Significant ghosting artifacts in UE5.1.1

[ms-dne]

Hi! We noticed severe ghosting artifacts while testing the FSR2.2 plugin for UE5.1.1 vanilla.

Steps to reproduce:

• Activate the FSR2.2 plugin in UE5
• In UE, create a Third Person sample map
• Start the map with -game
• Flags set: r.FidelityFX.FSR2.Enabled 1 r.FidelityFX.FSR2.QualityMode 1 r.Velocity.ForceOutput 1
• Disable motion blur with show motionblur
• Check that the FSR2 plugin is active with Ctrl+Shift+, there should be an entry in Scene>PostProcessing>ThirdParty FFSRTemporalUpscaler
• Check that velocity is defined everywhere with vis SceneVelocity
• Notice ghosting artefacts while slowly turning the camera around the player pawn.

We weren’t able to address this issue yet but here are some of our findings so far:

The issue only happens when the camera is moving.

After enabling FFX_FSR2_ENABLE_DEBUG_CHECKING, we got the warn "frameTimeDelta is less than 1.0f - this value should be milliseconds (~16.6f for 60fps)". In FSR2TemporalUpscaler.cpp:1599, it should probably be instead: Fsr2DispatchParams.frameTimeDelta = View.Family->Time.GetDeltaWorldTimeSeconds() * 1000.0f; Logs show some other warns concerning cameraNear and cameraFar, but these parameters aren’t used.

We noticed that the FSR2MotionVectorTexture was exhibiting an odd behaviour, where it would suddenly show negative values for a few frames at regular intervals (shown in red). The bottom 3 pixels of FSR2MotionVectorTexture are also wrong because it seems that SceneVelocity and SceneDepth are both zero here; this makes the analysis of the converted velocity a bit inconvenient.

In PostProcessFFX_FSR2ConvertVelocity.usf, we investigated the branch where ComputeStaticVelocity() is used and it pointed us towards this PR: https://github.com/EpicGames/UnrealEngine/pull/6933, but it wasn’t clear to us that the problem came from motion vectors specifically.

Analysing sequences of frames in PIX with slight camera movement showed that FSR2_LockStatus[LOCK_LIFETIME_REMAINING] was still increasing on the boundary of objects after disocclusion of the background with a significant depth separation:

We tried to disable the locking mechanism by force-killing locks in ffx_fsr2_accumulate.h/FinalizeLockStatus(), which greatly reduced ghosting artifacts, but not entirely:

Is this issue known? Can this be a configuration issue, or a motion vectors issue? Thanks for any help!

[kyriewxcode]

I have a same problem, have you solved it?

[ms-dne]

Hello, we were only able to reduce the artifacts by setting the following in FSR2TemporalUpscaler.cpp:

Fsr2DispatchParams.cameraNear = FLT_MAX;
Fsr2DispatchParams.cameraFar = View.ViewMatrices.ComputeNearPlane();

Also, the issue didn’t seem to happen after changing the material on the skeletal mesh to a black material. But with a lit, metallic material we still have minor 1-frame ghosting pixels that are quite noticeable.

[kyriewxcode]

Thank you for your quick answer. It looks like a bug in the FSR 2.2 plugin, version 2.1 is fine now

[patrick-han]

What GPU was this on? Curious because I am seeing similar artifacts.