of other columns.)


=head1 SPACING 2


This of course does not solve the problem of generating the
springs. This is an area that needs a lot of work, and the optimal
solution to find is not of a mathematical nature.

Gourlay's solution is used.