4 @c A menu is needed before every deeper *section nesting of @nodes
5 @c Run M-x texinfo-all-menus-update
6 @c to automagically fill in these menus
7 @c before saving changes
11 @chapter Advanced Topics
14 When translating the input to notation, there are number of distinct
15 phases. We list them here:
17 @c todo: moved from refman.
19 The purpose of LilyPond is explained informally by the term `music
20 typesetter'. This is not a fully correct name: Not only does the
21 program print musical symbols, it also makes aesthetic decisions.
22 Symbols and their placements are @emph{generated} from a high-level
23 musical description. In other words, LilyPond would be best described
24 to be a `music compiler' or `music to notation compiler'.
26 LilyPond is linked to GUILE, GNU's Scheme library for extension
27 programming. The Scheme library provides the glue that holds together
28 the low-level routines and separate modules which are written in C++.
30 When lilypond is run to typeset sheet music, the following happens:
33 @item GUILE initialization: Various scheme files are read.
35 @item Parsing: First standard @code{ly} initialization files are read,
36 then the user @file{ly} file is read.
38 @item Interpretation: The music in the file is processed `in playing
39 order', i.e., the order that you use to read sheet music, or the
40 order in which notes are played. The result of this step is a
41 typesetting specification.
43 @item Typesetting: The typesetting specification is solved: positions
44 and formatting is calculated.
46 @item The visible results ("virtual ink") are written to the output file.
49 During these stages different types of data play the the main role:
50 During parsing, @strong{Music} objects are created. During the
51 interpretation, @strong{contexts} are constructed, and with these
52 contexts a network of @strong{graphical objects} (`grobs') is
53 created. These grobs contain unknown variables, and the network forms a
54 set of equations. After solving the equations and filling in these
55 variables, the printed output is written to an output file.
57 These threemanship of tasks (parsing, translating, typesetting) and
58 data-structures (music, context, graphical objects) permeates the entire
59 design of the program.
66 The @code{ly} file is read and converted to a list of @code{Scores}, which
67 each contain @code{Music} and paper/midi-definitions. Here @code{Music},
68 @code{Pitch}, and @code{Duration} objects are created.
70 @item Interpreting music
71 @cindex interpreting music
73 All music events are `read' in the same order as they would be played
74 (or read from paper). At every step of the interpretation, musical
75 events are delivered to interpretation contexts,
77 which use them to build @code{Grob}s (or MIDI objects for MIDI output).
79 In this stage @code{Music_iterators} do a traversal of the @code{Music}
80 structure. The music events thus encountered are reported to
81 @code{Translator}s, a set of objects that collectively form interpretation
87 At places where line breaks may occur, clefs and bars are prepared for
88 a possible line break.
93 In this stage, all information that is needed to determine line breaking
96 @item Break calculation
98 The lines and horizontal positions of the columns are determined.
102 Relations between all grobs are modified to reflect line breaks: When a
103 spanner, e.g. a slur, crosses a line break, then the spanner is `broken
104 into pieces'; for every line that the spanner is in, a copy of the grob
105 is made. A substitution process redirects all grob references so that
106 each spanner grob will only reference other grobs in the same line.
110 All vertical dimensions and spanning objects are computed, and all grobs
111 are output, line by line. The output is encoded in the form of
116 The data types that are mentioned here are all discussed in this
120 @c FIXME: Note entry vs Music entry at top level menu is confusing.
123 * Interpretation context::
124 * Syntactic details::
130 @node Interpretation context
131 @section Interpretation context
134 * Creating contexts::
136 * Context properties::
137 * Engravers and performers::
138 * Changing context definitions::
139 * Defining new contexts::
143 Interpretation contexts are objects that only exist during a run of
144 LilyPond. During the interpretation phase of LilyPond (when it prints
145 @code{interpreting music} to standard output), the music expression in
146 a @code{\score} block is interpreted in time order. This is the same
147 order that humans hear and play the music.
149 During this interpretation, the interpretation context holds the
150 state for the current point within the music. It contains information
154 @item What notes are playing at this point?
