From 449fc597b5bfdc6e273c2225bed12cb9b796fe31 Mon Sep 17 00:00:00 2001
From: Wojtek Figat <wojtek@figat.pl>
Date: Mon, 2 Feb 2026 19:21:45 +0100
Subject: [PATCH] Fix crash when using overlapping instances

#3899
---
 Source/Engine/Foliage/Foliage.cpp        | 42 ++++++++++++++++++++----
 Source/Engine/Foliage/FoliageCluster.cpp |  1 +
 Source/Engine/Foliage/FoliageCluster.h   |  5 +++
 3 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/Source/Engine/Foliage/Foliage.cpp b/Source/Engine/Foliage/Foliage.cpp
index ddc3468f7..f8b9c7b0f 100644
--- a/Source/Engine/Foliage/Foliage.cpp
+++ b/Source/Engine/Foliage/Foliage.cpp
@@ -44,20 +44,39 @@ void Foliage::AddToCluster(ChunkedArray<FoliageCluster, FOLIAGE_CLUSTER_CHUNKS_S
     ASSERT(instance.Bounds.Radius > ZeroTolerance);
     ASSERT(cluster->Bounds.Intersects(instance.Bounds));
 
-    // Find target cluster
-    while (cluster->Children[0])
+    // Minor clusters don't use bounds intersection but try to find the first free cluster instead
+    if (cluster->IsMinor)
     {
+        // Insert into the first non-full child cluster or subdivide 1st child
+#define CHECK_CHILD(idx) \
+        if (cluster->Children[idx]->Instances.Count() < FOLIAGE_CLUSTER_CAPACITY) \
+        { \
+           cluster->Children[idx]->Instances.Add(&instance); \
+           return; \
+        }
+        CHECK_CHILD(3);
+        CHECK_CHILD(2);
+        CHECK_CHILD(1);
+        cluster = cluster->Children[0];
+#undef CHECK_CHILD
+    }
+    else
+    {
+        // Find target cluster
+        while (cluster->Children[0])
+        {
 #define CHECK_CHILD(idx) \
 			if (cluster->Children[idx]->Bounds.Intersects(instance.Bounds)) \
 			{ \
 				cluster = cluster->Children[idx]; \
 				continue; \
 			}
-        CHECK_CHILD(0);
-        CHECK_CHILD(1);
-        CHECK_CHILD(2);
-        CHECK_CHILD(3);
+            CHECK_CHILD(0);
+            CHECK_CHILD(1);
+            CHECK_CHILD(2);
+            CHECK_CHILD(3);
 #undef CHECK_CHILD
+        }
     }
 
     // Check if it's not full
@@ -79,11 +98,20 @@ void Foliage::AddToCluster(ChunkedArray<FoliageCluster, FOLIAGE_CLUSTER_CHUNKS_S
         // Setup children
         const Vector3 min = cluster->Bounds.Minimum;
         const Vector3 max = cluster->Bounds.Maximum;
-        const Vector3 size = cluster->Bounds.GetSize();
+        const Vector3 size = max - min;
         cluster->Children[0]->Init(BoundingBox(min, min + size * Vector3(0.5f, 1.0f, 0.5f)));
         cluster->Children[1]->Init(BoundingBox(min + size * Vector3(0.5f, 0.0f, 0.5f), max));
         cluster->Children[2]->Init(BoundingBox(min + size * Vector3(0.5f, 0.0f, 0.0f), min + size * Vector3(1.0f, 1.0f, 0.5f)));
         cluster->Children[3]->Init(BoundingBox(min + size * Vector3(0.0f, 0.0f, 0.5f), min + size * Vector3(0.5f, 1.0f, 1.0f)));
+        if (cluster->IsMinor || size.MinValue() < 1.0f)
+        {
+            // Mark children as minor to avoid infinite subdivision
+            cluster->IsMinor = true;
+            cluster->Children[0]->IsMinor = true;
+            cluster->Children[1]->IsMinor = true;
+            cluster->Children[2]->IsMinor = true;
+            cluster->Children[3]->IsMinor = true;
+        }
 
         // Move instances to a proper cells
         for (int32 i = 0; i < cluster->Instances.Count(); i++)
diff --git a/Source/Engine/Foliage/FoliageCluster.cpp b/Source/Engine/Foliage/FoliageCluster.cpp
index 1f76e5086..fd4c0f753 100644
--- a/Source/Engine/Foliage/FoliageCluster.cpp
+++ b/Source/Engine/Foliage/FoliageCluster.cpp
@@ -9,6 +9,7 @@ void FoliageCluster::Init(const BoundingBox& bounds)
     Bounds = bounds;
     TotalBounds = bounds;
     MaxCullDistance = 0.0f;
+    IsMinor = false;
 
     Children[0] = nullptr;
     Children[1] = nullptr;
diff --git a/Source/Engine/Foliage/FoliageCluster.h b/Source/Engine/Foliage/FoliageCluster.h
index 55cbeb027..c55305c5d 100644
--- a/Source/Engine/Foliage/FoliageCluster.h
+++ b/Source/Engine/Foliage/FoliageCluster.h
@@ -33,6 +33,11 @@ public:
     /// </summary>
     float MaxCullDistance;
 
+    /// <summary>
+    /// Flag used by clusters that are not typical quad-tree nodes but have no volume (eg. lots of instances placed on top of each other).
+    /// </summary>
+    int32 IsMinor : 1;
+
     /// <summary>
     /// The child clusters. If any element is valid then all are created.
     /// </summary>