Action Constraint: introduce a mix mode setting.

Currently the action channels are applied after the existing
transformation, as if the action controlled a child of the
bone. This is not very natural, but more importantly, the
transform tools are not designed to work conveniently with an
additional 'pseudo-child' transformation, resulting in effects
like an unexpected pivot location.

Implementing a Before mode that integrates the action channels
as if applied to a parent allows using the special transform
tool code intended for dealing with such constraints.

Note that in either mode, Action constraints should be added
in reverse order, putting a new constraint before the existing
ones that the Action was keyframed to work together.

In order to implement the option, extract a utility from
the Copy Transform constraint code for combining transforms
with special anti-shear scale handling that matches the
Aligned Inherit Scale mode.

The Before mode also requires switching the constraint to
the Local owner space, while the After mode can still use the
World space for efficiency as before. Since the constraint
doesn't have an Owner space option in the UI, this has to be
handled in an RNA setter.

For full backward compatibility, the original simple matrix
multiplication mode is preserved as the third option, but it
is not recommended due to creating shear.

Differential Revision: https://developer.blender.org/D6297

1 files changed, 21 insertions, 0 deletions

diff --git a/source/blender/blenlib/intern/math_matrix.c b/source/blender/blenlib/intern/math_matrix.c
index 5118d8a698e..4d8a2f72eca 100644
--- a/source/blender/blenlib/intern/math_matrix.c
+++ b/source/blender/blenlib/intern/math_matrix.c
@@ -1235,6 +1235,27 @@ bool invert_m4_m4(float inverse[4][4], const float mat[4][4])
 #endif
 }
 
+/**
+ * Combines transformations, handling scale separately in a manner equivalent
+ * to the Aligned Inherit Scale mode, in order to avoid creating shear.
+ * If A scale is uniform, the result is equivalent to ordinary multiplication.
+ */
+void mul_m4_m4m4_aligned_scale(float R[4][4], const float A[4][4], const float B[4][4])
+{
+  float loc_a[3], rot_a[3][3], size_a[3];
+  float loc_b[3], rot_b[3][3], size_b[3];
+  float loc_r[3], rot_r[3][3], size_r[3];
+
+  mat4_to_loc_rot_size(loc_a, rot_a, size_a, A);
+  mat4_to_loc_rot_size(loc_b, rot_b, size_b, B);
+
+  mul_v3_m4v3(loc_r, A, loc_b);
+  mul_m3_m3m3_uniq(rot_r, rot_a, rot_b);
+  mul_v3_v3v3(size_r, size_a, size_b);
+
+  loc_rot_size_to_mat4(R, loc_r, rot_r, size_r);
+}
+
 /****************************** Linear Algebra *******************************/
 
 void transpose_m3(float mat[3][3])