1. 概述
之所以写这个绘制简单三角形的实例其实是想知道如何在Unreal中通过代码绘制自定义Mesh，如果你会绘制一个三角形，那么自然就会绘制复杂的Mesh了。所以这是很多图形工作者的第一课。
2. 详论
2.1. 代码实现
Actor是Unreal的基本显示对象，有点类似于Unity中的GameObject或者OSG中的Node。因此，我们首先要实现一个继承自AActor的类
头文件CustomMeshActor.h：
#pragma once // clang-format off #include "CoreMinimal.h" #include "GameFramework/Actor.h" #include "CustomMeshActor.generated.h" // clang-format on UCLASS() class UESTUDY_API ACustomMeshActor : public AActor { GENERATED_BODY() public: // Sets default values for this actor's properties ACustomMeshActor(); protected: // Called when the game starts or when spawned virtual void BeginPlay() override; UStaticMesh* CreateMesh(); void CreateGeometry(FStaticMeshRenderData* RenderData); void CreateMaterial(UStaticMesh* mesh); public: // Called every frame virtual void Tick(float DeltaTime) override; UPROPERTY(VisibleAnywhere, BlueprintReadOnly) UStaticMeshComponent* staticMeshComponent; }; 实现CustomMeshActor.cpp：
#include "CustomMeshActor.h" #include "Output.h" // Sets default values ACustomMeshActor::ACustomMeshActor() { // Set this actor to call Tick() every frame. You can turn this off to // improve performance if you don't need it. PrimaryActorTick.bCanEverTick = true; } // Called when the game starts or when spawned void ACustomMeshActor::BeginPlay() { Super::BeginPlay(); staticMeshComponent = NewObject<UStaticMeshComponent>(this); staticMeshComponent->SetMobility(EComponentMobility::Stationary); SetRootComponent(staticMeshComponent); staticMeshComponent->RegisterComponent(); UStaticMesh* mesh = CreateMesh(); if (mesh) { staticMeshComponent->SetStaticMesh(mesh); } } UStaticMesh* ACustomMeshActor::CreateMesh() { UStaticMesh* mesh = NewObject<UStaticMesh>(staticMeshComponent); mesh->NeverStream = true; mesh->SetIsBuiltAtRuntime(true); TUniquePtr<FStaticMeshRenderData> RenderData = MakeUnique<FStaticMeshRenderData>(); CreateGeometry(RenderData.Get()); CreateMaterial(mesh); mesh->SetRenderData(MoveTemp(RenderData)); mesh->InitResources(); mesh->CalculateExtendedBounds(); //设置包围盒之后调用这个函数起效，否则会被视锥体剔除 return mesh; } void ACustomMeshActor::CreateMaterial(UStaticMesh* mesh) { UMaterial* material1 = (UMaterial*)StaticLoadObject( UMaterial::StaticClass(), nullptr, TEXT("Material'/Game/Materials/RedColor.RedColor'")); mesh->AddMaterial(material1); UMaterial* material2 = (UMaterial*)StaticLoadObject( UMaterial::StaticClass(), nullptr, TEXT("Material'/Game/Materials/GreenColor.GreenColor'")); mesh->AddMaterial(material2); } void ACustomMeshActor::CreateGeometry(FStaticMeshRenderData* RenderData) { RenderData->AllocateLODResources(1); FStaticMeshLODResources& LODResources = RenderData->LODResources[0]; int vertexNum = 4; TArray<FVector> xyzList; xyzList.Add(FVector(0, 0, 50)); xyzList.Add(FVector(100, 0, 50)); xyzList.Add(FVector(100, 100, 50)); xyzList.Add(FVector(0, 100, 50)); TArray<FVector2D> uvList; uvList.Add(FVector2D(0, 1)); uvList.Add(FVector2D(0, 0)); uvList.Add(FVector2D(1, 0)); uvList.Add(FVector2D(1, 1)); // 设置顶点数据 TArray<FStaticMeshBuildVertex> StaticMeshBuildVertices; StaticMeshBuildVertices.SetNum(vertexNum); for (int m = 0; m < vertexNum; m++) { StaticMeshBuildVertices[m].Position = xyzList[m]; StaticMeshBuildVertices[m].Color = FColor(255, 0, 0); StaticMeshBuildVertices[m].UVs[0] = uvList[m]; StaticMeshBuildVertices[m].TangentX = FVector(0, 1, 0); //切线 StaticMeshBuildVertices[m].TangentY = FVector(1, 0, 0); //副切线 StaticMeshBuildVertices[m].TangentZ = FVector(0, 0, 1); //法向量 } LODResources.bHasColorVertexData = false; //顶点buffer LODResources.VertexBuffers.PositionVertexBuffer.Init(StaticMeshBuildVertices); //法线，切线，贴图坐标buffer LODResources.VertexBuffers.StaticMeshVertexBuffer.Init( StaticMeshBuildVertices, 1); //设置索引数组 TArray<uint32> indices; int numTriangles = 2; int indiceNum = numTriangles * 3; indices.SetNum(indiceNum); indices[0] = 2; indices[1] = 1; indices[2] = 0; indices[3] = 3; indices[4] = 2; indices[5] = 0; LODResources.IndexBuffer.SetIndices(indices, EIndexBufferStride::Type::AutoDetect); LODResources.bHasDepthOnlyIndices = false; LODResources.bHasReversedIndices = false; LODResources.bHasReversedDepthOnlyIndices = false; // LODResources.bHasAdjacencyInfo = false; FStaticMeshLODResources::FStaticMeshSectionArray& Sections = LODResources.Sections; { FStaticMeshSection& section = Sections.AddDefaulted_GetRef(); section.bEnableCollision = false; section.MaterialIndex = 0; section.NumTriangles = 1; section.FirstIndex = 0; section.MinVertexIndex = 0; section.MaxVertexIndex = 2; } { FStaticMeshSection& section = Sections.AddDefaulted_GetRef(); section.bEnableCollision = false; section.MaterialIndex = 0; section.NumTriangles = 1; section.FirstIndex = 3; section.MinVertexIndex = 3; section.MaxVertexIndex = 5; } double boundArray[7] = {0, 0, 0, 200, 200, 200, 200}; //设置包围盒 FBoxSphereBounds BoundingBoxAndSphere; BoundingBoxAndSphere.Origin = FVector(boundArray[0], boundArray[1], boundArray[2]); BoundingBoxAndSphere.BoxExtent = FVector(boundArray[3], boundArray[4], boundArray[5]); BoundingBoxAndSphere.SphereRadius = boundArray[6]; RenderData->Bounds = BoundingBoxAndSphere; } // Called every frame void ACustomMeshActor::Tick(float DeltaTime) { Super::Tick(DeltaTime); } 然后将这个类对象ACustomMeshActor拖放到场景中，显示结果如下：
2.2. 解析：Component
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Actor只是一个空壳，具体的功能是通过各种类型的Component实现的（这一点与Unity不谋而合），这里使用的是UStaticMeshComponent，这也是Unreal场景中用的最多的Mesh组件。
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这里组件初始化是在BeginPlay()中创建的，如果在构造函数中创建，那么就不能使用NewObject，而应该使用如下方法：
// Sets default values ACustomMeshActor::ACustomMeshActor() { // Set this actor to call Tick() every frame. You can turn this off to // improve performance if you don't need it. PrimaryActorTick.bCanEverTick = true; staticMeshComponent = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("SceneRoot")); staticMeshComponent->SetMobility(EComponentMobility::Static); SetRootComponent(staticMeshComponent); UStaticMesh* mesh = CreateMesh(); if (mesh) { staticMeshComponent->SetStaticMesh(mesh); } } -
承接2，在BeginPlay()中创建和在构造函数中创建的区别就在于前者是运行时创建，而后者在程序运行之前就创建了，可以在未运行的编辑器状态下看到静态网格体和材质。
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承接2，在构造函数中创建的UStaticMeshComponent移动性被设置成Static了，这时运行会提示“光照需要重建”，也就是静态对象需要烘焙光照，在工具栏"构建"->"仅构建光照"烘培一下即可。这种方式运行时渲染效率最高。
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对比4，运行时创建的UStaticMeshComponent移动性可以设置成Stationary，表示这个静态物体不移动，启用缓存光照法，并且缓存动态阴影。
2.3. 解析：材质
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在UE编辑器分别创建了红色和绿色简单材质，注意材质是单面还是双面的，C++代码设置的要和材质蓝图中设置的要保持一致。最开始我参考的就是参考文献1中的代码，代码中设置成双面，但是我自己的材质蓝图中用的单面，程序启动直接崩溃了。
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如果场景中材质显示不正确，比如每次浏览场景时的效果都不一样，说明可能法向量没有设置，我最开始就没有注意这个问题以为是光照的问题。
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单面材质的话，正面是逆时针序还是顺时针序？从这个案例来看应该是逆时针。UE是个左手坐标系，X轴向前，法向量是(0, 0, 1)，从法向量的一边看过去，顶点顺序是(100, 100, 50)->(100, 0, 50)->(0, 0, 50)，明显是逆时针。
2.4. 解析：包围盒
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包围盒参数最好要设置，UE似乎默认实现了视景体裁剪，不在范围内的物体会不显示。如果在某些视角场景对象突然不显示了，可能包围盒参数没有设置正确，导致视景体裁剪错误地筛选掉了当前场景对象。
FBoxSphereBounds BoundingBoxAndSphere; //... RenderData->Bounds = BoundingBoxAndSphere; //... mesh->CalculateExtendedBounds(); //设置包围盒之后调用这个函数起效，否则会被视锥体剔除 -
即使是一个平面，包围盒的三个Size参数之一也不能为0，否则还是可能会在某些视角场景对象不显示。
2.5. 解析：Section
Mesh内部是可以进行划分的，划分成多少个section就使用多少个材质，比如这里划分了两个section，最后就使用了两个材质。如下代码所示：
FStaticMeshLODResources::FStaticMeshSectionArray& Sections = LODResources.Sections; { FStaticMeshSection& section = Sections.AddDefaulted_GetRef(); section.bEnableCollision = false; section.MaterialIndex = 0; section.NumTriangles = 1; section.FirstIndex = 0; section.MinVertexIndex = 0; section.MaxVertexIndex = 2; } { FStaticMeshSection& section = Sections.AddDefaulted_GetRef(); section.bEnableCollision = false; section.MaterialIndex = 0; section.NumTriangles = 1; section.FirstIndex = 3; section.MinVertexIndex = 3; section.MaxVertexIndex = 5; } 3. 其他
除了本文介绍的方法之外，也有其他的实现办法，具体可以参考文献3-5。实在是没有时间进行进一步的研究了，因此记录备份一下。另外，文献6-7可能对了解UE关于Mesh的内部实现有所帮助，笔者反正是看麻了。不得不说，这么一个微小的功能涉及到的内容还真不少，看来有的研究了。
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