Unreal学习笔记2-绘制简单三角形

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

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。
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。
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。
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

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。
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。
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

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;
}