Issue 3458: Document \alterBroken

diff --git a/Documentation/notation/changing-defaults.itely b/Documentation/notation/changing-defaults.itely
index f91cb0d2235eea67e4c46ebf5b97c45ff074b9e1..6174456582fabd04769dee2d03b51e98e9a7febf 100644 (file)
--- a/Documentation/notation/changing-defaults.itely
+++ b/Documentation/notation/changing-defaults.itely
@@ -3663,6 +3663,7 @@ appearance of the printed score.
 * Vertical grouping of grobs::
 * Modifying stencils::
 * Modifying shapes::
+* Modifying broken spanners::
 * Unpure-pure containers::
 @end menu
 
@@ -4316,6 +4317,112 @@ Internals Reference:
 @rinternals{TieColumn}.
 
 
+@node Modifying broken spanners
+@subsection Modifying broken spanners
+
+@menu
+* Using alterBroken::
+@end menu
+
+@node Using alterBroken
+@unnumberedsubsubsec Using @code{\alterBroken}
+
+@cindex spanners, modifying
+@cindex broken spanners, modifying
+@funindex \alterBroken
+
+When a spanner crosses a line break or breaks, each piece
+inherits the attributes of the original spanner.  Thus, ordinary
+tweaking of a broken spanner applies the same modifications to
+each of its segments.  In the example below, overriding
+@code{thickness} affects the slur on either side of the line
+break.
+
+@lilypond[verbatim,quote,ragged-right,relative=2]
+r2
+\once\override Slur.thickness = 10
+c8( d e f
+\break
+g8 f e d) r2
+@end lilypond
+
+Independently modifying the appearance of individual pieces
+of a broken spanner is possible with the @code{\alterBroken}
+command.  This command can produce either an @code{\override}
+or a @code{\tweak} of a spanner property.
+
+The syntax for @code{\alterBroken} is
+
+@example
+[-]@code{\alterBroken} @var{property} @var{values} @var{item}
+@end example
+
+The argument @var{values} is a list of values, one for each
+broken piece.  If @var{item} is a grob name like @code{Slur} or
+@code{Staff.PianoPedalBracket}, the result is an @code{\override}
+of the specified grob type.  If @var{item} is a music expression
+such as @qq{(} or @qq{[} the result is the same music expression
+with an appropriate tweak applied.
+
+The leading hyphen must be used with the @code{\tweak} form.  Do
+not add it when @code{\alterBroken} is used as an
+@code{\override}.
+
+In its @code{\override} usage, @code{\alterBroken} may be
+prefaced by @code{\once} or @code{\temporary} and reverted by
+using @code{\revert} with @var{property}.
+
+The following code applies an independent @code{\override} to
+each of the slur segments in the previous example:
+
+@lilypond[verbatim,quote,ragged-right,relative=2]
+r2
+\alterBroken thickness #'(10 1) Slur
+c8( d e f
+\break
+g8 f e d) r2
+@end lilypond
+
+The @code{\alterBroken} command may be used with any spanner
+object, including @code{Tie}, @code{PhrasingSlur}, @code{Beam}
+and @code{TextSpanner}.  For example, an editor preparing a
+scholarly edition may wish to indicate the absence of part of a
+phrasing slur in a source by dashing only the segment which has
+been added.  The following example illustrates how this can be
+done, in this case using the @code{\tweak} form of the command:
+
+@lilypond[verbatim,quote,ragged-right,relative=2]
+% The empty list is conveniently used below, because it is the
+% default setting of dash-definition, resulting in a solid curve.
+c2-\alterBroken dash-definition #'(() ((0 1.0 0.4 0.75))) \(e
+\break
+g2 e\)
+@end lilypond
+
+It is important to understand that @code{\alterBroken} will set
+each piece of a broken spanner to the corresponding value in
+@var{values}.  When there are fewer values than pieces, any
+additional piece will be assigned the empty list.  This may lead
+to undesired results if the layout property is not set to the
+empty list by default.  In such cases, each segment should be
+assigned an appropriate value.
+
+@knownissues
+Line breaks may occur in different places following changes in
+layout. Settings chosen for @code{\alterBroken} may be unsuitable
+for a spanner that is no longer broken or is split into more
+segments than before.  Explicit use of @code{\break} can guard
+against this situation.
+ 
+The @code{\alterBroken} command is ineffective for spanner
+properties accessed before line-breaking such as
+@code{direction}.
+
+@seealso
+Extending LilyPond:
+@rextend{Difficult tweaks}.
+
+
 @node Unpure-pure containers
 @subsection Unpure-pure containers