Add "Gravitation" option to "Force" type force fields

This adds an option to force fields of type "Force", which enables the
simulation of gravitational behavior (dist^-2 falloff).

Patch by @AndreasE

Reviewers: #physics, LucaRood, mont29

Reviewed By: #physics, LucaRood, mont29

Tags: #physics

Differential Revision: https://developer.blender.org/D2389

4 files changed, 17 insertions, 1 deletions

diff --git a/release/scripts/startup/bl_ui/properties_physics_common.py b/release/scripts/startup/bl_ui/properties_physics_common.py
index 277b59d187d..4478c6a4379 100644
--- a/release/scripts/startup/bl_ui/properties_physics_common.py
+++ b/release/scripts/startup/bl_ui/properties_physics_common.py
@@ -274,6 +274,8 @@ def basic_force_field_settings_ui(self, context, field):
         col.prop(field, "use_global_coords", text="Global")
     elif field.type == 'HARMONIC':
         col.prop(field, "use_multiple_springs")
+    if field.type == 'FORCE':
+        col.prop(field, "use_gravity_falloff",  text="Gravitation")
 
     split = layout.split()
 
diff --git a/source/blender/blenkernel/intern/effect.c b/source/blender/blenkernel/intern/effect.c
index fe8f5ebdca6..4eee24b378f 100644
--- a/source/blender/blenkernel/intern/effect.c
+++ b/source/blender/blenkernel/intern/effect.c
@@ -848,6 +848,14 @@ static void do_physical_effector(EffectorCache *eff, EffectorData *efd, Effected
 			break;
 		case PFIELD_FORCE:
 			normalize_v3(force);
+			if (pd->flag & PFIELD_GRAVITATION){ /* Option: Multiply by 1/distance^2 */
+				if (efd->distance < FLT_EPSILON){
+					strength = 0.0f;
+				}
+				else {
+					strength *= powf(efd->distance, -2.0f);
+				}
+			}
 			mul_v3_fl(force, strength * efd->falloff);
 			break;
 		case PFIELD_VORTEX:
diff --git a/source/blender/makesdna/DNA_object_force.h b/source/blender/makesdna/DNA_object_force.h
index 59acefeffe4..ed14c4b9311 100644
--- a/source/blender/makesdna/DNA_object_force.h
+++ b/source/blender/makesdna/DNA_object_force.h
@@ -372,6 +372,7 @@ typedef struct SoftBody {
 #define PFIELD_DO_ROTATION		(1<<15)
 #define PFIELD_GUIDE_PATH_WEIGHT (1<<16)	/* apply curve weights */
 #define PFIELD_SMOKE_DENSITY    (1<<17)		/* multiply smoke force by density */
+#define PFIELD_GRAVITATION		(1<<18)             /* used for (simple) force */
 
 /* pd->falloff */
 #define PFIELD_FALL_SPHERE		0
diff --git a/source/blender/makesrna/intern/rna_object_force.c b/source/blender/makesrna/intern/rna_object_force.c
index 1d89f7535c4..514fca1b011 100644
--- a/source/blender/makesrna/intern/rna_object_force.c
+++ b/source/blender/makesrna/intern/rna_object_force.c
@@ -1275,7 +1275,7 @@ static void rna_def_field(BlenderRNA *brna)
 	prop = RNA_def_property(srna, "falloff_power", PROP_FLOAT, PROP_NONE);
 	RNA_def_property_float_sdna(prop, NULL, "f_power");
 	RNA_def_property_range(prop, 0.0f, 10.0f);
-	RNA_def_property_ui_text(prop, "Falloff Power", "Falloff power (real gravitational falloff = 2)");
+	RNA_def_property_ui_text(prop, "Falloff Power", "");
 	RNA_def_property_update(prop, 0, "rna_FieldSettings_update");
 	
 	prop = RNA_def_property(srna, "distance_min", PROP_FLOAT, PROP_NONE);
@@ -1394,6 +1394,11 @@ static void rna_def_field(BlenderRNA *brna)
 	RNA_def_property_boolean_sdna(prop, NULL, "flag", PFIELD_SMOKE_DENSITY);
 	RNA_def_property_ui_text(prop, "Apply Density", "Adjust force strength based on smoke density");
 	RNA_def_property_update(prop, 0, "rna_FieldSettings_update");
+	prop = RNA_def_property(srna, "use_gravity_falloff", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "flag", PFIELD_GRAVITATION);
+	RNA_def_property_ui_text(prop, "Gravity Falloff", "Multiply force by 1/distance²");
+	RNA_def_property_update(prop, 0, "rna_FieldSettings_update");
+
 	
 	/* Pointer */