From 4798c1cadda5c6dfb57e5d170bc4aab8b765e560 Mon Sep 17 00:00:00 2001
From: Aaron Ruiz Mora <moraaar@amazon.com>
Date: Mon, 17 May 2021 12:28:08 +0100
Subject: [PATCH] Fix cloth receiving weird impulses when actors are spawned

---
 .../ClothComponentMesh/ClothComponentMesh.cpp | 22 ++++++++++---------
 .../ClothComponentMesh/ClothComponentMesh.h   |  4 ++--
 2 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.cpp b/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.cpp
index d88fcb65ce..0914e26cb7 100644
--- a/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.cpp
+++ b/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.cpp
@@ -36,6 +36,9 @@ namespace NvCloth
     AZ_CVAR(float, cloth_DistanceToTeleport, 0.5f, nullptr, AZ::ConsoleFunctorFlags::Null,
         "The amount of meters the entity has to move in a frame to consider it a teleport for cloth.");
 
+    AZ_CVAR(float, cloth_SecondsToDelaySimulationOnActorSpawned, 0.25f, nullptr, AZ::ConsoleFunctorFlags::Null,
+        "The amount of time in seconds the cloth simulation will be delayed to avoid sudden impulses when actors are spawned.");
+
     // Helper class to map an RPI buffer from a buffer asset view.
     template<typename T>
     class MappedBuffer
@@ -190,7 +193,7 @@ namespace NvCloth
             m_meshClothInfo.m_particles,
             m_cloth->GetParticles().size(),
             m_meshRemappedVertices);
-        m_numberOfClothSkinningUpdates = 0;
+        m_timeClothSkinningUpdates = 0.0f;
 
         m_clothConstraints = ClothConstraints::Create(
             m_meshClothInfo.m_motionConstraints,
@@ -248,7 +251,7 @@ namespace NvCloth
 
     void ClothComponentMesh::OnPreSimulation(
         [[maybe_unused]] ClothId clothId,
-        [[maybe_unused]] float deltaTime)
+        float deltaTime)
     {
         AZ_PROFILE_FUNCTION(AZ::Debug::ProfileCategory::Cloth);
 
@@ -256,7 +259,7 @@ namespace NvCloth
 
         if (m_actorClothSkinning)
         {
-            UpdateSimulationSkinning();
+            UpdateSimulationSkinning(deltaTime);
 
             UpdateSimulationConstraints();
         }
@@ -338,7 +341,7 @@ namespace NvCloth
         }
     }
 
-    void ClothComponentMesh::UpdateSimulationSkinning()
+    void ClothComponentMesh::UpdateSimulationSkinning(float deltaTime)
     {
         if (m_actorClothSkinning)
         {
@@ -349,22 +352,21 @@ namespace NvCloth
             // Since component activation order is not trivial, the actor's pose might not be updated
             // immediately. Because of this cloth will receive a sudden impulse when changing from
             // T pose to animated pose. To avoid this undesired effect we will override cloth simulation during
-            // a short amount of frames.
-            const AZ::u32 numberOfTicksToDoFullSkinning = 10;
-            m_numberOfClothSkinningUpdates++;
+            // a short amount of time.
+            m_timeClothSkinningUpdates += deltaTime;
 
             // While the actor is not visible the skinned joints are not updated. Then when
             // it becomes visible the jump to the new skinned positions causes a sudden
             // impulse to cloth simulation. To avoid this undesired effect we will override cloth simulation during
-            // a short amount of frames.
+            // a short amount of time.
             m_actorClothSkinning->UpdateActorVisibility();
             if (!m_actorClothSkinning->WasActorVisible() &&
                 m_actorClothSkinning->IsActorVisible())
             {
-                m_numberOfClothSkinningUpdates = 0;
+                m_timeClothSkinningUpdates = 0.0f;
             }
 
-            if (m_numberOfClothSkinningUpdates <= numberOfTicksToDoFullSkinning)
+            if (m_timeClothSkinningUpdates <= cloth_SecondsToDelaySimulationOnActorSpawned)
             {
                 // Update skinning for all particles and apply it to cloth 
                 AZStd::vector<SimParticleFormat> particles = m_cloth->GetParticles();
diff --git a/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.h b/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.h
index a1fb6d5572..f8772d2e93 100644
--- a/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.h
+++ b/Gems/NvCloth/Code/Source/Components/ClothComponentMesh/ClothComponentMesh.h
@@ -85,7 +85,7 @@ namespace NvCloth
 
     private:
         void UpdateSimulationCollisions();
-        void UpdateSimulationSkinning();
+        void UpdateSimulationSkinning(float deltaTime);
         void UpdateSimulationConstraints();
         void UpdateRenderData(const AZStd::vector<SimParticleFormat>& particles);
 
@@ -133,7 +133,7 @@ namespace NvCloth
 
         // Cloth Skinning from the character
         AZStd::unique_ptr<ActorClothSkinning> m_actorClothSkinning;
-        AZ::u32 m_numberOfClothSkinningUpdates = 0;
+        float m_timeClothSkinningUpdates = 0.0f;
 
         // Cloth Constraints
         AZStd::unique_ptr<ClothConstraints> m_clothConstraints;