Drivers: support decomposing rotation into swing followed by twist.

In order to correctly drive corrective shape keys from a freely
rotating organic joint it is very often found necessary to
decompose the rotation into separate bending and twisting
motions. This type of decomposition cannot be reproduced by
any Euler order or a single quaternion.

Instead this is done by using a helper bone with a Damped Track
constraint aimed at the tail of the control to pick up the bending,
and its helper child with Copy Transforms to separate the twist.

Requiring two additional bones to drive a shape key or a correction
bone seems inconvenient, so this implements the necessary math as new
options in the recently introduced Rotation Mode dropdown of the
Transform Channel driver variable type. The data is also accessible
as a Transformation constraint input.

The output is in the form of Quaternion-derived 'pseudo-angles',
which for `Swing and Y Twist` would represent the following:

* W: true bend angle, independent of bend direction.
* Y: true twist angle.
* X, Z: pseudo-angles representing the proportion of bending around X/Z.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5651

1 files changed, 15 insertions, 0 deletions

diff --git a/source/blender/makesrna/intern/rna_fcurve.c b/source/blender/makesrna/intern/rna_fcurve.c
index 4f8e63e975f..254f3bc3710 100644
--- a/source/blender/makesrna/intern/rna_fcurve.c
+++ b/source/blender/makesrna/intern/rna_fcurve.c
@@ -141,6 +141,21 @@ const EnumPropertyItem rna_enum_driver_target_rotation_mode_items[] = {
     {DTAR_ROTMODE_EULER_ZXY, "ZXY", 0, "ZXY Euler", "Euler using the ZXY rotation order"},
     {DTAR_ROTMODE_EULER_ZYX, "ZYX", 0, "ZYX Euler", "Euler using the ZYX rotation order"},
     {DTAR_ROTMODE_QUATERNION, "QUATERNION", 0, "Quaternion", "Quaternion rotation"},
+    {DTAR_ROTMODE_SWING_TWIST_X,
+     "SWING_TWIST_X",
+     0,
+     "Swing and X Twist",
+     "Decompose into a swing rotation to aim the X axis, followed by twist around it"},
+    {DTAR_ROTMODE_SWING_TWIST_Y,
+     "SWING_TWIST_Y",
+     0,
+     "Swing and Y Twist",
+     "Decompose into a swing rotation to aim the Y axis, followed by twist around it"},
+    {DTAR_ROTMODE_SWING_TWIST_Z,
+     "SWING_TWIST_Z",
+     0,
+     "Swing and Z Twist",
+     "Decompose into a swing rotation to aim the Z axis, followed by twist around it"},
     {0, NULL, 0, NULL, NULL},
 };