Drivers: add an Average Scale option to the Transform Channel driver vars.

Unlike location and rotation, there is a meaningful definition of
overall/average scaling via the total change in the volume. This
adds an option to retrieve that via a single driver variable,
instead of having to use three and an expression.

Using the determinant to compute the volume scaling also allows
detecting flipping due to negative scale - this is impossible
to do via the three variable approach.

The volume_scale functions are added purely for code readability:
'volume scale factor' is easier to understand than determinant.

Differential Revision: https://developer.blender.org/D4803

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/fcurve.c b/source/blender/blenkernel/intern/fcurve.c
index 8c95e4c7ff3..e2d0a479792 100644
--- a/source/blender/blenkernel/intern/fcurve.c
+++ b/source/blender/blenkernel/intern/fcurve.c
@@ -1707,6 +1707,11 @@ static float dvar_eval_transChan(ChannelDriver *driver, DriverVar *dvar)
     /* not valid channel */
     return 0.0f;
   }
+  else if (dtar->transChan == DTAR_TRANSCHAN_SCALE_AVG) {
+    /* Cubic root of the change in volume, equal to the geometric mean
+     * of scale over all three axes unless the matrix includes shear. */
+    return cbrtf(mat4_to_volume_scale(mat));
+  }
   else if (dtar->transChan >= DTAR_TRANSCHAN_SCALEX) {
     /* Extract scale, and choose the right axis,
      * inline 'mat4_to_size'. */