* Sound output:: Sound output
* midilist:: midilist
* Grobs:: Graphical objects
+* Molecule:: Molecules
* Pre-defined Identifiers:: Pre-defined Identifiers
@c May be fragile. Better make single link to generated doco?
* Interpretation contexts:(lilypond-internals)LilyPond interpretation contexts.
+@unnumberedsubsec Comments
+
@cindex comment
@indexcode{%}
Block comments are started by `@code{%@{}' and ended by `@code{%@}}'.
They cannot be nested.
+@unnumberedsubsec Scheme
+
@indexcode{#}
LilyPond contains a Scheme interpreter (the GUILE library) for
[refer appendix/ online intro on Scheme]
+@unnumberedsubsec Keywords
+
@cindex keyword
Keywords start with a backslash, followed by a number of lower case
transpose type
@end example
+@unnumberedsubsec Integers
+
@cindex integer
Formed from an optional minus sign followed by digits. Arithmetic
operations cannot be done with integers, and integers cannot be mixed
with reals.
+@unnumberedsubsec Reals
+
@cindex real
is the internal representation of dimensions.
-
+@unnumberedsubsec
@cindex string
-
Begins and ends with the `@code{"}' character. To include a `@code{"}'
character in a string write `@code{\"}'. Various other backslash
sequences have special interpretations as in the C language. A string
depending on the situation. Strings can be concatenated with the
`@code{+}' operator.
-
The tokenizer accepts the following commands. They have no grammatical
function, hence they can appear anywhere in the input.
This command is used in init files to signal that the user file must
be read. This command cannot be used in a user file.
+@unnumberedsubsec file inclusion
+
@example
\include@keyindex{include} @var{file}
@end example
unquoted string will not work here!) or a string identifier. The full
filename including the @file{.ly} extension must be given,
+@unnumberedsubsec Version information
+
@example
\version@keyindex{version} @var{string} ;
@end example
@end table
@node Grobs, , , Reference Manual
+@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::
+@end menu
+
+@node What is a grob?, , , Grobs
+
+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 @code{Line_of_score}
+@ref{(lilypond-internals)Element 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 NoteCollision @ref{(lilypond-internals)Element
+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)Elements}
+
+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{Molecule}) is extracted from
+the network. These are then dumped into the output file
+
+@node Callbacks, , , Grobs
+
+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, , , Grobs
+
+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.
+
+@node Items and Spanners, , , Grobs
+
+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 invisible.
+@node Pointer substitution, , , Grobs
+
+
+@node Molecule, , , Reference Manual
+
+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.}. 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
+(refer to the C++ code for more details). All visible,
+ie. non-transparent, 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.
projects / lilypond.git / blobdiff