Copy Scale: support raising the copied scale to an arbitrary power.

Since scale is multiplicative, the appropriate way to partially copy
it is to use power. However, the influence slider of constraints uses
linear interpolation. Thus, there is no way to correctly split scale
via constraints without adding this feature.

In addition, this allows inverting scale by using negative powers,
fulfilling the function of Copy Rotation's Invert checkboxes.

release/scripts/startup/bl_ui/properties_constraint.py

@@ -430,6 +430,8 @@ class ConstraintButtonsPanel:
         row.prop(con, "use_y", text="Y")
         row.prop(con, "use_z", text="Z")
 
+        layout.prop(con, "power")
+
         row = layout.row()
         row.prop(con, "use_offset")
         row = row.row()

source/blender/blenkernel/intern/constraint.c

@@ -1882,6 +1882,7 @@ static void sizelike_new_data(void *cdata)
   bSizeLikeConstraint *data = (bSizeLikeConstraint *)cdata;
 
   data->flag = SIZELIKE_X | SIZELIKE_Y | SIZELIKE_Z | SIZELIKE_MULTIPLY;
+  data->power = 1.0f;
 }
 
 static void sizelike_id_looper(bConstraint *con, ConstraintIDFunc func, void *userdata)
@@ -1929,6 +1930,10 @@ static void sizelike_evaluate(bConstraint *con, bConstraintOb *cob, ListBase *ta
     mat4_to_size(size, ct->matrix);
     mat4_to_size(obsize, cob->matrix);
 
+    for (int i = 0; i < 3; i++) {
+      size[i] = powf(size[i], data->power);
+    }
+
     if (data->flag & SIZELIKE_OFFSET) {
       /* Scale is a multiplicative quantity, so adding it makes no sense.
        * However, the additive mode has to stay for backward compatibility. */

source/blender/blenloader/intern/versioning_280.c

@@ -702,6 +702,16 @@ static void do_version_constraints_maintain_volume_mode_uniform(ListBase *lb)
   }
 }
 
+static void do_version_constraints_copy_scale_power(ListBase *lb)
+{
+  for (bConstraint *con = lb->first; con; con = con->next) {
+    if (con->type == CONSTRAINT_TYPE_SIZELIKE) {
+      bSizeLikeConstraint *data = (bSizeLikeConstraint *)con->data;
+      data->power = 1.0f;
+    }
+  }
+}
+
 void do_versions_after_linking_280(Main *bmain)
 {
   bool use_collection_compat_28 = true;
@@ -3348,6 +3358,18 @@ void blo_do_versions_280(FileData *fd, Library *UNUSED(lib), Main *bmain)
   }
 
   {
+    /* Added a power option to Copy Scale. */
+    if (!DNA_struct_elem_find(fd->filesdna, "bSizeLikeConstraint", "float", "power")) {
+      LISTBASE_FOREACH (Object *, ob, &bmain->objects) {
+        do_version_constraints_copy_scale_power(&ob->constraints);
+        if (ob->pose) {
+          LISTBASE_FOREACH (bPoseChannel *, pchan, &ob->pose->chanbase) {
+            do_version_constraints_copy_scale_power(&pchan->constraints);
+          }
+        }
+      }
+    }
+
     /* Versioning code until next subversion bump goes here. */
   }
 }

source/blender/makesdna/DNA_constraint_types.h

@@ -289,7 +289,7 @@ typedef struct bLocateLikeConstraint {
 typedef struct bSizeLikeConstraint {
   struct Object *tar;
   int flag;
-  int reserved1;
+  float power;
   /** MAX_ID_NAME-2. */
   char subtarget[64];
 } bSizeLikeConstraint;

source/blender/makesrna/intern/rna_constraint.c

@@ -1327,6 +1327,13 @@ static void rna_def_constraint_size_like(BlenderRNA *brna)
   RNA_def_property_ui_text(prop, "Copy Z", "Copy the target's Z scale");
   RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
 
+  prop = RNA_def_property(srna, "power", PROP_FLOAT, PROP_NONE);
+  RNA_def_property_float_sdna(prop, NULL, "power");
+  RNA_def_property_float_default(prop, 1.0f);
+  RNA_def_property_ui_range(prop, -FLT_MAX, FLT_MAX, 1, 3);
+  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_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");