156 @item What symbols will be printed at this point?
158 @item What is the current key signature, time signature, point within
162 Contexts are grouped hierarchically: A @internalsref{Voice} context is
163 contained in a @internalsref{Staff} context (because a staff can contain
164 multiple voices at any point), a @internalsref{Staff} context is contained in
165 @internalsref{Score}, @internalsref{StaffGroup}, or
166 @internalsref{ChoirStaff} context.
168 Contexts associated with sheet music output are called @emph{notation
169 contexts}, those for sound output are called @emph{performance
170 contexts}. The default definitions of the standard notation and
171 performance contexts can be found in @file{ly/engraver-init.ly} and
172 @file{ly/performer-init.ly}, respectively.
175 @node Creating contexts
176 @subsection Creating contexts
177 @cindex @code{\context}
178 @cindex context selection
180 Contexts for a music expression can be selected manually, using the
181 following music expression.
184 \context @var{contexttype} [= @var{contextname}] @var{musicexpr}
188 This instructs lilypond to interpret @var{musicexpr} within the context
189 of type @var{contexttype} and with name @var{contextname}. If this
190 context does not exist, it will be created.
192 @lilypond[verbatim,singleline]
194 \notes \relative c'' {
195 c4 <d4 \context Staff = "another" e4> f
201 In this example, the @code{c} and @code{d} are printed on the
202 default staff. For the @code{e}, a context Staff called
203 @code{another} is specified; since that does not exist, a new
204 context is created. Within @code{another}, a (default) Voice context
205 is created for the @code{e4}. When all music referring to a
206 context is finished, the context is ended as well. So after the
207 third quarter, @code{another} is removed.
210 @node Default contexts
211 @subsection Default contexts
213 Most music expressions don't need an explicit @code{\context}
214 declaration: they inherit the notation context from their parent. Each
215 note is a music expression, and as you can see in the following example,
216 only the sequential music enclosing the three notes has an explicit context.
218 @lilypond[verbatim,singleline]
219 \score { \notes \context Voice = goUp { c'4 d' e' } }
222 There are some quirks that you must keep in mind when dealing with
225 First, every top level music is interpreted by the Score context; in other
226 words, you may think of @code{\score} working like
230 \context Score @var{music}
234 Second, contexts are created automatically to be able to interpret the
235 music expressions. Consider the following example.
237 @lilypond[verbatim, singleline]
238 \score { \context Score \notes { c'4-( d' e'-) } }
242 The sequential music is interpreted by the Score context initially
243 (notice that the @code{\context} specification is redundant), but when a
244 note is encountered, contexts are setup to accept that note. In this
245 case, a Thread, Voice, and Staff context are created. The rest of the
246 sequential music is also interpreted with the same Thread, Voice, and
247 Staff context, putting the notes on the same staff, in the same voice.
249 @node Context properties
250 @subsection Context properties
252 Notation contexts have properties. These properties are from
253 the @file{.ly} file using the following expression:
254 @cindex @code{\property}
257 \property @var{contextname}.@var{propname} = @var{value}
261 Sets the @var{propname} property of the context @var{contextname} to the
262 specified Scheme expression @var{value}. All @var{propname} and
263 @var{contextname} are strings, which are typically unquoted.
265 Properties that are set in one context are inherited by all of the
266 contained contexts. This means that a property valid for the
267 @internalsref{Voice} context can be set in the @internalsref{Score} context
268 (for example) and thus take effect in all @internalsref{Voice} contexts.
270 @cindex @code{Current}
271 If you don't wish to specify the name of the context in the
272 @code{\property}-expression itself, you can refer to the abstract context
273 name, @code{Current}. The @code{Current} context is the latest
274 used context. This will typically mean the @internalsref{Thread}
275 context, but you can force another context with the
276 @code{\property}-command. Hence the expressions
279 \property @var{contextname}.@var{propname} = @var{value}
286 \context @var{contextname}
287 \property Current.@var{propname} = @var{value}
292 The main use for this is in macros -- allowing the specification of a
293 property-setting without restriction to a specific context.
