虚幻引擎如何添加一个MeshPass

发表于 分类于 ， 阅读次数： 本文字数： 1.5k 阅读时长 ≈ 5 分钟
本文是我学习如何添加一个MeshPass的笔记。以如何通过MeshPass来渲染出一张特定物体的蒙版为例。

这一章中，我希望通过添加一个MeshPass的形式把标记了特定Tag的物体渲染成一张特定的RT作为蒙版。虽然CustomDepth之类的功能也能方便地做到这一点，但这里主要是想通过这个来学习如何添加一个MeshPass
这里参考了EMeshPass::EditorLevelInstance的实现。

首先，我们先注册我们的MeshPass以及MeshProcessor

在MeshPass的相关的内容里把我们的MeshPass加上

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
namespace EMeshPass  
{  
   enum Type : uint8  
   {  
     ...
  
#if WITH_EDITOR  
      HitProxy,  
      HitProxyOpaqueOnly,  
      EditorLevelInstance,  
      EditorSelection,  
      //Texwood Begin Add, For Add A MeshPass  
      SpecialTagElement,  
      //Texwood End Add, For Add A MeshPass  
#endif  
      Num,  
      NumBits = 6,  
   };}  
  ...
inline const TCHAR* GetMeshPassName(EMeshPass::Type MeshPass)  
{  
   switch (MeshPass)  
   {   ...
#if WITH_EDITOR  
   case EMeshPass::HitProxy: return TEXT("HitProxy");  
   case EMeshPass::HitProxyOpaqueOnly: return TEXT("HitProxyOpaqueOnly");  
   case EMeshPass::EditorLevelInstance: return TEXT("EditorLevelInstance");  
   case EMeshPass::EditorSelection: return TEXT("EditorSelection");  
   //Texwood Begin Add, For Add A MeshPass  
   case EMeshPass::SpecialTagElement: return TEXT("SpecialTagElement");  
   //Texwood End Add, For Add A MeshPass  
#endif  
   }  
  
#if WITH_EDITOR  
   //Texwood Begin Add, For Add A MeshPass  
   //static_assert(EMeshPass::Num == 29 + 4, "Need to update switch(MeshPass) after changing EMeshPass"); // GUID to prevent incorrect auto-resolves, please change when changing the expression: {A6E82589-44B3-4DAD-AC57-8AF6BD50DF43}   
   //这里的数量记得要一起改
   static_assert(EMeshPass::Num == 30 + 4, "Need to update switch(MeshPass) after changing EMeshPass"); // GUID to prevent incorrect auto-resolves, please change when changing the expression: {A6E82589-44B3-4DAD-AC57-8AF6BD50DF43}  
   //Texwood End Add, For Add A MeshPass
#else  
   static_assert(EMeshPass::Num == 29, "Need to update switch(MeshPass) after changing EMeshPass"); // GUID to prevent incorrect auto-resolves, please change when changing the expression: {A6E82589-44B3-4DAD-AC57-8AF6BD50DF43}  
#endif  
  
   checkf(0, TEXT("Missing case for EMeshPass %u"), (uint32)MeshPass);  
   return nullptr;  
}

class ENGINE_API FPSOCollectorCreateManager  
{  
public:  
//Texwood0935 Begin Add ,For Add A Mesh Pass  
   //constexpr static uint32 MaxPSOCollectorCount = 33;  
   //这里也别忘了+1 
   constexpr static uint32 MaxPSOCollectorCount = 34;  
//Texwood0935 Begin Add ,For Add A Mesh Pass  
  
  ...
};

然后仿照FEditorLevelInstanceMeshProcessor写一个MeshProcessor

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
//Texwood0935 Begin Add, For Add A MeshPass  
class FSpecialTagElementMeshProcessor : public FSceneRenderingAllocatorObject<FSpecialTagElementMeshProcessor>, public FMeshPassProcessor  
{  
public:  
  
   FSpecialTagElementMeshProcessor(const FScene* Scene, const FSceneView* InViewIfDynamicMeshCommand, FMeshPassDrawListContext* InDrawListContext);  
  
   virtual void AddMeshBatch(const FMeshBatch& RESTRICT MeshBatch, uint64 BatchElementMask, const FPrimitiveSceneProxy* RESTRICT PrimitiveSceneProxy, int32 StaticMeshId = -1) override final;  
  
   FMeshPassProcessorRenderState PassDrawRenderState;  
  
private:  
   bool TryAddMeshBatch(const FMeshBatch& RESTRICT MeshBatch, uint64 BatchElementMask, const FPrimitiveSceneProxy* RESTRICT PrimitiveSceneProxy, int32 StaticMeshId, const FMaterialRenderProxy* MaterialRenderProxy, const FMaterial* Material);  
  
   bool Process(  
      const FMeshBatch& MeshBatch,  
      uint64 BatchElementMask,  
      int32 StaticMeshId,  
      const FPrimitiveSceneProxy* RESTRICT PrimitiveSceneProxy,  
      const FMaterialRenderProxy& RESTRICT MaterialRenderProxy,  
      const FMaterial& RESTRICT MaterialResource,  
      ERasterizerFillMode MeshFillMode,  
      ERasterizerCullMode MeshCullMode);  
  
   int32 GetStencilValue(const FSceneView* View, const FPrimitiveSceneProxy* PrimitiveSceneProxy);  
};  
//Texwood0935 End Add, For Add A MeshPass

