Sculpt: Use plasticity for softbody influence

Previously the softbody strength property was controlling the strength
of the constraints that pin all vertices to the original location. This
was causing problems when the forces were trying to deform the vertices
too much, like when using gravity or grab brushes.

Now softbody is implemented with plasticity, which creates constraints to
a separate coordinates array. These coordinates are deformed with the
simulation, and the plasticity parameter controls how much the
simulation moves the coordinates (plasticity 0), or the coordinates move
the simulation back to its previous position (plasticity 1).

This creates much better and predictable results and adding softbody
plasticity to the brushes can increase its control and the stability of
the simulation.

Reviewed By: sergey, zeddb

Differential Revision: https://developer.blender.org/D9187

3 files changed, 73 insertions, 19 deletions

diff --git a/source/blender/editors/sculpt_paint/sculpt.c b/source/blender/editors/sculpt_paint/sculpt.c
index 39b675de3c0..c4c9db69dcd 100644
--- a/source/blender/editors/sculpt_paint/sculpt.c
+++ b/source/blender/editors/sculpt_paint/sculpt.c
@@ -5747,7 +5747,8 @@ static void do_brush_action(Sculpt *sd, Object *ob, Brush *brush, UnifiedPaintSe
 
     if (brush->deform_target == BRUSH_DEFORM_TARGET_CLOTH_SIM) {
       if (!ss->cache->cloth_sim) {
-        ss->cache->cloth_sim = SCULPT_cloth_brush_simulation_create(ss, 1.0f, 0.0f, false, true);
+        ss->cache->cloth_sim = SCULPT_cloth_brush_simulation_create(
+            ss, 1.0f, 0.0f, 0.0f, false, true);
         SCULPT_cloth_brush_simulation_init(ss, ss->cache->cloth_sim);
       }
       SCULPT_cloth_brush_store_simulation_state(ss, ss->cache->cloth_sim);
diff --git a/source/blender/editors/sculpt_paint/sculpt_cloth.c b/source/blender/editors/sculpt_paint/sculpt_cloth.c
index 2bca5e2e1f0..84d156122f3 100644
--- a/source/blender/editors/sculpt_paint/sculpt_cloth.c
+++ b/source/blender/editors/sculpt_paint/sculpt_cloth.c
@@ -229,7 +229,7 @@ static void cloth_brush_add_softbody_constraint(SculptClothSimulation *cloth_sim
   length_constraint->node = node_index;
 
   length_constraint->elem_position_a = cloth_sim->pos[v];
-  length_constraint->elem_position_b = cloth_sim->init_pos[v];
+  length_constraint->elem_position_b = cloth_sim->softbody_pos[v];
 
   length_constraint->type = SCULPT_CLOTH_CONSTRAINT_SOFTBODY;
 
@@ -242,6 +242,33 @@ static void cloth_brush_add_softbody_constraint(SculptClothSimulation *cloth_sim
   cloth_brush_reallocate_constraints(cloth_sim);
 }
 
+static void cloth_brush_add_pin_constraint(SculptClothSimulation *cloth_sim,
+                                           const int node_index,
+                                           const int v,
+                                           const float strength)
+{
+  SculptClothLengthConstraint *length_constraint =
+      &cloth_sim->length_constraints[cloth_sim->tot_length_constraints];
+
+  length_constraint->elem_index_a = v;
+  length_constraint->elem_index_b = v;
+
+  length_constraint->node = node_index;
+
+  length_constraint->elem_position_a = cloth_sim->pos[v];
+  length_constraint->elem_position_b = cloth_sim->init_pos[v];
+
+  length_constraint->type = SCULPT_CLOTH_CONSTRAINT_PIN;
+
+  length_constraint->length = 0.0f;
+  length_constraint->strength = strength;
+
+  cloth_sim->tot_length_constraints++;
+
+  /* Reallocation if the array capacity is exceeded. */
+  cloth_brush_reallocate_constraints(cloth_sim);
+}
+
 static void cloth_brush_add_deformation_constraint(SculptClothSimulation *cloth_sim,
                                                    const int node_index,
                                                    const int v,
@@ -323,9 +350,8 @@ static void do_cloth_brush_build_constraints_task_cb_ex(
       }
       SCULPT_VERTEX_NEIGHBORS_ITER_END(ni);
 