295 Properties can be unset using the following expression:
298 \property @var{contextname}.@var{propname} \unset
301 @cindex properties, unsetting
302 @cindex @code{\unset}
305 This removes the definition of @var{propname} in @var{contextname}. If
306 @var{propname} was not defined in @var{contextname} (but was inherited
307 from a higher context), then this has no effect.
311 The syntax of @code{\unset} is asymmetric: @code{\property \unset} is not
312 the inverse of @code{\property \set}.
315 @node Engravers and performers
316 @subsection Engravers and performers
320 Basic building blocks of translation are called engravers; they are
324 @node Changing context definitions
325 @subsection Changing context definitions
326 @cindex context definition
327 @cindex translator definition
329 The most common way to define a context is by extending an existing
330 context. You can change an existing context from the paper block by
331 first initializing a translator with an existing context identifier:
336 @var{context-identifier}
342 Then you can add and remove engravers using the following syntax:
345 \remove @var{engravername}
346 \consists @var{engravername}
350 Here @var{engravername} is a string, the name of an engraver in the
353 @lilypond[verbatim,singleline]
361 \remove Clef_engraver
369 You can also set properties in a translator definition. The syntax is as
373 @var{propname} = @var{value}
374 @var{propname} \set @var{grob-propname} = @var{pvalue}
375 @var{propname} \override @var{grob-propname} = @var{pvalue}
376 @var{propname} \revert @var{grob-propname}
380 @var{propname} is a string, @var{grob-propname} a symbol, @var{value}
381 and @code{pvalue} are Scheme expressions. These types of property
382 assignments happen before interpretation starts, so a @code{\property}
383 command will override any predefined settings.
385 To simplify editing translators, all standard contexts have standard
386 identifiers called @var{name}@code{Context}, e.g. @code{StaffContext},
387 @code{VoiceContext}; see @file{ly/engraver-init.ly}.
390 @node Defining new contexts
391 @subsection Defining new contexts
393 If you want to build a context from scratch, you must also supply the
394 following extra information:
397 @item A name, specified by @code{\name @var{contextname}}.
399 @item A cooperation module. This is specified by @code{\type
407 \type "Engraver_group_engraver"
410 \consists "Staff_symbol_engraver"
411 \consists "Note_head_engraver"
412 \consistsend "Axis_group_engraver"
417 The argument of @code{\type} is the name for a special engraver that
418 handles cooperation between simple engravers such as
419 @code{Note_head_engraver} and @code{Staff_symbol_engraver}. Alternatives
420 for this engraver are the following:
423 @cindex @code{Engraver_group_engraver}
425 @item @code{Engraver_group_engraver}
426 The standard cooperation engraver.
428 @cindex @code{Score_engraver}
429 @item @code{Score_engraver}
430 This is a cooperation module that should be in the top level context.
436 @item @code{\alias} @var{alternate-name}:
437 This specifies a different name. In the above example,
438 @code{\property Staff.X = Y} will also work on @code{SimpleStaff}s
440 @item @code{\consistsend} @var{engravername}:
441 Analogous to @code{\consists}, but makes sure that
442 @var{engravername} is always added to the end of the list of
445 Some engraver types need to be at the end of the list; this
446 insures they stay there even if a user adds or removes engravers.
447 End-users generally don't need this command.
449 @item @code{\accepts} @var{contextname}:
450 Add @var{contextname} to the list of contexts this context can
451 contain in the context hierarchy. The first listed context is the
452 context to create by default.
454 @item @code{\denies}:
455 The opposite of @code{\accepts}. Added for
456 completeness, but is never used in practice.
