Integrated limits for volume preservation in stretch-to constraints.

Currently the volume variation feature in stretch constraints is
unlimited. This has to be compensated by riggers by adding scale limit
constraints, but these are unaware of the stretch orientation and can
lead to flipping. Also the stretch calculation itself is not working
properly and can lead to collapsing volume.

The patch fixes this with several modifications:
- Interpret the volume variation factor as exponent, which works better
  with large values for artistic purposes.
- Add integrated limits to the volume "bulge" factor, so secondary
  constraints for compensation become unnecessary
- Add a smoothness factor to make limits less visible.

Eventually a generic volume preservation constraint would be nicer,
because multiple constraints currently implement volume variation of
their own. This feature could actually work very nicely independent from
other constraint features.

Differential Revision: https://developer.blender.org/D826

1 files changed, 25 insertions, 0 deletions

diff --git a/source/blender/makesrna/intern/rna_constraint.c b/source/blender/makesrna/intern/rna_constraint.c
index 5519b192ca4..aab2d35c0e4 100644
--- a/source/blender/makesrna/intern/rna_constraint.c
+++ b/source/blender/makesrna/intern/rna_constraint.c
@@ -1365,6 +1365,31 @@ static void rna_def_constraint_stretch_to(BlenderRNA *brna)
 	RNA_def_property_range(prop, 0.0, 100.f);
 	RNA_def_property_ui_text(prop, "Volume Variation", "Factor between volume variation and stretching");
 	RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
+
+	prop = RNA_def_property(srna, "use_bulge_min", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag", STRETCHTOCON_USE_BULGE_MIN);
+	RNA_def_property_ui_text(prop, "Use Volume Variation Minimum", "Use lower limit for volume variation");
+	RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
+
+	prop = RNA_def_property(srna, "use_bulge_max", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag", STRETCHTOCON_USE_BULGE_MAX);
+	RNA_def_property_ui_text(prop, "Use Volume Variation Maximum", "Use upper limit for volume variation");
+	RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
+
+	prop = RNA_def_property(srna, "bulge_min", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_range(prop, 0.0, 100.f);
+	RNA_def_property_ui_text(prop, "Volume Variation Minimum", "Minimum volume stretching factor");
+	RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
+
+	prop = RNA_def_property(srna, "bulge_max", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_range(prop, 0.0, 100.f);
+	RNA_def_property_ui_text(prop, "Volume Variation Maximum", "Maximum volume stretching factor");
+	RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
+
+	prop = RNA_def_property(srna, "bulge_smooth", PROP_FLOAT, PROP_FACTOR);
+	RNA_def_property_range(prop, 0.0, 1.0f);
+	RNA_def_property_ui_text(prop, "Volume Variation Smoothness", "");
+	RNA_def_property_update(prop, NC_OBJECT | ND_CONSTRAINT, "rna_Constraint_update");
 }
 
 static void rna_def_constraint_rigid_body_joint(BlenderRNA *brna)