VSM r.Shadow.Virtual.UseReceiverMaskDirectional Breaks Cache Behavior Overrides

Development Rendering
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1

Hello, since 5.7 this CVAR has been turned on by default and I’d like to understand why and if it is intentional that it breaks / prevents shadow cache behaviour to be set as we used to in 5.6 on meshes.

r.Shadow.Virtual.UseReceiverMaskDirectional 1

Most of my work is performance related and the VSM cache viewmode has always been a great tool to find issues and reduce shadow costs, but now it can’t be used unless that CVAR is off by default.

I found a forum post that discusses this but it dismisses the issue as by design.

I have attached a video explaining and showing what 5.7 projects now behave like in detail - I believe this is not the intended design but would really appreciate some clarity on the topic.

Thanks

VSM_Issue.mp4(58.1 MB)

2

Steps to Reproduce
Open a Vanilla 5.7 project

Check VSM Cache view mode -> Observe that all dynamic pages are invalidated and the map is entirely blue.

Take an Object with WPO and set its invalidation behavior to be static. -> Observe shadows continue to move.

Set:

r.Shadow.Virtual.UseReceiverMaskDirectional 0

Observe cache behavior behaves as expected.

3

Hello,

Thanks for reporting this, this could be an unintended regression and it looks like it’s possible to address this behavior by changing VirtualShadowMapPageCacheCommon.ush, but I need to verify with my colleague who is more familiar with this area if the current behavior is intended as you mentioned. They’re currently out of office (national holiday) so I should have more information next week.

bool ShouldCacheInstanceAsStatic(uint InstanceId, bool bViewUncached, bool bAllowWPO, int SceneRendererPrimaryViewId)
{
	if (bViewUncached)
	{
		return false;
	}
    
	FInstanceSceneData InstanceData = GetInstanceSceneDataUnchecked(InstanceId);
	FPrimitiveSceneData PrimitiveData = GetPrimitiveData(InstanceData.PrimitiveId);
 
	// If WPO is allowed but material invalidations are disabled (Static behavior),
 	// still cache as static to match the intended behavior
	if (bAllowWPO)
	{
		if ((PrimitiveData.Flags & PRIMITIVE_SCENE_DATA_FLAG_DISABLE_MATERIAL_INVALIDATIONS) == 0u)
		{
			return false;
		}
	}
4

Quick update: I’ve created https://issues.unrealengine.com/issue/UE\-380433 to track this issue and it should be visible publicly soon.

What was happening in the repro project was that the mesh was in the “guard-band” of clipmap levels where it is cached as dynamic such that it doesn’t cause a static invalidation when it starts animating and the logic wasn’t handling the cache as static flag correctly as you noted.

5

Thanks so much Alex for getting back to me and investigating further ! Glad to hear you were able to find the root cause !