article(LilyPond internals)(Han-Wen Nienhuys)()

This 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 page may be
a little incoherent.  Unfortunately, it is also quite outdated.  A
more thorough  and understandable document is in the works.

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


sect(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.

description(

dit(Parsing:)

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

dit(Interpreting music)

The music is walked through in time-order. The iterators which do the
walking report Music 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 Music (mostly atomic gobs called 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 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 Note_heads_engraver broadcasts its heads, and the Stem_engraver 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 Beam requests of both instruments will be merged).

dit(Prebreaking)

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

dit(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), 

dit(Break calculation:)

The lines and horizontal positions of the columns are determined.

dit(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.

dit(Postprocesing:)

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

dit(Output paper)

)

sect(mudela)

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'

description(
dit(code(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.
dit(code(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.
dit(code(Rest_req))
    Typeset a rest.
dit(code(Span_req))
    This type of request typically results in the creation of a code(Spanner)
dit(code(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.
dit(code(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  code(Dynamic) request carries a time, measured
    from the start of its note.
)

sect(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 input

verb(
	\type Staff < % chord
		{ \meter 2/4; [c8 c8] }
		{\meter 2/4;  [e8 e8] }
	>
)

Both the cs and es are part of a staff (they are in the same
Voice_group), so they should share meters, but the two [ ] pairs
should be merged.

The judge in this "allocation" problem a set of brokers: the requests
are transmitted to so-called engravers which respond if they want to
accept a request eg, the code(Notehead_engraver) will accept
code(Note_req)s, and turn down code(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 code(Item)
or code(Spanner)). If a code(Request_engraver) creates something, it
tells the enclosing context. If all items/spanners have been created,
then each Engraver is notified of any created Score_element, via a
broadcasting system.

nsubsect(example:)

verb(
	c4
)

produces:

verb(
	Note_request (duration 1/4)
	Stem_request (duration 1/4)
)

Note_request will be taken by a code(Notehead_engraver), stem_request
will be taken by a code(Stem_beam_engraver). code(Notehead_engraver)
creates a code(Notehead), code(Stem_beam_engraver) creates a
code(Stem). Both announce this to the Staff_engraver. Staff_engraver
will tell code(Stem_beam_engraver) about the code(Notehead), which
will add the code(Notehead) to the code(Stem) it just created.

To decide on merging, several engravers have been grouped. Please
check file(init/engraver.ly).


sect(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.

sect(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 code(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.

sect(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.

sect(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:

verb(
	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 

verb(
        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.)