大部分内容直接copyFEditorLevelInstanceMeshProcessor的实现就行了，注意我们不需要判断物体是否是LevelInstance以及我们的stencil值不同(这里我们先暂时都返回1)。
流程重点就是我们通过MeshPassProcessor的AddMeshBatch接口传来的MeshBatch以及其他信息，构建一个我们想要的MeshDrawCommands。
（直接用FEditorLevelInstanceMeshProcessor实现的好处就是shader啥的都不用我们自己写了，简而言之就是偷个懒×）
参考FEditorLevelInstanceMeshProcessor的实现时，我们还会发现一行很重要的代码：

1
FRegisterPassProcessorCreateFunction RegisterEditorLevelInstancePass(&CreateEditorLevelInstancePassProcessor, EShadingPath::Deferred, EMeshPass::EditorLevelInstance, EMeshPassFlags::MainView);

这个代码声明了一个全局变量，这个全局变量被声明的时候，通过构造函数把MeshPass和相应的MeshPassProcessor绑定到一起(一点吐槽：看UE代码的时候发现，UE还挺喜欢像这样在构造函数里面搞点东西)

然后，让标记了特定Tag的物体渲染成一张特定的蒙版

在不修改任何东西的前提下，到我们的MeshPassProcessor的AddMeshBatch是无法知道对应物体的Tag的，因此我们首先把Tag的值传进来，我们可以通过PrimitiveSceneProxy来完成这个目的。
在PrimitiveSceneProxy增加一个变量，并且在构造PrimitiveSceneProxy的时候把Component的Tags赋值给那个变量。
然后在FSpecialTagElementMeshProcessor::GetStencilValue中把有特殊Tag的物体的stencil值设置为1。

做完上述工作之后，让标记了特定Tag的物体渲染成一张特定的蒙版的任务还差一步：把Pass加到我们的渲染流程里。
我们把这个Pass加到后处理Pass里。同样仿照添加EMeshPass::EditorLevelInstance的AddVisualizeLevelInstancePass。
在两者流程基本相同，这里暂时不考虑nanite物体(nanite物体因为和普通物体的渲染流程不同，要做额外处理)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
//Texwood0935 Begin Add, For Add A MeshPass  
BEGIN_SHADER_PARAMETER_STRUCT(FVisualizeSpecialTagElementPassParameters, )  
   SHADER_PARAMETER_STRUCT_INCLUDE(FSceneTextureShaderParameters, SceneTextures)  
   SHADER_PARAMETER_STRUCT_INCLUDE(FInstanceCullingDrawParams, InstanceCullingDrawParams)  
   RENDER_TARGET_BINDING_SLOTS()  
END_SHADER_PARAMETER_STRUCT()  
  
void AddVisualizeSpecialTagElementPass(FRDGBuilder& GraphBuilder, const FViewInfo& View,  
   const FVisualizeLevelInstanceInputs& Inputs)  
{  
   check(Inputs.SceneColor.IsValid());  
   check(Inputs.SceneDepth.IsValid());  
  
   RDG_EVENT_SCOPE(GraphBuilder, "Editor Visualize Special Tag Element");  
  
   const uint32 NumSamples = View.GetSceneTexturesConfig().NumSamples;  
  
   // Patch uniform buffers with updated state for rendering the outline mesh draw commands.  
   const FViewInfo* EditorView = CreateEditorPrimitiveView(View, Inputs.SceneColor.ViewRect, NumSamples);  
  
   FRDGTextureRef DepthStencilTexture = nullptr;  
  
   // Generate custom depth / stencil for outline shapes.  
   {  
      {         FRDGTextureDesc DepthStencilDesc = Inputs.SceneColor.Texture->Desc;  
         DepthStencilDesc.Reset();  
         DepthStencilDesc.Format = PF_DepthStencil;  
         // This is a reversed Z depth surface, so 0.0f is the far plane.  
         DepthStencilDesc.ClearValue = FClearValueBinding((float)ERHIZBuffer::FarPlane, 0);  
         DepthStencilDesc.Flags = TexCreate_DepthStencilTargetable | TexCreate_ShaderResource;  
         DepthStencilDesc.NumSamples = NumSamples;  
  
