LilyPond Beginnershandleiding

Hoofdstuk 4: Tweaking output 142
standard LilyPond predefined commands, such as \stemUp and \slurDotted. You will see that
these are nothing more than definitions of variables containing one or a group of \override
commands. For example, /tieDotted is defined to be:
tieDotted = {
\override Tie.dash-period = #0.75
\override Tie.dash-fraction = #0.1
}
If you do not like the default values these predefined commands can be redefined easily, just
like any other variable, at the head of your input file.
The following are the most useful files to be found in ‘ly/’:
Filename Contents
‘ly/engraver-init.ly’ Definitions of engraver Contexts
‘ly/paper-defaults-init.ly’ Specifications of paper-related defaults
‘ly/performer-init.ly’ Definitions of performer Contexts
‘ly/property-init.ly’ Definitions of all common predefined commands
‘ly/spanner-init.ly’ Definitions of spanner-related predefined commands
Other settings (such as the definitions of markup commands) are stored as ‘.scm’ (Scheme)
files. The Scheme programming language is used to provide a programmable interface into Lily-
Pond internal operation. Further explanation of these files is currently outside the scope of this
manual, as a knowledge of the Scheme language is required. Users should be warned that a
substantial amount of technical knowledge or time is required to understand Scheme and these
files (see Sectie “Scheme tutorial” in Uitbreidingen).
If you have this knowledge, the Scheme files which may be of interest are:
Filename Contents
‘scm/auto-beam.scm’ Sub-beaming defaults
‘scm/define-grobs.scm’ Default settings for grob properties
‘scm/define-markup-commands.scm’ Specify all markup commands
‘scm/midi.scm’ Default settings for MIDI output
‘scm/output-lib.scm’ Settings that affect appearance of frets, colors, accidentals,
bar lines, etc
‘scm/parser-clef.scm’ Definitions of supported clefs
‘scm/script.scm’ Default settings for articulations
4.6.5 Advanced tweaks with Scheme
Although many things are possible with the \override and \tweak commands, an even more
powerful way of modifying the action of LilyPond is available through a programmable interface
to the LilyPond internal operation. Code written in the Scheme programming language can be
incorporated directly in the internal operation of LilyPond. Of course, at least a basic knowledge
of programming in Scheme is required to do this, and an introduction is provided in the Sectie
“Scheme tutorial” in Uitbreidingen.
As an illustration of one of the many possibilities, instead of setting a property to a constant
it can be set to a Scheme procedure which is then called whenever that property is accessed by
LilyPond. The property can then be set dynamically to a value determined by the procedure at
the time it is called. In this example we color the note head in accordance with its position on
the staff.
#(define (color-notehead grob)
"Color the notehead according to its position on the staff."
(let ((mod-position (modulo (ly:grob-property grob 'staff-position)