Add mesh index buffer optimization based on meshoptimizer library

Work similar to existing impl but yields better results with even less overdraw.

Source/Engine/Graphics/Models/ModelData.Tool.cpp

@@ -10,11 +10,12 @@
 #include "Engine/Core/Collections/BitArray.h"
 #include "Engine/Tools/ModelTool/ModelTool.h"
 #include "Engine/Tools/ModelTool/VertexTriangleAdjacency.h"
-#include "Engine/Profiler/ProfilerCPU.h"
 #include "Engine/Platform/Platform.h"
 #include "Engine/Platform/MessageBox.h"
+#include "Engine/Profiler/ProfilerCPU.h"
+#include <ThirdParty/meshoptimizer/meshoptimizer.h>
 #define USE_MIKKTSPACE 1
-#include "ThirdParty/MikkTSpace/mikktspace.h"
+#include <ThirdParty/MikkTSpace/mikktspace.h>
 #if USE_ASSIMP
 #define USE_SPATIAL_SORT 1
 #define ASSIMP_BUILD_NO_EXPORT
@@ -25,6 +26,26 @@
 #endif
 #include <stack>
 
+bool InitedMeshOpt = false;
+
+void* MeshOptAllocate(size_t size)
+{
+    return Allocator::Allocate(size);
+}
+
+void MeshOptDeallocate(void* ptr)
+{
+    Allocator::Free(ptr);
+}
+
+void InitMeshOpt()
+{
+    if (InitedMeshOpt)
+        return;
+    InitedMeshOpt = true;
+    meshopt_setAllocator(MeshOptAllocate, MeshOptDeallocate);
+}
+
 #if PLATFORM_WINDOWS
 
 // Import UVAtlas library
@@ -934,6 +955,23 @@ void MeshData::ImproveCacheLocality()
         LOG(Info, "Generated {3} for mesh in {0}s ({1} vertices, {2} indices)", time, vertexCount, indexCount, TEXT("optimized indices"));
 }
 
+void MeshData::Optimize()
+{
+    if (Indices.IsEmpty() || Positions.IsEmpty())
+        return;
+    PROFILE_CPU();
+    InitMeshOpt();
+
+    // Vertex cache optimization (https://meshoptimizer.org/#vertex-cache-optimization)
+    Array<uint32> tmpIndices;
+    tmpIndices.Resize(Indices.Count());
+    meshopt_optimizeVertexCache(tmpIndices.Get(), Indices.Get(), Indices.Count(), Positions.Count());
+
+    // Overdraw optimization (https://meshoptimizer.org/#overdraw-optimization)
+    const float overdrawThreshold = 1.05f;
+    meshopt_optimizeOverdraw(Indices.Get(), tmpIndices.Get(), Indices.Count(), (const float*)Positions.Get(), Positions.Count(), sizeof(Float3), overdrawThreshold);
+}
+
 float MeshData::CalculateTrianglesArea() const
 {
     PROFILE_CPU();

Source/Engine/Graphics/Models/ModelData.h

@@ -228,9 +228,16 @@ public:
 
     /// <summary>
     /// Reorders all triangles for improved vertex cache locality. It tries to arrange all triangles to fans and to render triangles which share vertices directly one after the other.
+    /// [Deprecated in v1.13]
     /// </summary>
+    DEPRECATED("Use Optimize instead as it does more advanced optimizations on index buffer.")
     void ImproveCacheLocality();
 
+    /// <summary>
+    /// Optimizes mesh vertex and index buffers for runtime rendering. Uses 'meshoptimizer' library.
+    /// </summary>
+    void Optimize();
+
     /// <summary>
     /// Sums the area of all triangles in the mesh.
     /// </summary>

Source/Engine/Tools/ModelTool/ModelTool.Assimp.cpp

@@ -437,6 +437,12 @@ bool ProcessMesh(ModelData& result, AssimpImporterData& data, const aiMesh* aMes
             }
         }
     }
+
+    if (data.Options.OptimizeMeshes)
+    {
+        mesh.Optimize();
+    }
+
     return false;
 }
 
@@ -730,7 +736,7 @@ bool ModelTool::ImportDataAssimp(const String& path, ModelData& data, Options& o
         if (options.ReverseWindingOrder)
             flags &= ~aiProcess_FlipWindingOrder;
         if (options.OptimizeMeshes)
-            flags |= aiProcess_OptimizeMeshes | aiProcess_SplitLargeMeshes | aiProcess_ImproveCacheLocality;
+            flags |= aiProcess_OptimizeMeshes;
         if (options.MergeMeshes)
             flags |= aiProcess_RemoveRedundantMaterials;
     }

Source/Engine/Tools/ModelTool/ModelTool.OpenFBX.cpp

@@ -988,7 +988,7 @@ bool ProcessMesh(ModelData& result, OpenFbxImporterData& data, const ofbx::Mesh*
 
     if (data.Options.OptimizeMeshes)
     {
-        mesh.ImproveCacheLocality();
+        mesh.Optimize();
     }
 
     // Apply FBX Mesh geometry transformation

Source/Engine/Tools/ModelTool/ModelTool.cpp

@@ -45,6 +45,7 @@
 #include "Editor/Utilities/EditorUtilities.h"
 #include "Engine/Animations/Graph/AnimGraph.h"
 #include <ThirdParty/meshoptimizer/meshoptimizer.h>
+extern void InitMeshOpt();
 #endif
 
 ModelSDFHeader::ModelSDFHeader(const ModelBase::SDFData& sdf, const GPUTextureDescription& desc)
@@ -940,16 +941,6 @@ void OptimizeCurve(LinearCurve<T>& curve)
     }
 }
 
-void* MeshOptAllocate(size_t size)
-{
-    return Allocator::Allocate(size);
-}
-
-void MeshOptDeallocate(void* ptr)
-{
-    Allocator::Free(ptr);
-}
-
 void TrySetupMaterialParameter(MaterialInstance* instance, Span<const Char*> paramNames, const Variant& value, MaterialParameterType type)
 {
     for (const Char* name : paramNames)
@@ -1954,8 +1945,8 @@ bool ModelTool::ImportModel(const String& path, ModelData& data, Options& option
     // Automatic LOD generation
     if (options.GenerateLODs && options.LODCount > 1 && data.LODs.HasItems() && options.TriangleReduction < 1.0f - ZeroTolerance)
     {
+        InitMeshOpt();
         auto lodStartTime = DateTime::NowUTC();
-        meshopt_setAllocator(MeshOptAllocate, MeshOptDeallocate);
         float triangleReduction = Math::Saturate(options.TriangleReduction);
         int32 lodCount = Math::Max(options.LODCount, data.LODs.Count());
         int32 baseLOD = Math::Clamp(options.BaseLOD, 0, lodCount - 1);