SceneCapture that running on mainRenderer make skeletalMesh LOD flickering

Development Rendering
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1

[investigation]

When SceneCapture2D uses the main renderer, it gets added to the CustomRenderPass based on what it captures.

// SceneCaptureRendering.cpp
// Func : FScene::UpdateSceneCaptureContents(USceneCaptureComponent2D* CaptureComponent)
// As optimization for depth capture modes, render scene capture as additional render passes inside the main renderer.
if (GSceneCaptureAllowRenderInMainRenderer && 
    CaptureComponent->bRenderInMainRenderer && 
    (CaptureComponent->CaptureSource == ESceneCaptureSource::SCS_SceneDepth || CaptureComponent->CaptureSource == ESceneCaptureSource::SCS_DeviceDepth ||
     CaptureComponent->CaptureSource == ESceneCaptureSource::SCS_BaseColor || CaptureComponent->CaptureSource == ESceneCaptureSource::SCS_Normal)
    )
{
// ... other things...
AddCustomRenderPass(nullptr, PassInput);
}

In AllViews, the FViewInfo from CustomRenderPassInfos is added after the original FViewInfo of the main renderer.

// SceneRendering.cpp
// Func : FSceneRenderer::FSceneRenderer(const FSceneViewFamily* InViewFamily, FHitProxyConsumer* HitProxyConsumer)
AllViews.Empty(Views.Num() + NumAdditionalViews);
for (int32 i = 0; i < Views.Num(); ++i)
{
    AllViews.Add(&Views[i]);
}
for (FCustomRenderPassInfo& PassInfo : CustomRenderPassInfos)
{
    for (FViewInfo& View : PassInfo.Views)
    {
       AllViews.Add(&View);
    }
}

The visibility tasks are processed sequentially using a for loop, so the viewInfo added by CustomRenderPass is processed afterwards. As a result, the ViewInfo that comes later ends up being used as the final one.

// SceneVisibility.cpp
// Func : void FVisibilityTaskData::LaunchVisibilityTasks(const UE::Tasks::FTask& BeginInitVisibilityPrerequisites)
for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ++ViewIndex)
{
    // Each view gets its own visibility task packet which contains all the state to manage the task graph for a view.
    FVisibilityViewPacket& ViewPacket = ViewPackets.Emplace_GetRef(*this, Scene, *Views[ViewIndex], ViewIndex);
    ...
}

The View information is passed to SkeletalMeshSceneProxy and used for LOD calculation.

// SkeletalMesh.cpp
FPrimitiveViewRelevance FSkeletalMeshSceneProxy::GetViewRelevance(const FSceneView* View) const

I want to use MainRenderer for SceneCapture due to optimization concerns.

To solve this issue, I’m trying to add a flag member to the FSceneView structure that determines whether it should be used for LOD calculations.

I found a value bUseFieldOfViewForLOD in FViewInfo, but it doesn’t seem to be used anywhere.

I would appreciate any advice regarding this issue.

Thanks.

2

재현 방법

  1. Place a SceneCapture2D and use option `Render In Main Renderer`.
  2. Set option `BaseColor` in `RGB for Capture Source`.
  3. Make sure the SceneCapture2D capture skeletal mesh kind of characters.
  4. Run the game, check Mesh LOD coloration and you’ll notice flickering.

I’ve attached a reproducible project so you can just run game and check LODs.

I use 5.5.4 launch version for pasted project.

3

We’ve identified the issue, and have a fix. There are two parts to the fix, if you want to build the engine locally to apply it. The first is to FSceneRenderer::FSceneRenderer in Engine/Source/Runtime/Renderer/Private/SceneRendering.cpp, where the following frame values need to be initialized, on the line after CustomRenderPassInfo->ViewFamily.Time is initialized:

		CustomRenderPassInfo->ViewFamily.FrameNumber = ViewFamily.FrameNumber;
		CustomRenderPassInfo->ViewFamily.FrameCounter = ViewFamily.FrameCounter;

The second is to change this line in Engine/Source/Runtime/Engine/Private/SkeletalRender.cpp:

	// When rendering multiple views we need to guard the assignment with a mutex since relevance can occur in parallel.
	const bool bMultiView = View->Family->Views.Num() > 1;

To this, so it includes the Custom Render Pass views when deciding whether multi-view thread safety logic needs to run:

const bool bMultiView = View->Family->AllViews.Num() > 1;This code fix will be in 5.8, but the relevant code is the same in any older version of the engine which includes the “Render In Main Renderer” flag, so you should be able to apply the fix to any version.

Sorry for the inconvenience!

--Jason Hoerner

4

I applied it and it seems to work well. Thanks.