【UE5】ランタイムでStaticMesh同士を結合してドローコールを削減しよう

for () // LODのループ
{
	for () // セクションのループ
	{
		PDI->DrawMesh();
	}
}

PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "EnhancedInput", "MeshDescription" , "StaticMeshDescription" });

#include "MeshDescription.h"
#include "StaticMeshAttributes.h"

FMeshDescription MergeStaticMeshToMeshDescription(const UStaticMesh* meshA, const UStaticMesh* meshB)
{
    // LOD 0 のリソースを取得
    const FStaticMeshLODResources& LODResourceA = meshA->GetRenderData()->LODResources[0];
    const FStaticMeshLODResources& LODResourceB = meshB->GetRenderData()->LODResources[0];
    const FStaticMeshSection& SectionA = LODResourceA.Sections[0];
    const FStaticMeshSection& SectionB = LODResourceB.Sections[0];

    FMeshDescription MeshDescription;

    FStaticMeshAttributes AttributeGetter(MeshDescription);
    AttributeGetter.Register();

    TPolygonGroupAttributesRef<FName> PolygonGroupNames = AttributeGetter.GetPolygonGroupMaterialSlotNames();
    TVertexAttributesRef<FVector3f> VertexPositions = AttributeGetter.GetVertexPositions();
    TVertexInstanceAttributesRef<float> BinormalSigns = AttributeGetter.GetVertexInstanceBinormalSigns();
    TVertexInstanceAttributesRef<FVector3f> Normals = AttributeGetter.GetVertexInstanceNormals();
    TVertexInstanceAttributesRef<FVector4f> Colors = AttributeGetter.GetVertexInstanceColors();
    TVertexInstanceAttributesRef<FVector2f> UVs = AttributeGetter.GetVertexInstanceUVs();

    const int32 NumSections = 1;
    int32 VertexCount = LODResourceA.VertexBuffers.PositionVertexBuffer.GetNumVertices() + LODResourceB.VertexBuffers.PositionVertexBuffer.GetNumVertices();
    int32 VertexInstanceCount = SectionA.NumTriangles * 3 + SectionB.NumTriangles * 3;
    int32 PolygonCount = SectionA.NumTriangles + SectionB.NumTriangles;

    TArray<FPolygonGroupID> PolygonGroupForSection;
    PolygonGroupForSection.Reserve(NumSections);

    // 頂点やインスタンスの合計数を計算
    MeshDescription.ReserveNewVertices(VertexCount);
    MeshDescription.ReserveNewVertexInstances(VertexInstanceCount);
    MeshDescription.ReserveNewPolygons(PolygonCount);
    MeshDescription.ReserveNewEdges(PolygonCount * 2);
    UVs.SetNumChannels(1);

    // 新しいポリゴングループを作成
    FPolygonGroupID newPolygonGroupID = MeshDescription.CreatePolygonGroup();
    PolygonGroupNames[newPolygonGroupID] = "";
    PolygonGroupForSection.Add(newPolygonGroupID);

    FPolygonGroupID PolygonGroupID = PolygonGroupForSection[0];

    // 頂点を作成
    int32 NumVertex = VertexCount;
    TMap<int32, FVertexID> VertexIndexToVertexID;
    VertexIndexToVertexID.Reserve(NumVertex);
    for (uint32 i = 0; i < LODResourceA.VertexBuffers.PositionVertexBuffer.GetNumVertices(); ++i)
    {
        const FVertexID VertexID = MeshDescription.CreateVertex();
        VertexPositions[VertexID] = FVector3f(LODResourceA.VertexBuffers.PositionVertexBuffer.VertexPosition(i));
        VertexIndexToVertexID.Add(i, VertexID);
    }
    for (uint32 i = 0; i < LODResourceB.VertexBuffers.PositionVertexBuffer.GetNumVertices(); ++i)
    {
        const FVertexID VertexID = MeshDescription.CreateVertex();
        VertexPositions[VertexID] = FVector3f(LODResourceB.VertexBuffers.PositionVertexBuffer.VertexPosition(i));
        VertexIndexToVertexID.Add(i + LODResourceA.VertexBuffers.PositionVertexBuffer.GetNumVertices(), VertexID);
    }

