Windy foliage jittering in Path Tracer

Development Rendering
1

Sorry if this is the wrong place for this.
I’m working on a short (8 minute) horror short for fun and as a portfolio piece. The entire scene is completed and I’ve rendered it out, however I quickly noticed that the foliage wasn’t moving in any of the renders despite me including a SimpleGrassWind setup for the World Position Offset value in the materials.

For some of the VFX in the scene I need Path Tracing enabled, so I did some research on the issue and found several settings that ended up resulting in the foliage moving in Path Traced Renders, which I’ve added below.

With these enabled the foliage is now moving properly in Path Traced Renders - However, some of the foliage is very obviously jittering and jumping in the renders, and I’ve not been able to find any solutions to fix this. To get the foliage to show up in renders in the first place I had to enable the r.ForceLod 0 and foliage.ForceLOD 0 settings, so it’s possible that’s having an impact? I don’t know enough about setting up LODs and console variables to solve the issue by troubleshooting it myself, so any advice or settings that might help stop the issue would be helpful.


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2

I do not have much experience with foliage in general but it might have something to do with Anti-aliasing. Do you only use spatial samples? If so, it could help to add some temporal samples.

3

I had the same flickering issue with plants in Path Tracer. After testing, I realized the problem wasn’t my console commands — those were working fine. The issue was mainly with large amounts of painted foliage.

What worked for me was this:

Create groups of around 500–1500 plants (If your PC allows it, you can do more)
Convert those groups into separate instances/actors
Place them manually by sections in the scene
Enable Evaluate World Position Offset in Ray Tracing for those closer plants

For cinematics, this workflow works well because only what is close to the camera really needs animated wind. I keep foreground foliage moving, while background foliage stays as normal static foliage.

Also, background foliage and small grass covering the ground stay static as well. Since ground grass is usually small, the lack of movement is barely noticeable

I would also recommend using this command:

r.RayTracing.Geometry.StaticMeshes.WPO.CullingRadius 100000

(1000m distance)

This allows objects farther away from the camera to keep their WPO animation active, which helps if you have converted actors farther in the background.

One important thing: after converting foliage into static mesh actors, remove those same plants from the foliage tool / painted foliage system. If you keep both versions at the same time, it can create overlapping instances that may look like flickering or cause strange behavior.

My recommendation is to keep your foliage tool mostly for static support foliage only — background plants, filler vegetation, and grass that helps populate the scene. Then only convert and instance the foliage that actually needs movement. In cinematics, only what is near the camera usually matters, so this approach keeps scenes looking dense while avoiding unnecessary animation calculations.

An extra benefit is that this approach saves a lot of resources and makes Path Tracer scenes render significantly faster.