         DepthStencilTexture = GraphBuilder.CreateTexture(DepthStencilDesc, TEXT("LevelInstanceDepth"));  
      }  
      FScene* Scene = View.Family->Scene->GetRenderScene();  
  
      const FScreenPassTextureViewport SceneColorViewport(Inputs.SceneColor);  
  
      auto* PassParameters = GraphBuilder.AllocParameters<FVisualizeSpecialTagElementPassParameters>();  
      const_cast<FViewInfo&>(View).ParallelMeshDrawCommandPasses[EMeshPass::SpecialTagElement].BuildRenderingCommands(GraphBuilder, Scene->GPUScene, PassParameters->InstanceCullingDrawParams);  
  
      PassParameters->SceneTextures = Inputs.SceneTextures;  
      PassParameters->RenderTargets.DepthStencil = FDepthStencilBinding(  
         DepthStencilTexture,  
         ERenderTargetLoadAction::EClear,  
         ERenderTargetLoadAction::EClear,  
         FExclusiveDepthStencil::DepthWrite_StencilWrite);  
  
      GraphBuilder.AddPass(  
         RDG_EVENT_NAME("LevelInstanceDepth %dx%d", SceneColorViewport.Rect.Width(), SceneColorViewport.Rect.Height()),  
         PassParameters,         ERDGPassFlags::Raster,  
         [&View, SceneColorViewport, DepthStencilTexture, PassParameters](FRHICommandListImmediate& RHICmdList)  
      {         RHICmdList.SetViewport(SceneColorViewport.Rect.Min.X, SceneColorViewport.Rect.Min.Y, 0.0f, SceneColorViewport.Rect.Max.X, SceneColorViewport.Rect.Max.Y, 1.0f);  
  
         {            SCOPED_DRAW_EVENT(RHICmdList, EditorLevelInstance);  
  
            // Run LevelInstance pass on static elements  
            View.ParallelMeshDrawCommandPasses[EMeshPass::SpecialTagElement].DispatchDraw(nullptr, RHICmdList, &PassParameters->InstanceCullingDrawParams);  
         }  
      });   }}  
//Texwood0935 End Add, For Add A MeshPass

然后在后处理流程里调用我们的函数

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
void AddPostProcessingPasses(  
   FRDGBuilder& GraphBuilder,  
   const FViewInfo& View,   
int32 ViewIndex,  
   bool bAnyLumenActive,  
   EReflectionsMethod ReflectionsMethod,  
   const FPostProcessingInputs& Inputs,  
   const Nanite::FRasterResults* NaniteRasterResults,  
   FInstanceCullingManager& InstanceCullingManager,  
   FVirtualShadowMapArray* VirtualShadowMapArray,   
FLumenSceneFrameTemporaries& LumenFrameTemporaries,  
   const FSceneWithoutWaterTextures& SceneWithoutWaterTextures,  
   FScreenPassTexture TSRMoireInput)  
{
...
if (PassSequence.IsEnabled(EPass::VisualizeSpecialTagElement))  
{  
   FVisualizeSpecialTagElement PassInputs;  
   PassSequence.AcceptOverrideIfLastPass(EPass::VisualizeSpecialTagElement, PassInputs.OverrideOutput);  
   PassInputs.SceneColor = SceneColor;  
   PassInputs.SceneDepth = SceneDepth;  
   PassInputs.SceneTextures.SceneTextures = Inputs.SceneTextures;  
   SceneColor = AddVisualizeSpecialTagElementPass(GraphBuilder, View, PassInputs);  
}
...
}

同时别忘了在计算可见性的地方让Mesh能够加入我们的MeshPass

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
void MarkRelevant(){
...
{  
   DrawCommandPacket.AddCommandsForMesh(PrimitiveIndex, PrimitiveSceneInfo, StaticMeshRelevance, StaticMesh, Scene, bCanCache, EMeshPass::SpecialTagElement);  
}
...
}

void ComputeDynamicMeshRelevance(EShadingPath ShadingPath, bool bAddLightmapDensityCommands, const FPrimitiveViewRelevance& ViewRelevance, const FMeshBatchAndRelevance& MeshBatch, FViewInfo& View, FMeshPassMask& PassMask, FPrimitiveSceneInfo* PrimitiveSceneInfo, const FPrimitiveBounds& Bounds)
{
if (ViewRelevance.bDrawRelevance)  
{  
   ...  
   //Texwood0935 Begin Add, For Add A MeshPass  
   PassMask.Set(EMeshPass::SpecialTagElement);  
   View.NumVisibleDynamicMeshElements[EMeshPass::SpecialTagElement] += NumElements;  
   //Texwood0935 End Add, For Add A MeshPass  
}
}

编译运行，看看我们的结果

备注：调试的时候遇到一个很麻烦的bug，VS输出的SV_Position值有问题，是NAN导致结果出错。目前排查到的原因是LocalToWorld矩阵的值有问题，具体是哪步的值开始传出问题的还没继续排查。因此把Pass改成在BasePass的时候调用了(这个时候的值没问题)。
找到原因了，和后处理的顺序有关，把相应的Pass的顺序提前就好了

最后，代码添加一个MeshPass学习笔记代码记录