From 830db22dcc306618f87768326d0c8df8d98c48b5 Mon Sep 17 00:00:00 2001
From: Chandler Cox <mr.chandlercox@gmail.com>
Date: Fri, 9 Aug 2024 09:05:53 -0500
Subject: [PATCH] Blend out of anim slots when stopped.

---
 .../Animations/Graph/AnimGroup.Animation.cpp  | 65 ++++++++++++++++---
 Source/Engine/Level/Actors/AnimatedModel.cpp  |  2 +-
 2 files changed, 58 insertions(+), 9 deletions(-)

diff --git a/Source/Engine/Animations/Graph/AnimGroup.Animation.cpp b/Source/Engine/Animations/Graph/AnimGroup.Animation.cpp
index 7538e06b5..6763f6d5d 100644
--- a/Source/Engine/Animations/Graph/AnimGroup.Animation.cpp
+++ b/Source/Engine/Animations/Graph/AnimGroup.Animation.cpp
@@ -2237,8 +2237,9 @@ void AnimGraphExecutor::ProcessGroupAnimation(Box* boxBase, Node* nodeBase, Valu
                 {
                     // Start playing animation
                     bucket.Index = i;
-                    bucket.TimePosition = 0.0f;
-                    bucket.BlendInPosition = 0.0f;
+                    // Keep bucket time position and blend in time for if blending between two anims in the same slot.
+                    bucket.TimePosition = bucket.TimePosition;
+                    bucket.BlendInPosition = bucket.BlendInPosition;
                     bucket.BlendOutPosition = 0.0f;
                     bucket.LoopsDone = 0;
                     bucket.LoopsLeft = slot.LoopCount;
@@ -2248,6 +2249,12 @@ void AnimGraphExecutor::ProcessGroupAnimation(Box* boxBase, Node* nodeBase, Valu
             if (bucket.Index == -1 || !slots[bucket.Index].Animation->IsLoaded())
             {
                 value = tryGetValue(node->GetBox(1), Value::Null);
+                // Reset times if time is left over from playing between different anims in the same slot.
+                if (bucket.BlendInPosition > 0)
+                {
+                    bucket.TimePosition = 0;
+                    bucket.BlendInPosition = 0;
+                }
                 return;
             }
         }
@@ -2256,12 +2263,6 @@ void AnimGraphExecutor::ProcessGroupAnimation(Box* boxBase, Node* nodeBase, Valu
         auto& slot = slots[bucket.Index];
         Animation* anim = slot.Animation;
         ASSERT(slot.Animation && slot.Animation->IsLoaded());
-        if (slot.Reset)
-        {
-            // Start from the begining
-            slot.Reset = false;
-            bucket.TimePosition = 0.0f;
-        }
         const float deltaTime = slot.Pause ? 0.0f : context.DeltaTime * slot.Speed;
         const float length = anim->GetLength();
         const bool loop = bucket.LoopsLeft != 0;
@@ -2285,6 +2286,54 @@ void AnimGraphExecutor::ProcessGroupAnimation(Box* boxBase, Node* nodeBase, Valu
         // Speed is accounted for in the new time pos, so keep sample speed at 1
         value = SampleAnimation(node, loop, length, 0.0f, bucket.TimePosition, newTimePos, anim, 1);
         bucket.TimePosition = newTimePos;
+
+        // On animation slot stop
+        if (slot.Reset)
+        {
+            // Blend between last anim and new anim if found, otherwise blend back to input.
+            Animation* sAnim = nullptr;
+            for (int32 i = 0; i < slots.Count(); i++)
+            {
+                auto& s = slots[i];
+                if (s.Animation && s.Name == slotName && s.Animation != slot.Animation)
+                {
+                    sAnim = s.Animation;
+                }
+            }
+            float oldTimePos = bucket.BlendOutPosition;
+            bucket.BlendOutPosition += deltaTime;
+            bucket.BlendInPosition = bucket.BlendOutPosition;
+            const float alpha = bucket.BlendOutPosition / slot.BlendOutTime;
+            if (sAnim != nullptr)
+            {
+                auto sValue = SampleAnimation(node, false, sAnim->GetLength(), 0.0f, oldTimePos, bucket.BlendInPosition, sAnim, 1);
+                //value = SampleAnimationsWithBlend(node, false, length, 0.0f, bucket.TimePosition, newTimePos, anim, sAnim, 1, 1, alpha);
+                value = Blend(node, value, sValue, alpha, AlphaBlendMode::HermiteCubic);
+            }
+            else
+            {
+                auto input = tryGetValue(node->GetBox(1), Value::Null);
+                value = Blend(node, value, input, alpha, AlphaBlendMode::HermiteCubic);
+            }
+
+            if (bucket.BlendOutPosition >= slot.BlendOutTime)
+            {
+                // Start from the beginning or the blend in position if next anim found.
+                slot.Animation = nullptr;
+                slot.Reset = false;
+                if (!sAnim)
+                {
+                    bucket.TimePosition = 0;
+                    bucket.BlendInPosition = 0;
+                }
+                else
+                {
+                    bucket.TimePosition = bucket.BlendInPosition;
+                }
+            }
+            break;
+        }
+        
         if (bucket.LoopsLeft == 0 && slot.BlendOutTime > 0.0f && length - slot.BlendOutTime < bucket.TimePosition)
         {
             // Blend out
diff --git a/Source/Engine/Level/Actors/AnimatedModel.cpp b/Source/Engine/Level/Actors/AnimatedModel.cpp
index 2c0e8474d..0eab292c5 100644
--- a/Source/Engine/Level/Actors/AnimatedModel.cpp
+++ b/Source/Engine/Level/Actors/AnimatedModel.cpp
@@ -506,7 +506,7 @@ void AnimatedModel::StopSlotAnimation(const StringView& slotName, Animation* ani
     {
         if (slot.Animation == anim && slot.Name == slotName)
         {
-            slot.Animation = nullptr;
+            //slot.Animation = nullptr;
             slot.Reset = true;
             break;
         }