Fix T81904: Cloth brush simulation failing with local area and mirror

When using local area, all nodes need to build their constraints first
before activating them for simulation. THis ensures that nodes get their
structural constraints from the initial location of each symmetry pass.

Reviewed By: sergey

Maniphest Tasks: T81904

Differential Revision: https://developer.blender.org/D9303

1 files changed, 31 insertions, 6 deletions

diff --git a/source/blender/editors/sculpt_paint/sculpt_cloth.c b/source/blender/editors/sculpt_paint/sculpt_cloth.c
index 1a1200bb6c2..67129d5da50 100644
--- a/source/blender/editors/sculpt_paint/sculpt_cloth.c
+++ b/source/blender/editors/sculpt_paint/sculpt_cloth.c
@@ -1127,14 +1127,27 @@ void SCULPT_cloth_sim_activate_nodes(SculptClothSimulation *cloth_sim,
   }
 }
 
+static void sculpt_cloth_ensure_constraints_in_simulation_area(Sculpt *sd,
+                                                               Object *ob,
+                                                               PBVHNode **nodes,
+                                                               int totnode)
+{
+  SculptSession *ss = ob->sculpt;
+  Brush *brush = BKE_paint_brush(&sd->paint);
+  const float radius = ss->cache->initial_radius;
+  const float limit = radius + (radius * brush->cloth_sim_limit);
+  float sim_location[3];
+  cloth_brush_simulation_location_get(ss, brush, sim_location);
+  SCULPT_cloth_brush_ensure_nodes_constraints(
+      sd, ob, nodes, totnode, ss->cache->cloth_sim, sim_location, limit);
+}
+
 /* Main Brush Function. */
 void SCULPT_do_cloth_brush(Sculpt *sd, Object *ob, PBVHNode **nodes, int totnode)
 {
   SculptSession *ss = ob->sculpt;
   Brush *brush = BKE_paint_brush(&sd->paint);
 
-  /* In the first brush step of each symmetry pass, build the constraints for the vertices in all
-   * nodes inside the simulation's limits. */
   /* Brushes that use anchored strokes and restore the mesh can't rely on symmetry passes and steps
    * count as it is always the first step, so the simulation needs to be created when it does not
    * exist for this stroke. */
@@ -1151,14 +1164,26 @@ void SCULPT_do_cloth_brush(Sculpt *sd, Object *ob, PBVHNode **nodes, int totnode
           SCULPT_is_cloth_deform_brush(brush));
       SCULPT_cloth_brush_simulation_init(ss, ss->cache->cloth_sim);
     }
+
+    if (brush->cloth_simulation_area_type == BRUSH_CLOTH_SIMULATION_AREA_LOCAL) {
+      /* When using simulation a fixed local simulation area, constraints are created only using
+       * the initial stroke position and initial radius (per symmetry pass) instead of per node.
+       * This allows to skip unnecessary constraints that will never be simulated, making the
+       * solver faster. When the simulation starts for a node, the node gets activated and all its
+       * constraints are considered final. As the same node can be included inside the brush radius
+       * from multiple symmetry passes, the cloth brush can't activate the node for simulation yet
+       * as this will cause the ensure constraints function to skip the node in the next symmetry
+       * passes. It needs to build the constraints here and skip simulating the first step, so all
+       * passes can add their constraints to all affected nodes. */
+      sculpt_cloth_ensure_constraints_in_simulation_area(sd, ob, nodes, totnode);
+    }
+    /* The first step of a symmetry pass is never simulated as deformation modes need valid delta
+     * for brush tip alignement. */
     return;
   }
 
   /* Ensure the constraints for the nodes. */
-  const float radius = ss->cache->initial_radius;
-  const float limit = radius + (radius * brush->cloth_sim_limit);
-  SCULPT_cloth_brush_ensure_nodes_constraints(
-      sd, ob, nodes, totnode, ss->cache->cloth_sim, ss->cache->location, limit);
+  sculpt_cloth_ensure_constraints_in_simulation_area(sd, ob, nodes, totnode);
 
   /* Store the initial state in the simulation. */
   SCULPT_cloth_brush_store_simulation_state(ss, ss->cache->cloth_sim);