Add exact inversion

4 files changed, 43 insertions, 0 deletions

diff --git a/Documentation/notation/pitches.itely b/Documentation/notation/pitches.itely
index 78a222794d..4ee236cfda 100644
--- a/Documentation/notation/pitches.itely
+++ b/Documentation/notation/pitches.itely
@@ -594,6 +594,7 @@ This section discusses how to modify pitches.
 @menu
 * Octave checks::
 * Transpose::
+* Inversion::
 * Modal transformations::
 @end menu
 
@@ -813,6 +814,22 @@ The relative conversion will not affect @code{\transpose},
 To use relative mode within transposed music, an additional
 @code{\relative} must be placed inside @code{\transpose}.
 
+@node Inversion
+@unnumberedsubsubsec Inversion
+
+@cindex inversion
+
+A music expression can be inverted with @code{\inversion}.
+
+@example
+\inversion @var{frompitch} @var{topitch} @var{musicexpr}
+@end example
+
+@noindent
+This means that @code{@var{musicexpr}} is inverted
+interval-by-interval, and transposition is chosen so that
+@code{@var{frompitch}} is mapped to @code{@var{topitch}}.
+
 @node Modal transformations
 @unnumberedsubsubsec Modal transformations
 
diff --git a/input/regression/modal-transforms.ly b/input/regression/modal-transforms.ly
index 9070601510..7487f2b8bf 100644
--- a/input/regression/modal-transforms.ly
+++ b/input/regression/modal-transforms.ly
@@ -22,12 +22,14 @@ motif = {
         \modalTranspose c' f' \cOctatonicScale \motif
         \retrograde \motif
         \modalInversion aes' b' \cOctatonicScale \motif
+	\inversion aes' b' \motif
       }
       {
         s1-"Octatonic motif" |
         s1-"motif transposed from c to f" |
         s1-"motif in retrograde" |
         s1-"motif inverted around aes to b" |
+	s1-"motif inverted exactly"
       }
     >>
   }
diff --git a/ly/music-functions-init.ly b/ly/music-functions-init.ly
index 34261c114b..ac62bffa9a 100644
--- a/ly/music-functions-init.ly
+++ b/ly/music-functions-init.ly
@@ -480,6 +480,13 @@ using @var{scale}.")
       (change-pitches music transposer)
       music))
 
+inversion =
+#(define-music-function
+   (parser location around to music) (ly:music? ly:music? ly:music?)
+   (_i "Invert @var{music} about @var{around} and
+transpose from @var{around} to @var{to}.")
+   (music-invert around to music))
+
 musicMap =
 #(define-music-function (parser location proc mus) (procedure? ly:music?)
    (_i "Apply @var{proc} to @var{mus} and all of the music it contains.")
diff --git a/scm/modal-transforms.scm b/scm/modal-transforms.scm
index cdaa015ccc..151fb8c3be 100644
--- a/scm/modal-transforms.scm
+++ b/scm/modal-transforms.scm
@@ -220,3 +220,20 @@ Typically used to construct a scale for input to transposer-factory
     (map retrograde-music reversed)
 
     music))
+
+(define-public (pitch-invert around to music)
+  "If @var{music} is a single pitch, inverts it about @var{around}
+and transposes from @var{around} to @var{to}."
+  (let ((p (ly:music-property music 'pitch)))
+    (if (ly:pitch? p)
+	(ly:music-set-property!
+	 music 'pitch
+	 (ly:pitch-transpose to (ly:pitch-diff around p))))
+    music))
+
+(define-public (music-invert around-pitch to-pitch music)
+  "Applies pitch-invert to all pitches in @var{music}."
+  (let ((around (car (extract-pitch-sequence around-pitch)))
+	(to (car (extract-pitch-sequence to-pitch))))
+     (music-map (lambda (x) (pitch-invert around to x)) music)))
+