Copy Scale: support copying arbitrary non-uniform scale as uniform.

Uniform scale is superior to non-uniform scale in that it works with
parenting without causing shear. Thus it is a valid desire in some
cases to turn arbitrary scale into guaranteed uniform scale.

Implementing this in the Copy Scale constraint allows one for instance
to 'inherit scale as uniform' by disabling Inherit Scale, and using
Copy Scale from parent with Offset and Make Uniform.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5614

4 files changed, 44 insertions, 4 deletions

diff --git a/release/scripts/startup/bl_ui/properties_constraint.py b/release/scripts/startup/bl_ui/properties_constraint.py
index 6fa953574cb..d8eb0db62fc 100644
--- a/release/scripts/startup/bl_ui/properties_constraint.py
+++ b/release/scripts/startup/bl_ui/properties_constraint.py
@@ -430,6 +430,7 @@ class ConstraintButtonsPanel:
         row.prop(con, "use_z", text="Z")
 
         layout.prop(con, "power")
+        layout.prop(con, "use_make_uniform")
 
         row = layout.row()
         row.prop(con, "use_offset")
diff --git a/source/blender/blenkernel/intern/constraint.c b/source/blender/blenkernel/intern/constraint.c
index e1e4d138fd9..1a3c562941e 100644
--- a/source/blender/blenkernel/intern/constraint.c
+++ b/source/blender/blenkernel/intern/constraint.c
@@ -1927,9 +1927,39 @@ static void sizelike_evaluate(bConstraint *con, bConstraintOb *cob, ListBase *ta
   if (VALID_CONS_TARGET(ct)) {
     float obsize[3], size[3];
 
-    mat4_to_size(size, ct->matrix);
     mat4_to_size(obsize, cob->matrix);
 
+    /* Compute one uniform scale factor to apply to all three axes. */
+    if (data->flag & SIZELIKE_UNIFORM) {
+      const int all_axes = SIZELIKE_X | SIZELIKE_Y | SIZELIKE_Z;
+      float total = 1.0f;
+
+      /* If all axes are selected, use the determinant. */
+      if ((data->flag & all_axes) == all_axes) {
+        total = fabsf(mat4_to_volume_scale(ct->matrix));
+      }
+      /* Otherwise multiply individual values. */
+      else {
+        mat4_to_size(size, ct->matrix);
+
+        if (data->flag & SIZELIKE_X) {
+          total *= size[0];
+        }
+        if (data->flag & SIZELIKE_Y) {
+          total *= size[1];
+        }
+        if (data->flag & SIZELIKE_Z) {
+          total *= size[2];
+        }
+      }
+
+      copy_v3_fl(size, cbrt(total));
+    }
+    /* Regular per-axis scaling. */
+    else {
+      mat4_to_size(size, ct->matrix);
+    }
+
     for (int i = 0; i < 3; i++) {
       size[i] = powf(size[i], data->power);
     }
@@ -1948,13 +1978,13 @@ static void sizelike_evaluate(bConstraint *con, bConstraintOb *cob, ListBase *ta
       }
     }
 
-    if ((data->flag & SIZELIKE_X) && (obsize[0] != 0)) {
+    if ((data->flag & (SIZELIKE_X | SIZELIKE_UNIFORM)) && (obsize[0] != 0)) {
       mul_v3_fl(cob->matrix[0], size[0] / obsize[0]);
     }
-    if ((data->flag & SIZELIKE_Y) && (obsize[1] != 0)) {
+    if ((data->flag & (SIZELIKE_Y | SIZELIKE_UNIFORM)) && (obsize[1] != 0)) {
       mul_v3_fl(cob->matrix[1], size[1] / obsize[1]);
     }
-    if ((data->flag & SIZELIKE_Z) && (obsize[2] != 0)) {
+    if ((data->flag & (SIZELIKE_Z | SIZELIKE_UNIFORM)) && (obsize[2] != 0)) {
       mul_v3_fl(cob->matrix[2], size[2] / obsize[2]);
     }
   }
diff --git a/source/blender/makesdna/DNA_constraint_types.h b/source/blender/makesdna/DNA_constraint_types.h
index eda52fdf14a..17ce279f1b7 100644
--- a/source/blender/makesdna/DNA_constraint_types.h
+++ b/source/blender/makesdna/DNA_constraint_types.h
@@ -748,6 +748,7 @@ typedef enum eCopyScale_Flags {
   SIZELIKE_Z = (1 << 2),
   SIZELIKE_OFFSET = (1 << 3),
   SIZELIKE_MULTIPLY = (1 << 4),
+  SIZELIKE_UNIFORM = (1 << 5),
 } eCopyScale_Flags;
 
 /* bTransformConstraint.to/from */
diff --git a/source/blender/makesrna/intern/rna_constraint.c b/source/blender/makesrna/intern/rna_constraint.c
index 08d2a03b2a9..539e2a66296 100644
--- a/source/blender/makesrna/intern/rna_constraint.c
+++ b/source/blender/makesrna/intern/rna_constraint.c
@@ -1364,6 +1364,14 @@ static void rna_def_constraint_size_like(BlenderRNA *brna)
   RNA_def_property_ui_text(prop, "Power", "Raise the target's scale to the specified power");
   RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
 
+  prop = RNA_def_property(srna, "use_make_uniform", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flag", SIZELIKE_UNIFORM);
+  RNA_def_property_ui_text(prop,
+                           "Make Uniform",
+                           "Redistribute the copied change in volume equally "
+                           "between the three axes of the owner");
+  RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
+
   prop = RNA_def_property(srna, "use_offset", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, NULL, "flag", SIZELIKE_OFFSET);
   RNA_def_property_ui_text(prop, "Offset", "Combine original scale with copied scale");