    // インスタンスを作成
    int32 NumIndices = VertexInstanceCount;
    int32 NumTri = PolygonCount;
    TMap<int32, FVertexInstanceID> IndiceIndexToVertexInstanceID;
    IndiceIndexToVertexInstanceID.Reserve(NumIndices);
    for (uint32 i = 0; i < SectionA.NumTriangles * 3; i++)
    {
        const int32 VertexIndex = LODResourceA.IndexBuffer.GetIndex(i);
        const FVertexID VertexID = VertexIndexToVertexID[VertexIndex];
        const FVertexInstanceID VertexInstanceID = MeshDescription.CreateVertexInstance(VertexID);
        IndiceIndexToVertexInstanceID.Add(i, VertexInstanceID);

        Normals[VertexInstanceID] = (FVector3f)LODResourceA.VertexBuffers.StaticMeshVertexBuffer.VertexTangentZ(VertexIndex);
        BinormalSigns[VertexInstanceID] = 1.f;
        Colors[VertexInstanceID] = FLinearColor(1, 1, 1, 1);
        UVs.Set(VertexInstanceID, 0, FVector2f(LODResourceA.VertexBuffers.StaticMeshVertexBuffer.GetVertexUV(VertexIndex, 0)));
    }
    for (uint32 i = 0; i < SectionB.NumTriangles * 3; i++)
    {
        const int32 VertexIndex = LODResourceB.IndexBuffer.GetIndex(i);
        const FVertexID VertexID = VertexIndexToVertexID[VertexIndex + LODResourceA.VertexBuffers.PositionVertexBuffer.GetNumVertices()];
        const FVertexInstanceID VertexInstanceID = MeshDescription.CreateVertexInstance(VertexID);
        IndiceIndexToVertexInstanceID.Add(i + SectionA.NumTriangles * 3, VertexInstanceID);

        Normals[VertexInstanceID] = (FVector3f)LODResourceB.VertexBuffers.StaticMeshVertexBuffer.VertexTangentZ(VertexIndex);
        BinormalSigns[VertexInstanceID] = 1.f;
        Colors[VertexInstanceID] = FLinearColor(1, 1, 1, 1);
        UVs.Set(VertexInstanceID, 0, FVector2f(LODResourceB.VertexBuffers.StaticMeshVertexBuffer.GetVertexUV(VertexIndex, 0)));
    }

    // ポリゴンを作成
    for (int32 TriIdx = 0; TriIdx < PolygonCount; TriIdx++)
    {
        TArray<FVertexInstanceID> VertexInstanceIDs;
        VertexInstanceIDs.SetNum(3);

        VertexInstanceIDs[0] = IndiceIndexToVertexInstanceID[(TriIdx * 3)];
        VertexInstanceIDs[1] = IndiceIndexToVertexInstanceID[(TriIdx * 3) + 1];
        VertexInstanceIDs[2] = IndiceIndexToVertexInstanceID[(TriIdx * 3) + 2];

        MeshDescription.CreatePolygon(PolygonGroupID, VertexInstanceIDs);
    }
    return MeshDescription;
}

UStaticMesh* MergeStaticMesh(const UStaticMesh* meshA, const UStaticMesh* meshB)
{
	if (meshA == nullptr || meshB == nullptr) {
		return nullptr;
	}

    FMeshDescription meshDesc = MergeStaticMeshToMeshDescription(meshA, meshB);

    // ビルド設定
    UStaticMesh::FBuildMeshDescriptionsParams mdParams;
    mdParams.bUseHashAsGuid = false;
    mdParams.bMarkPackageDirty = true;
    mdParams.bBuildSimpleCollision = false;
    mdParams.bCommitMeshDescription = true;
    mdParams.bAllowCpuAccess = true;

    // 非Editorビルドでは必須
    mdParams.bFastBuild = true;

    TArray<const FMeshDescription*> MeshDescs;
    MeshDescs.Emplace(&meshDesc);
    UStaticMesh* MainStaticMesh = NewObject<UStaticMesh>();

    // FMeshDescriptionからStaticMeshへビルド
    MainStaticMesh->BuildFromMeshDescriptions(MeshDescs, mdParams);

    // マテリアルの設定(両StaticMeshともに同じマテリアルなので片方のみ利用して設定)
    MainStaticMesh->GetStaticMaterials().Add(FStaticMaterial(meshA->GetMaterial(0)));

	return MainStaticMesh;
}