458 @item @code{\name} @var{contextname}:
459 This sets the type name of the context, e.g. @internalsref{Staff},
460 @internalsref{Voice}. If the name is not specified, the translator won't do
464 In the @code{\paper} block, it is also possible to define translator
465 identifiers. Like other block identifiers, the identifier can only
466 be used as the very first item of a translator. In order to define
467 such an identifier outside of @code{\score}, you must do
471 foo = \translator @{ @dots{} @}
478 \translator @{ \foo @dots{} @}
483 @cindex paper types, engravers, and pre-defined translators
486 @node Syntactic details
487 @section Syntactic details
488 @cindex Syntactic details
490 This section describes details that were too boring to be put elsewhere.
494 * Music expressions::
495 * Manipulating music expressions::
503 @subsection Identifiers
507 What has this section got to do with identifiers?
508 It seems more appropriate in the introduction to Chapter 4,
514 All of the information in a LilyPond input file is internally
515 represented as a Scheme value. In addition to normal Scheme data types
516 (such as pair, number, boolean, etc.), LilyPond has a number of
517 specialized data types,
528 @item Music_output_def
529 @item Moment (rational number)
532 LilyPond also includes some transient object types. Objects of these
533 types are built during a LilyPond run, and do not `exist' per se within
534 your input file. These objects are created as a result of your input
535 file, so you can include commands in the input to manipulate them,
536 during a LilyPond run.
539 @item Grob: short for `Graphical object'.
541 @item Molecule: Device-independent page output object,
542 including dimensions. Produced by some Grob functions.
544 @item Translator: An object that produces audio objects or Grobs. This is
545 not yet user-accessible.
547 @item Font_metric: An object representing a font.
551 @node Music expressions
552 @subsection Music expressions
553 @cindex music expressions
555 Music in LilyPond is entered as a music expression. Notes, rests, lyric
556 syllables are music expressions, and you can combine music expressions
557 to form new ones, for example by enclosing a list of expressions in
558 @code{\sequential @{ @}} or @code{< >}. In the following example, a
559 compound expression is formed out of the quarter note @code{c} and a
560 quarter note @code{d}:
563 \sequential @{ c4 d4 @}
566 @cindex Sequential music
567 @cindex @code{\sequential}
568 @cindex sequential music
571 @cindex Simultaneous music
572 @cindex @code{\simultaneous}
574 The two basic compound music expressions are simultaneous and
578 \sequential @code{@{} @var{musicexprlist} @code{@}}
579 \simultaneous @code{@{} @var{musicexprlist} @code{@}}
582 For both, there is a shorthand:
585 @code{@{} @var{musicexprlist} @code{@}}
592 @code{<} @var{musicexprlist} @code{>}
596 for simultaneous music.
597 In principle, the way in which you nest sequential and simultaneous to
598 produce music is not relevant. In the following example, three chords
599 are expressed in two different ways:
601 @lilypond[fragment,verbatim,center]
602 \notes \context Voice {
603 <a c'> <b d'> <c' e'>
604 < { a b c' } { c' d' e' } >
607 However, using @code{<} and @code{>} for chords turns up various
608 syntactical peculiarities. For this reason, a special syntax for
609 chords was introduced in version 1.7: @code{<< >>}.
615 Other compound music expressions include
618 \transpose @var{from} @var{to} @var{expr}
619 \apply @var{func} @var{expr}
620 \context @var{type} = @var{id} @var{expr}
621 \times @var{fraction} @var{expr}
625 @c . {Manipulating music expressions}
626 @node Manipulating music expressions
627 @subsection Manipulating music expressions
629 The @code{\apply} mechanism gives you access to the internal
630 representation of music. You can write Scheme-functions that operate
631 directly on it. The syntax is
634 \apply #@var{func} @var{music}
638 This means that @var{func} is applied to @var{music}. The function
639 @var{func} should return a music expression.
641 This example replaces the text string of a script. It also shows a dump
642 of the music it processes, which is useful if you want to know more
643 about how music is stored.
645 @lilypond[verbatim,singleline]
646 #(define (testfunc x)
647 (if (equal? (ly:get-mus-property x 'text) "foo")
648 (ly:set-mus-property! x 'text "bar"))
650 (ly:set-mus-property! x 'elements
651 (map testfunc (ly:get-mus-property x 'elements)))
657 \apply #testfunc { c'4_"foo" }
661 For more information on what is possible, see the automatically
662 generated documentation.
