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03-UnrealEngine/Rendering/Shader/UE的透明渲染功能笔记.md
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175
03-UnrealEngine/Rendering/Shader/UE的透明渲染功能笔记.md
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---
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title: UE的透明渲染功能笔记
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date: 2022-10-30 21:36:08
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excerpt:
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tags: Rendering
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rating: ⭐
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---
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其主函数为FDeferredShadingSceneRenderer::RenderTranslucency()，位于FXSystem与头发合成之后，渲染Distortion、速度之前。
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```c++
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// Draw translucency.
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if (bCanOverlayRayTracingOutput && TranslucencyViewsToRender != ETranslucencyView::None)
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{ RDG_CSV_STAT_EXCLUSIVE_SCOPE(GraphBuilder, RenderTranslucency);
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SCOPE_CYCLE_COUNTER(STAT_TranslucencyDrawTime);
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// Raytracing doesn't need the distortion effect.
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const bool bShouldRenderDistortion = TranslucencyViewsToRender != ETranslucencyView::RayTracing;
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#if RHI_RAYTRACING
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if (EnumHasAnyFlags(TranslucencyViewsToRender, ETranslucencyView::RayTracing))
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{ RenderRayTracingTranslucency(GraphBuilder, SceneColorTexture);
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EnumRemoveFlags(TranslucencyViewsToRender, ETranslucencyView::RayTracing);
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}#endif
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// Render all remaining translucency views.
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AddSetCurrentStatPass(GraphBuilder, GET_STATID(STAT_CLM_Translucency));
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RenderTranslucency(GraphBuilder, SceneColorTexture, SceneDepthTexture, HairDatas, &SeparateTranslucencyTextures, TranslucencyViewsToRender);
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AddServiceLocalQueuePass(GraphBuilder);
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TranslucencyViewsToRender = ETranslucencyView::None;
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...
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}
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```
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## Pass Event String
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static const TCHAR* TranslucencyPassToString(ETranslucencyPass::Type TranslucencyPass)
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{
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switch (TranslucencyPass)
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{ case ETranslucencyPass::TPT_StandardTranslucency:
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return TEXT("Standard");
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case ETranslucencyPass::TPT_TranslucencyAfterDOF:
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return TEXT("AfterDOF");
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case ETranslucencyPass::TPT_TranslucencyAfterDOFModulate:
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return TEXT("AfterDOFModulate");
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case ETranslucencyPass::TPT_AllTranslucency:
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return TEXT("All");
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} checkNoEntry();
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return TEXT("");
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}
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##
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半透明渲染分为可分离与单Pass渲染。
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- 可分离渲染调用RenderViewTranslucencyInner()渲染内部，之后调用AddEndSeparateTranslucencyTimerPass()完成渲染。
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- 单Pass渲染完所有物体。调用RenderViewTranslucencyInner()渲染内部，之后调用AddEndSeparateTranslucencyTimerPass()完成渲染。
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Shader变量使用FTranslucentBasePassParameters。
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MeshDrawPass名为。
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```c++
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EMeshPass::TranslucencyStandard
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EMeshPass::TranslucencyAfterDOF
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EMeshPass::TranslucencyAfterDOFModulate
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EMeshPass::TranslucencyAfterMotionBlur
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EMeshPass::TranslucencyAll
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```
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最后在AddPass()调用RenderViewTranslucencyInner()进行实际MeshDraw渲染：
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```c++
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if (bRenderInParallel)
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{
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GraphBuilder.AddPass(
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RDG_EVENT_NAME("Translucency(%s Parallel) %dx%d",
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TranslucencyPassToString(TranslucencyPass),
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int32(View.ViewRect.Width() * ViewportScale),
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int32(View.ViewRect.Height() * ViewportScale)),
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PassParameters,
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ERDGPassFlags::Raster | ERDGPassFlags::SkipRenderPass,
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[&SceneRenderer, &View, PassParameters, ViewportScale, Viewport, TranslucencyPass](FRHICommandListImmediate& RHICmdList)
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{
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FRDGParallelCommandListSet ParallelCommandListSet(RHICmdList, GET_STATID(STAT_CLP_Translucency), SceneRenderer, View, FParallelCommandListBindings(PassParameters), ViewportScale);
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RenderViewTranslucencyInner(RHICmdList, SceneRenderer, View, Viewport, ViewportScale, TranslucencyPass, &ParallelCommandListSet, PassParameters->InstanceCullingDrawParams);
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});
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}
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else
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{
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GraphBuilder.AddPass(
