Copy Transforms: implement Remove Target Shear and more Mix options.

This constraint can be naturally viewed as a prototype for a future
4x4 matrix math node (or subset thereof), since its basic semantics
already is matrix assignment. Thus it makes sense to add math options
to this constraint to increase flexibility in the meantime.

This patch adds support for several operations that would be useful:

- An option to remove shear in the incoming target matrix.

  Shear is known to cause issues for various mathematical operations,
  so an option to remove it at key points is useful.

  Constraints based on Euler like Copy Rotation and Limit Rotation
  already have always enabled shear removal built in, because their
  math doesn't work correctly with shear.

  In the future node system shear removal would be a separate node
  (and currently Limit Rotation can be used as a Remove Shear constraint).
  However removing shear from the result of the target space conversion
  before mixing (similar to Copy Rotation) has to be built into
  Copy Transforms itself as an option.

- More ways to combine the target and owner matrices.

  Similar to multiple Inherit Scale modes for parenting, there are
  multiple ways one may want to combine matrices based on context.
  This implements 3 variants for each of the Before/After modes
  (one of them already existing).

  - Full implements regular matrix multiplication as the most basic
    option. The downside is the risk of creating shear.
  - Aligned emulates the 'anti-shear' Aligned Inherit Scale mode,
    and basically uses Full for location, and Split for rotation/scale.
    (This choice already existed.)
  - Split Channels combines location, rotation and scale separately.

  Looking at D7547 there is demand for Split Channels in some cases,
  so I think it makes sense to include it in Copy Transforms too, so that
  the Mix menu items can be identical for it and the Action constraint.

Differential Revision: https://developer.blender.org/D9469

2 files changed, 23 insertions, 0 deletions

diff --git a/source/blender/blenlib/BLI_math_matrix.h b/source/blender/blenlib/BLI_math_matrix.h
index 54df88ca541..0c3a184d302 100644
--- a/source/blender/blenlib/BLI_math_matrix.h
+++ b/source/blender/blenlib/BLI_math_matrix.h
@@ -211,6 +211,7 @@ void mul_transposed_mat3_m4_v3(const float M[4][4], float r[3]);
 void mul_m3_v3_double(const float M[3][3], double r[3]);
 
 void mul_m4_m4m4_aligned_scale(float R[4][4], const float A[4][4], const float B[4][4]);
+void mul_m4_m4m4_split_channels(float R[4][4], const float A[4][4], const float B[4][4]);
 
 void mul_m3_fl(float R[3][3], float f);
 void mul_m4_fl(float R[4][4], float f);
diff --git a/source/blender/blenlib/intern/math_matrix.c b/source/blender/blenlib/intern/math_matrix.c
index 5eb0125062d..88bef854213 100644
--- a/source/blender/blenlib/intern/math_matrix.c
+++ b/source/blender/blenlib/intern/math_matrix.c
@@ -1290,6 +1290,9 @@ bool invert_m4_m4(float inverse[4][4], const float mat[4][4])
  * 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.
+ *
+ * Note: this effectively takes output location from simple multiplication,
+ *       and uses mul_m4_m4m4_split_channels for rotation and scale.
  */
 void mul_m4_m4m4_aligned_scale(float R[4][4], const float A[4][4], const float B[4][4])
 {
@@ -1307,6 +1310,25 @@ void mul_m4_m4m4_aligned_scale(float R[4][4], const float A[4][4], const float B
   loc_rot_size_to_mat4(R, loc_r, rot_r, size_r);
 }
 
+/**
+ * Separately combines location, rotation and scale of the input matrices.
+ */
+void mul_m4_m4m4_split_channels(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);
+
+  add_v3_v3v3(loc_r, loc_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 R[3][3])