664 Directly accessing internal representations is dangerous: The
665 implementation is subject to changes, so you should avoid this feature
668 A final example is a function that reverses a piece of music in time:
670 @lilypond[verbatim,singleline]
671 #(define (reverse-music music)
672 (let* ((elements (ly:get-mus-property music 'elements))
673 (reversed (reverse elements))
674 (span-dir (ly:get-mus-property music 'span-direction)))
675 (ly:set-mus-property! music 'elements reversed)
676 (if (ly:dir? span-dir)
677 (ly:set-mus-property! music 'span-direction (- span-dir)))
678 (map reverse-music reversed)
681 music = \notes { c'4 d'4( e'4 f'4 }
686 \apply #reverse-music \music
691 More examples are given in the distributed example files in
696 @subsection Assignments
699 Identifiers allow objects to be assigned to names during the parse
700 stage. To assign an identifier, use @var{name}@code{=}@var{value}.
701 To refer to an identifier, precede its name with a backslash:
702 `@code{\}@var{name}'. @var{value} is any valid Scheme value or any of
703 the input-types listed above. Identifier assignments can appear at top
704 level in the LilyPond file, but also in @code{\paper} blocks.
706 An identifier can be created with any string for its name, but you will
707 only be able to refer to identifiers whose names begin with a letter,
708 being entirely alphabetical. It is impossible to refer to an identifier
709 whose name is the same as the name of a keyword.
711 The right hand side of an identifier assignment is parsed completely
712 before the assignment is done, so it is allowed to redefine an
713 identifier in terms of its old value, e.g.
719 When an identifier is referenced, the information it points to is
720 copied. For this reason, an identifier reference must always be the
721 first item in a block.
726 \paperIdent % wrong and invalid
732 \paperIdent % correct
740 @subsection Lexical modes
741 @cindex Lexical modes
744 @cindex @code{\notes}
745 @cindex @code{\chords}
746 @cindex @code{\lyrics}
748 To simplify entering notes, lyrics, and chords, LilyPond has three
749 special input modes in addition to the default mode: note, lyrics, and
750 chords mode. These input modes change the way that normal, unquoted
751 words are interpreted: For example, the word @code{cis} may be
752 interpreted as a C-sharp, as a lyric syllable `cis' or as a C-sharp
753 major triad respectively.
755 A mode switch is entered as a compound music expression
758 @code{\notes} @var{musicexpr}
759 @code{\chords} @var{musicexpr}
760 @code{\lyrics} @var{musicexpr}
764 In each of these cases, these expressions do not add anything to the
765 meaning of their arguments. They just instruct the parser in what mode
766 to parse their arguments.
768 Different input modes may be nested.
773 @subsection Ambiguities
777 The grammar contains a number of ambiguities. We hope to resolve them at
788 is interpreted as the string identifier assignment. However,
789 it can also be interpreted as making a string identifier @code{\foo}
790 containing @code{"bar"}, or a music identifier @code{\foo} containing
791 the syllable `bar'. The former interpretation is chosen.
793 @item If you do a nested repeat like
802 then it is ambiguous to which @code{\repeat} the
803 @code{\alternative} belongs. This is the classic if-then-else
804 dilemma. It may be solved by using braces.
808 @c . {Lexical details}
809 @node Lexical details
810 @section Lexical details
812 Even more boring details, now on the lexical side of the input parser.
822 @subsection Direct Scheme
825 @cindex Scheme, in-line code
826 @cindex accessing Scheme
827 @cindex evaluating Scheme
830 LilyPond internally uses GUILE, a Scheme-interpreter. Scheme is a
831 language from the LISP family. You can learn more about Scheme at
832 @uref{http://www.scheme.org}. It is used to represent data throughout
833 the whole program. The hash-sign (@code{#}) accesses GUILE directly: The
834 code following the hash-sign is evaluated as Scheme. The boolean value
835 @var{true} is @code{#t} in Scheme, so for LilyPond @var{true} looks like
838 LilyPond contains a Scheme interpreter (the GUILE library) for
839 internal use. In some places, Scheme expressions also form valid syntax:
840 Wherever it is allowed,
847 evaluates the specified Scheme code. Example:
850 \property Staff.TestObject \override #'foobar = #(+ 1 2)
853 @code{\override} expects two Scheme expressions.
