From 59f9a5e6ac6f5bff914204ef299cba27bb5016eb Mon Sep 17 00:00:00 2001
From: Robert Sheldon <rsheldiii>
Date: Thu, 15 Jul 2021 15:58:34 +0200
Subject: Fix T88188: Allow keyframing vertex mass in cloth sim

Update vertex weights between simulation steps if they have changed.
This allows for animated vertex weights in the cloth sim.

Reviewed By: Sebastian Parborg

Differential Revision: http://developer.blender.org/D11640
---
 source/blender/blenkernel/intern/cloth.c             | 9 ++++++++-
 source/blender/makesrna/intern/rna_cloth.c           | 1 -
 source/blender/simulation/SIM_mass_spring.h          | 2 ++
 source/blender/simulation/intern/SIM_mass_spring.cpp | 6 +++++-
 4 files changed, 15 insertions(+), 3 deletions(-)

(limited to 'source/blender')

diff --git a/source/blender/blenkernel/intern/cloth.c b/source/blender/blenkernel/intern/cloth.c
index 8678a659c0a..0fa58a74f2b 100644
--- a/source/blender/blenkernel/intern/cloth.c
+++ b/source/blender/blenkernel/intern/cloth.c
@@ -262,17 +262,19 @@ static bool do_init_cloth(Object *ob, ClothModifierData *clmd, Mesh *result, int
 static int do_step_cloth(
     Depsgraph *depsgraph, Object *ob, ClothModifierData *clmd, Mesh *result, int framenr)
 {
+  /* simulate 1 frame forward */
   ClothVertex *verts = NULL;
   Cloth *cloth;
   ListBase *effectors = NULL;
   MVert *mvert;
   unsigned int i = 0;
   int ret = 0;
+  bool vert_mass_changed = false;
 
-  /* simulate 1 frame forward */
   cloth = clmd->clothObject;
   verts = cloth->verts;
   mvert = result->mvert;
+  vert_mass_changed = verts->mass != clmd->sim_parms->mass;
 
   /* force any pinned verts to their constrained location. */
   for (i = 0; i < clmd->clothObject->mvert_num; i++, verts++) {
@@ -283,6 +285,11 @@ static int do_step_cloth(
     /* Get the current position. */
     copy_v3_v3(verts->xconst, mvert[i].co);
     mul_m4_v3(ob->obmat, verts->xconst);
+
+    if (vert_mass_changed) {
+      verts->mass = clmd->sim_parms->mass;
+      SIM_mass_spring_set_implicit_vertex_mass(cloth->implicit, i, verts->mass);
+    }
   }
 
   effectors = BKE_effectors_create(depsgraph, ob, NULL, clmd->sim_parms->effector_weights, false);
diff --git a/source/blender/makesrna/intern/rna_cloth.c b/source/blender/makesrna/intern/rna_cloth.c
index 9e57368f8f9..2bc00dd5af5 100644
--- a/source/blender/makesrna/intern/rna_cloth.c
+++ b/source/blender/makesrna/intern/rna_cloth.c
@@ -652,7 +652,6 @@ static void rna_def_cloth_sim_settings(BlenderRNA *brna)
   prop = RNA_def_property(srna, "mass", PROP_FLOAT, PROP_UNIT_MASS);
   RNA_def_property_range(prop, 0.0f, FLT_MAX);
   RNA_def_property_ui_text(prop, "Vertex Mass", "The mass of each vertex on the cloth material");
-  RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
   RNA_def_property_update(prop, 0, "rna_cloth_update");
 
   prop = RNA_def_property(srna, "vertex_group_mass", PROP_STRING, PROP_NONE);
diff --git a/source/blender/simulation/SIM_mass_spring.h b/source/blender/simulation/SIM_mass_spring.h
index 43de8b155cf..b3299258209 100644
--- a/source/blender/simulation/SIM_mass_spring.h
+++ b/source/blender/simulation/SIM_mass_spring.h
@@ -45,6 +45,8 @@ void SIM_mass_spring_solver_free(struct Implicit_Data *id);
 int SIM_mass_spring_solver_numvert(struct Implicit_Data *id);
 
 int SIM_cloth_solver_init(struct Object *ob, struct ClothModifierData *clmd);
+void SIM_mass_spring_set_implicit_vertex_mass(struct Implicit_Data *data, int index, float mass);
+
 void SIM_cloth_solver_free(struct ClothModifierData *clmd);
 int SIM_cloth_solve(struct Depsgraph *depsgraph,
                     struct Object *ob,
diff --git a/source/blender/simulation/intern/SIM_mass_spring.cpp b/source/blender/simulation/intern/SIM_mass_spring.cpp
index cf654ebff07..ca01120eecb 100644
--- a/source/blender/simulation/intern/SIM_mass_spring.cpp
+++ b/source/blender/simulation/intern/SIM_mass_spring.cpp
@@ -203,7 +203,7 @@ int SIM_cloth_solver_init(Object *UNUSED(ob), ClothModifierData *clmd)
   cloth->implicit = id = SIM_mass_spring_solver_create(cloth->mvert_num, nondiag);
 
   for (i = 0; i < cloth->mvert_num; i++) {
-    SIM_mass_spring_set_vertex_mass(id, i, verts[i].mass);
+    SIM_mass_spring_set_implicit_vertex_mass(id, i, verts[i].mass);
   }
 
   for (i = 0; i < cloth->mvert_num; i++) {
@@ -213,6 +213,10 @@ int SIM_cloth_solver_init(Object *UNUSED(ob), ClothModifierData *clmd)
   return 1;
 }
 
+void SIM_mass_spring_set_implicit_vertex_mass(Implicit_Data *data, int index, float mass){
+  SIM_mass_spring_set_vertex_mass(data, index, mass);
+}
+
 void SIM_cloth_solver_free(ClothModifierData *clmd)
 {
   Cloth *cloth = clmd->clothObject;
-- 
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