-      if (brush->cloth_constraint_softbody_strength > 0.0f) {
-        cloth_brush_add_softbody_constraint(
-            data->cloth_sim, node_index, vd.index, brush->cloth_constraint_softbody_strength);
+      if (data->cloth_sim->softbody_strength > 0.0f) {
+        cloth_brush_add_softbody_constraint(data->cloth_sim, node_index, vd.index, 1.0f);
       }
 
       /* As we don't know the order of the neighbor vertices, we create all possible combinations
@@ -379,8 +405,7 @@ static void do_cloth_brush_build_constraints_task_cb_ex(
       if (sim_falloff < 1.0f) {
         /* Create constraints with more strength the closer the vertex is to the simulation
          * boundary. */
-        cloth_brush_add_softbody_constraint(
-            data->cloth_sim, node_index, vd.index, 1.0f - sim_falloff);
+        cloth_brush_add_pin_constraint(data->cloth_sim, node_index, vd.index, 1.0f - sim_falloff);
       }
     }
   }
@@ -809,14 +834,26 @@ static void cloth_brush_satisfy_constraints(SculptSession *ss,
                                0.5f;
       }
 
-      madd_v3_v3fl(cloth_sim->pos[v1],
-                   correction_vector_half,
-                   1.0f * mask_v1 * sim_factor_v1 * constraint->strength * deformation_strength);
-      if (v1 != v2) {
-        madd_v3_v3fl(cloth_sim->pos[v2],
+      if (constraint->type == SCULPT_CLOTH_CONSTRAINT_SOFTBODY) {
+        const float softbody_plasticity = brush ? brush->cloth_constraint_softbody_strength : 0.0f;
+        madd_v3_v3fl(cloth_sim->pos[v1],
+                     correction_vector_half,
+                     1.0f * mask_v1 * sim_factor_v1 * constraint->strength * softbody_plasticity);
+        madd_v3_v3fl(cloth_sim->softbody_pos[v1],
                      correction_vector_half,
-                     -1.0f * mask_v2 * sim_factor_v2 * constraint->strength *
-                         deformation_strength);
+                     -1.0f * mask_v1 * sim_factor_v1 * constraint->strength *
+                         (1.0f - softbody_plasticity));
+      }
+      else {
+        madd_v3_v3fl(cloth_sim->pos[v1],
+                     correction_vector_half,
+                     1.0f * mask_v1 * sim_factor_v1 * constraint->strength * deformation_strength);
+        if (v1 != v2) {
+          madd_v3_v3fl(cloth_sim->pos[v2],
+                       correction_vector_half,
+                       -1.0f * mask_v2 * sim_factor_v2 * constraint->strength *
+                           deformation_strength);
+        }
       }
     }
   }
@@ -954,6 +991,7 @@ static void cloth_sim_initialize_default_node_state(SculptSession *ss,
 SculptClothSimulation *SCULPT_cloth_brush_simulation_create(SculptSession *ss,
                                                             const float cloth_mass,
                                                             const float cloth_damping,
+                                                            const float cloth_softbody_strength,
                                                             const bool use_collisions,
                                                             const bool needs_deform_coords)
 {
@@ -984,8 +1022,14 @@ SculptClothSimulation *SCULPT_cloth_brush_simulation_create(SculptSession *ss,
         totverts, sizeof(float), "cloth sim deformation strength");
   }
 
