ue4-UDecalComponent组件，紧贴Mesh的材质

在4.11版本后的 TopDown 模板中，多了个紧跟光标移动的一个图案，会贴在不同凹凸面的Mesh中，就是使用了一个新增的组件 UDecalComponent

直接看 TopDown 模板中使用

1. c++ 中的 Character 中加个 UDecalComponent 组件

   /** A decal that projects to the cursor location. */
   UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = Camera, meta = (AllowPrivateAccess = "true"))
   class UDecalComponent* CursorToWorld;
   
   //构造函数中加载一个蓝图材质set到 UDecalComponent 组件中
   AMyTestCharacter::AMyTestCharacter()
   {
       // Create a decal in the world to show the cursor's location
       CursorToWorld = CreateDefaultSubobject<UDecalComponent>("CursorToWorld");
       CursorToWorld->SetupAttachment(RootComponent);
       static ConstructorHelpers::FObjectFinder<UMaterial> DecalMaterialAsset(TEXT("Material'/Game/TopDownCPP/Blueprints/M_Cursor_Decal.M_Cursor_Decal'"));
       if (DecalMaterialAsset.Succeeded())
       {
           CursorToWorld->SetDecalMaterial(DecalMaterialAsset.Object);
       }
       CursorToWorld->DecalSize = FVector(16.0f, 32.0f, 32.0f);
       CursorToWorld->SetRelativeRotation(FRotator(90.0f, 0.0f, 0.0f).Quaternion());
   }

2. 蓝图子类可以对这个组件的 Decal Material 材质 进行重新赋值
   
   

3. 每帧tick获取当前屏幕光标在当前摄像机对应 world 中的位置 Loaction，再将 UDecalComponent 组件set到那个 Location

   void AMyTestCharacter::Tick(float DeltaSeconds)
   {
       Super::Tick(DeltaSeconds);
       if (APlayerController* PC = Cast<APlayerController>(GetController()))
       {
           FHitResult TraceHitResult;
           PC->GetHitResultUnderCursor(ECC_Visibility, true, TraceHitResult);
           FVector CursorFV = TraceHitResult.ImpactNormal;
           FRotator CursorR = CursorFV.Rotation();
           CursorToWorld->SetWorldLocation(TraceHitResult.Location);
           CursorToWorld->SetWorldRotation(CursorR);
       }
   }

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