UE4 3DUI及曲面UI实现浅析和拓展

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最近在工作中遇见了游戏场景内3Dui和曲面ui的需求，大致看了下ue4 4.27的实现方式，简单来说就是将widget渲染到一个RT上，然后将RT赋给一个动态材质，在场景代理中动态构建一个mesh并使用这个动态材质。

看看实现，首先在beginplay或者component onregister时初始化窗口，这里就用的创建2dui的方式。

void UWidgetComponent::InitWidget()
{
       // Don't do any work if Slate is not initialized
       if ( FSlateApplication::IsInitialized() )
       {
              UWorld* World = GetWorld();
              if ( WidgetClass && Widget == nullptr && World &&  !World->bIsTearingDown)
              {
                     Widget = CreateWidget(GetWorld(), WidgetClass);
                     SetTickMode(TickMode);
              }
。。。。。。
       }
}

然后在tick中更新各组件

更新widget，将widget放入一个slatewindow以进行渲染，注意在最开始首先创建了动态材质，并在材质中将RT作为参数赋了进去。这里很有迷惑性，因为这时RT还没有值是空，真正的的RT要等后面updateRenderTarget时再赋值进去。

void UWidgetComponent::UpdateWidget()
{
。。。。。
                     TSharedPtr<SWidget> NewSlateWidget;
                     if (Widget)
                     {
                           NewSlateWidget = Widget->TakeWidget();
                     }
                     // Create the SlateWindow if it doesn't exists
                     bool bNeededNewWindow = false;
                     if (!SlateWindow.IsValid())
                     {
                           UpdateMaterialInstance();
                           SlateWindow =  SNew(SVirtualWindow).Size(CurrentDrawSize);
                           SlateWindow->SetIsFocusable(bWindowFocusable);
                           SlateWindow->SetVisibility(ConvertWindowVisibilityToVisibility(WindowVisibility));
                           RegisterWindow();
                           bNeededNewWindow = true;
                     }
。。。。。
                     // We Get here if we have a UMG Widget
                     if (NewSlateWidget.IsValid())
                     {
                           if (NewSlateWidget != CurrentSlateWidget ||  bNeededNewWindow)
                           {
                                  CurrentSlateWidget = NewSlateWidget;
                                  SlateWindow->SetContent(NewSlateWidget.ToSharedRef());
。。。。。
                           }
                     }
。。。。。
}void UWidgetComponent::UpdateMaterialInstance()
{
。。。。。
       UMaterialInterface* BaseMaterial = GetMaterial(0);
       MaterialInstance = UMaterialInstanceDynamic::Create(BaseMaterial, this);
       UpdateMaterialInstanceParameters();
}void UWidgetComponent::UpdateMaterialInstanceParameters()
{
       if ( MaterialInstance )
       {
              MaterialInstance->SetTextureParameterValue("SlateUI", RenderTarget);
              MaterialInstance->SetVectorParameterValue("TintColorAndOpacity",  TintColorAndOpacity);
              MaterialInstance->SetScalarParameterValue("OpacityFromTexture",  OpacityFromTexture);
       }
}

更新完窗口后就将窗口绘制到RT上，核心函数就高亮的两个，在Update中创建和初始化RT，然后将其赋给material，再将窗口使用FWindowRender绘制到RT上。

void UWidgetComponent::DrawWidgetToRenderTarget(float DeltaTime)
{
。。。。。
       UpdateRenderTarget(CurrentDrawSize);
       // The render target could be null if the current draw size is zero
       if(RenderTarget)
       {
              bRedrawRequested = false;
              WidgetRenderer->DrawWindow(
                     RenderTarget,
                     SlateWindow->GetHittestGrid(),
                     SlateWindow.ToSharedRef(),
                     DrawScale,
                     CurrentDrawSize,
                     DeltaTime);
。。。。。
       }
}void UWidgetComponent::UpdateRenderTarget(FIntPoint DesiredRenderTargetSize)
{
。。。。。
              if ( RenderTarget == nullptr )
              {
                     RenderTarget = NewObject<UTextureRenderTarget2D>(this);
。。。。。
                     if ( MaterialInstance )
                     {
                           MaterialInstance->SetTextureParameterValue("SlateUI",  RenderTarget);
                     }
              }
。。。。。
}

到这里是将窗口画面绘制到RT到，而生成并渲染到3Dmesh上是通过代理，meshbuilder实现的。代理构造时获取窗口的材质和各参数。

详细看看平面窗口（plane）和曲面窗口（cylinder)是怎么计算的。

FWidget3DSceneProxy( UWidgetComponent* InComponent, ISlate3DRenderer& InRenderer )
              : FPrimitiveSceneProxy( InComponent )
              ,  ArcAngle(FMath::DegreesToRadians(InComponent->GetCylinderArcAngle()))
              , Pivot( InComponent->GetPivot() )
              , Renderer( InRenderer )
              , RenderTarget( InComponent->GetRenderTarget() )
              , MaterialInstance( InComponent->GetMaterialInstance() )
              , BlendMode( InComponent->GetBlendMode() )
              , GeometryMode(InComponent->GetGeometryMode())
              , BodySetup(InComponent->GetBodySetup())
       {
              bWillEverBeLit = false;
              MaterialRelevance =  MaterialInstance->GetRelevance_Concurrent(GetScene().GetFeatureLevel());
       }virtual void GetDynamicMeshElements(const TArray<const FSceneView*>& Views, const  FSceneViewFamily& ViewFamily, uint32 VisibilityMap, FMeshElementCollector&  Collector) const override
       {
#if WITH_EDITOR
              const bool bWireframe = AllowDebugViewmodes() &&  ViewFamily.EngineShowFlags.Wireframe;
              auto WireframeMaterialInstance = new FColoredMaterialRenderProxy(
                     GEngine->WireframeMaterial ?  GEngine->WireframeMaterial->GetRenderProxy() : nullptr,

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