854 The first one is a symbol (@code{foobar}), the second one
855 an integer (namely, 3).
857 In-line Scheme may be used at the top level. In this case the result is
860 Scheme is a full-blown programming language, and a full discussion is
861 outside the scope of this document. Interested readers are referred to
862 the website @uref{http://www.schemers.org/} for more information on
870 Formed from an optional minus sign and a sequence of digits followed
871 by a @emph{required} decimal point and an optional exponent such as
872 @code{-1.2e3}. Reals can be built up using the usual operations:
873 `@code{+}', `@code{-}', `@code{*}', and
874 `@code{/}', with parentheses for grouping.
882 A real constant can be followed by one of the dimension keywords:
883 @code{\mm} @code{\pt}, @code{\in}, or @code{\cm}, for millimeters,
884 points, inches and centimeters, respectively. This converts the number
885 that is the internal representation of that dimension.
893 Begins and ends with the @code{"} character. To include a @code{"}
894 character in a string write @code{\"}. Various other backslash
895 sequences have special interpretations as in the C language. A string
896 that contains no spaces can be written without the quotes. Strings can
897 be concatenated with the @code{+} operator.
900 @c . {Output details}
902 @section Output details
904 LilyPond's default output format is @TeX{}. Using the option @option{-f}
905 (or @option{--format}) other output formats can be selected also, but
906 currently none of them reliably work.
908 At the beginning of the output file, various global parameters are defined.
909 It also contains a large @code{\special} call to define PostScript routines
910 to draw items not representable with @TeX{}, mainly slurs and ties. A DVI
911 driver must be able to understand such embedded PostScript, or the output
912 will be rendered incompletely.
914 Then the file @file{lilyponddefs.tex} is loaded to define the macros used
915 in the code which follows. @file{lilyponddefs.tex} includes various other
916 files, partially depending on the global parameters.
918 Now the music is output system by system (a `system' consists of all
919 staves belonging together). From @TeX{}'s point of view, a system is an
920 @code{\hbox} which contains a lowered @code{\vbox} so that it is centered
921 vertically on the baseline of the text. Between systems,
922 @code{\interscoreline} is inserted vertically to have stretchable space.
923 The horizontal dimension of the @code{\hbox} is given by the
924 @code{linewidth} parameter from LilyPond's @code{\paper} block (using the
925 natural line width if its value is@w{ }@minus{}1).
927 After the last system LilyPond emits a stronger variant of
928 @code{\interscoreline} only if the macro @code{\lilypondpaperlastpagefill}
929 is not defined (flushing the systems to the top of the page). You can
930 avoid that manually by saying
933 \def\lilypondpaperlastpagefill@{1@}
937 or by setting the variable @code{lastpagefill} in LilyPond's @code{\paper}
940 It is possible to fine-tune the vertical offset further by defining the
941 macro @code{\lilypondscoreshift}. Example:
944 \def\lilypondscoreshift@{0.25\baselineskip@}
948 @code{\baselineskip} is the distance from one text line to the next.
950 The code produced by LilyPond can be used by both @TeX{} and La@TeX{}.
952 Here an example how to embed a small LilyPond file @code{foo.ly} into
953 running La@TeX{} text without using the @code{lilypond-book} script
954 (@pxref{Integrating text and music with lilypond-book}).
957 \documentclass@{article@}
959 \def\lilypondpaperlastpagefill@{@}
961 \def\lilypondscoreshift@{0.25\baselineskip@}
964 This is running text which includes an example music file
970 The file @file{foo.tex} has been simply produced with
976 It is important to set the @code{indent} parameter to zero in the
977 @code{\paper} block of @file{foo.ly}.
979 The call to @code{\lineskip} assures that there is enough vertical space
980 between the LilyPond box and the surrounding text lines.