+  if (cloth_softbody_strength > 0.0f) {
+    cloth_sim->softbody_pos = MEM_calloc_arrayN(
+        totverts, sizeof(float[3]), "cloth sim softbody pos");
+  }
+
   cloth_sim->mass = cloth_mass;
   cloth_sim->damping = cloth_damping;
+  cloth_sim->softbody_strength = cloth_softbody_strength;
 
   if (use_collisions) {
     cloth_sim->collider_list = cloth_brush_collider_cache_create(ss->depsgraph);
@@ -1037,6 +1081,7 @@ void SCULPT_cloth_brush_simulation_init(SculptSession *ss, SculptClothSimulation
 {
   const int totverts = SCULPT_vertex_count_get(ss);
   const bool has_deformation_pos = cloth_sim->deformation_pos != NULL;
+  const bool has_softbody_pos = cloth_sim->softbody_pos != NULL;
   for (int i = 0; i < totverts; i++) {
     copy_v3_v3(cloth_sim->last_iteration_pos[i], SCULPT_vertex_co_get(ss, i));
     copy_v3_v3(cloth_sim->init_pos[i], SCULPT_vertex_co_get(ss, i));
@@ -1045,6 +1090,9 @@ void SCULPT_cloth_brush_simulation_init(SculptSession *ss, SculptClothSimulation
       copy_v3_v3(cloth_sim->deformation_pos[i], SCULPT_vertex_co_get(ss, i));
       cloth_sim->deformation_strength[i] = 1.0f;
     }
+    if (has_softbody_pos) {
+      copy_v3_v3(cloth_sim->softbody_pos[i], SCULPT_vertex_co_get(ss, i));
+    }
   }
 }
 
@@ -1086,6 +1134,7 @@ void SCULPT_do_cloth_brush(Sculpt *sd, Object *ob, PBVHNode **nodes, int totnode
           ss,
           brush->cloth_mass,
           brush->cloth_damping,
+          brush->cloth_constraint_softbody_strength,
           (brush->flag2 & BRUSH_CLOTH_USE_COLLISION),
           SCULPT_is_cloth_deform_brush(brush));
       SCULPT_cloth_brush_simulation_init(ss, ss->cache->cloth_sim);
@@ -1121,6 +1170,7 @@ void SCULPT_cloth_simulation_free(struct SculptClothSimulation *cloth_sim)
   MEM_SAFE_FREE(cloth_sim->length_constraints);
   MEM_SAFE_FREE(cloth_sim->length_constraint_tweak);
   MEM_SAFE_FREE(cloth_sim->deformation_pos);
+  MEM_SAFE_FREE(cloth_sim->softbody_pos);
   MEM_SAFE_FREE(cloth_sim->init_pos);
   MEM_SAFE_FREE(cloth_sim->deformation_strength);
   MEM_SAFE_FREE(cloth_sim->node_state);
@@ -1455,6 +1505,7 @@ static int sculpt_cloth_filter_invoke(bContext *C, wmOperator *op, const wmEvent
       ss,
       cloth_mass,
       cloth_damping,
+      0.0f,
       use_collisions,
       cloth_filter_is_deformation_filter(filter_type));
 
diff --git a/source/blender/editors/sculpt_paint/sculpt_intern.h b/source/blender/editors/sculpt_paint/sculpt_intern.h
index 84a47ab86ba..a690e426770 100644
--- a/source/blender/editors/sculpt_paint/sculpt_intern.h
+++ b/source/blender/editors/sculpt_paint/sculpt_intern.h
@@ -365,11 +365,13 @@ void SCULPT_do_cloth_brush(struct Sculpt *sd,
 
 void SCULPT_cloth_simulation_free(struct SculptClothSimulation *cloth_sim);
 
-struct SculptClothSimulation *SCULPT_cloth_brush_simulation_create(struct SculptSession *ss,
-                                                                   const float cloth_mass,
-                                                                   const float cloth_damping,
-                                                                   const bool use_collisions,
-                                                                   const bool needs_deform_coords);
+struct SculptClothSimulation *SCULPT_cloth_brush_simulation_create(
+    struct SculptSession *ss,
+    const float cloth_mass,
+    const float cloth_damping,
+    const float cloth_softbody_strength,
+    const bool use_collisions,
+    const bool needs_deform_coords);
 void SCULPT_cloth_brush_simulation_init(struct SculptSession *ss,
                                         struct SculptClothSimulation *cloth_sim);