Unreal學習筆記2-繪製簡單三角形

charlee44發表於2023-02-01

1. 概述

之所以寫這個繪製簡單三角形的例項其實是想知道如何在Unreal中透過程式碼繪製自定義Mesh,如果你會繪製一個三角形,那麼自然就會繪製複雜的Mesh了。所以這是很多圖形工作者的第一課。

2. 詳論

2.1. 程式碼實現

Actor是Unreal的基本顯示物件,有點類似於Unity中的GameObject或者OSG中的Node。因此,我們首先要實現一個繼承自AActor的類

標頭檔案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;
};

實現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();  //設定包圍盒之後呼叫這個函式起效,否則會被視錐體剔除
  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));

  // 設定頂點資料
  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);  //切線
    StaticMeshBuildVertices[m].TangentY = FVector(1, 0, 0);  //副切線
    StaticMeshBuildVertices[m].TangentZ = FVector(0, 0, 1);  //法向量
  }

  LODResources.bHasColorVertexData = false;

  //頂點buffer
  LODResources.VertexBuffers.PositionVertexBuffer.Init(StaticMeshBuildVertices);

  //法線,切線,貼圖座標buffer
  LODResources.VertexBuffers.StaticMeshVertexBuffer.Init(
      StaticMeshBuildVertices, 1);

  //設定索引陣列
  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};

  //設定包圍盒
  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); }

然後將這個類物件ACustomMeshActor拖放到場景中,顯示結果如下:

imglink1

2.2. 解析:Component

  1. Actor只是一個空殼,具體的功能是透過各種型別的Component實現的(這一點與Unity不謀而合),這裡使用的是UStaticMeshComponent,這也是Unreal場景中用的最多的Mesh元件。

  2. 這裡元件初始化是在BeginPlay()中建立的,如果在建構函式中建立,那麼就不能使用NewObject,而應該使用如下方法:

    // 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);
        }
    }
    
  3. 承接2,在BeginPlay()中建立和在建構函式中建立的區別就在於前者是執行時建立,而後者在程式執行之前就建立了,可以在未執行的編輯器狀態下看到靜態網格體和材質。

  4. 承接2,在建構函式中建立的UStaticMeshComponent移動性被設定成Static了,這時執行會提示“光照需要重建”,也就是靜態物件需要烘焙光照,在工具欄"構建"->"僅構建光照"烘培一下即可。這種方式執行時渲染效率最高。

  5. 對比4,執行時建立的UStaticMeshComponent移動性可以設定成Stationary,表示這個靜態物體不移動,啟用快取光照法,並且快取動態陰影。

2.3. 解析:材質

  1. 在UE編輯器分別建立了紅色和綠色簡單材質,注意材質是單面還是雙面的,C++程式碼設定的要和材質藍圖中設定的要保持一致。最開始我參考的就是參考文獻1中的程式碼,程式碼中設定成雙面,但是我自己的材質藍圖中用的單面,程式啟動直接崩潰了。

  2. 如果場景中材質顯示不正確,比如每次瀏覽場景時的效果都不一樣,說明可能法向量沒有設定,我最開始就沒有注意這個問題以為是光照的問題。

  3. 單面材質的話,正面是逆時針序還是順時針序?從這個案例來看應該是逆時針。UE是個左手座標系,X軸向前,法向量是(0, 0, 1),從法向量的一邊看過去,頂點順序是(100, 100, 50)->(100, 0, 50)->(0, 0, 50),明顯是逆時針。

2.4. 解析:包圍盒

  1. 包圍盒引數最好要設定,UE似乎預設實現了視景體裁剪,不在範圍內的物體會不顯示。如果在某些視角場景物件突然不顯示了,可能包圍盒引數沒有設定正確,導致視景體裁剪錯誤地篩選掉了當前場景物件。

    FBoxSphereBounds BoundingBoxAndSphere;
    //...
    RenderData->Bounds = BoundingBoxAndSphere;
    //...
    mesh->CalculateExtendedBounds();  //設定包圍盒之後呼叫這個函式起效,否則會被視錐體剔除
    
  2. 即使是一個平面,包圍盒的三個Size引數之一也不能為0,否則還是可能會在某些視角場景物件不顯示。

2.5. 解析:Section

Mesh內部是可以進行劃分的,劃分成多少個section就使用多少個材質,比如這裡劃分了兩個section,最後就使用了兩個材質。如下程式碼所示:

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

除了本文介紹的方法之外,也有其他的實現辦法,具體可以參考文獻3-5。實在是沒有時間進行進一步的研究了,因此記錄備份一下。另外,文獻6-7可能對了解UE關於Mesh的內部實現有所幫助,筆者反正是看麻了。不得不說,這麼一個微小的功能涉及到的內容還真不少,看來有的研究了。

4. 參考

  1. UE4繪製簡單三角形(二)
  2. UE4之座標系
  3. [UE4 C++]三種方式繪製三角形
  4. Building a StaticMesh in C++ during runtime
  5. Build static mesh from description
  6. 虛幻 – StaticMesh 分析
  7. Creating a Custom Mesh Component in UE4

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