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RDG_EVENT_NAME("Translucency(%s) %dx%d",
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TranslucencyPassToString(TranslucencyPass),
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int32(View.ViewRect.Width() * ViewportScale),
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int32(View.ViewRect.Height() * ViewportScale)),
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PassParameters,
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ERDGPassFlags::Raster,
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[&SceneRenderer, &View, ViewportScale, Viewport, TranslucencyPass, PassParameters](FRHICommandListImmediate& RHICmdList)
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{
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RenderViewTranslucencyInner(RHICmdList, SceneRenderer, View, Viewport, ViewportScale, TranslucencyPass, nullptr, PassParameters->InstanceCullingDrawParams);
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});
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}
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```
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最后使用FBasePassMeshProcessor。
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```c++
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DrawDynamicMeshPass(View, RHICmdList,
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[&View, &DrawRenderState, TranslucencyPass](FDynamicPassMeshDrawListContext* DynamicMeshPassContext)
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{
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FBasePassMeshProcessor PassMeshProcessor(
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View.Family->Scene->GetRenderScene(),
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View.GetFeatureLevel(),
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&View,
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DrawRenderState,
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DynamicMeshPassContext,
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FBasePassMeshProcessor::EFlags::CanUseDepthStencil,
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TranslucencyPass);
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const uint64 DefaultBatchElementMask = ~0ull;
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for (int32 MeshIndex = 0; MeshIndex < View.ViewMeshElements.Num(); MeshIndex++)
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{ const FMeshBatch& MeshBatch = View.ViewMeshElements[MeshIndex];
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PassMeshProcessor.AddMeshBatch(MeshBatch, DefaultBatchElementMask, nullptr);
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}});
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```
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## 透明物体判断
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IsTranslucentBlendMode()位于MaterialShared.h：
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```c++
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inline bool IsTranslucentBlendMode(enum EBlendMode BlendMode)
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{
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return BlendMode != BLEND_Opaque && BlendMode != BLEND_Masked;
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}
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```
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位于`bool FBasePassMeshProcessor::ShouldDraw(const FMaterial& Material)`
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```c++
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const bool bIsTranslucent = IsTranslucentBlendMode(BlendMode);
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if (bTranslucentBasePass)
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{
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if (bIsTranslucent && !Material.IsDeferredDecal())
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{
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switch (TranslucencyPassType)
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{
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case ETranslucencyPass::TPT_StandardTranslucency:
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bShouldDraw = !Material.IsTranslucencyAfterDOFEnabled();
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break;
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case ETranslucencyPass::TPT_TranslucencyAfterDOF:
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bShouldDraw = Material.IsTranslucencyAfterDOFEnabled();
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break;
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// only dual blended or modulate surfaces need background modulation
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case ETranslucencyPass::TPT_TranslucencyAfterDOFModulate:
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bShouldDraw = Material.IsTranslucencyAfterDOFEnabled() && (Material.IsDualBlendingEnabled(GetFeatureLevelShaderPlatform(FeatureLevel)) || BlendMode == BLEND_Modulate);
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break;
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case ETranslucencyPass::TPT_AllTranslucency:
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bShouldDraw = true;
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break;
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}
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}
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}
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```
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## 为什么MultiDraw无法同时渲染透明与不透明材质
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首先透明会使用FBasePassMeshProcessor进行渲染（不同BasePass会有不同的设置），针对图元创建MeshProcessor时是以每个图元为单位进行的。
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```c++
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void FMaterialRenderProxy::UpdateUniformExpressionCacheIfNeeded(ERHIFeatureLevel::Type InFeatureLevel) const
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{
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if (!UniformExpressionCache[InFeatureLevel].bUpToDate)
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{ // Don't cache uniform expressions if an entirely different FMaterialRenderProxy is going to be used for rendering
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const FMaterial* Material = GetMaterialNoFallback(InFeatureLevel);
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if (Material)
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{ FMaterialRenderContext MaterialRenderContext(this, *Material, nullptr);
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MaterialRenderContext.bShowSelection = GIsEditor;
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EvaluateUniformExpressions(UniformExpressionCache[InFeatureLevel], MaterialRenderContext);
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}
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}
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}
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```
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