UE 5.7 - Set Niagara Vector4 Array only works during Simulate?

Development Programming & Scripting
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1

I’m using a GPU Niagara Grid2D Simulation Stage to write to a Render Target.

The system works perfectly during PIE/Simulate, but not while evaluating in the editor or during Movie Render Queue.

What I’ve verified:

  • UE 5.7
  • GPU Compute Sim
  • Grid2D Collection
  • Simulation Stage = Always
  • Niagara Component active
  • Niagara Life Cycle Track added
  • Blueprint updates every Sequencer frame using a Repeater Event
  • Niagara Set Vector4 Array (User.LocationsAndSizes) is called every frame

Observed behaviour:

  • Simulate: Grid2D updates, Render Target updates, grass bends.
  • Outside Simulate: Render Target never updates.

I also tested:

  • Niagara Set Vector4 Array
  • immediately followed by
  • Niagara Get Vector4 Array

Inside Simulate this behaves as expected.

Outside Simulate it appears the array never becomes available.

Is UNiagaraDataInterfaceArrayFunctionLibrary::SetNiagaraArrayVector4 expected to work outside a simulated world, or are Array Data Interface updates only propagated while the Niagara Component is actively simulating?

2

the simulate only behavior has a simple root cause: a GPU compute sim stage only executes when the niagara component actually ticks the simulation. in PIE the game loop ticks it every frame so your grid writes land. the plain editor viewport does not run the live sim (that is what the Simulate button on the asset is for, expected), and during MRQ the default deferred pipeline effectively pauses the game loop between frames, so the component never ticks and the render target writes never execute even though your repeater fires and the system looks active.

what fixes it in practice:

for sequencer/MRQ, the owning actor has to be allowed to tick while the game is paused between frames: the actor details has the tickable when paused setting (SetTickableWhenPaused in code). with that on, the component keeps simulating during MRQ shots and the RT updates per frame. activating on camera cut plus a few warmup ticks also helps the first frames not be empty.

drive the system once, not every sequencer frame: set your vector4 array parameter once on activate instead of pushing it in a repeater each frame, then let the sim run. the repeater approach works in PIE mostly because PIE ticks everything, which is why it reads as a sequencer bug when it is a ticking one.

on the variable side there is a second wall hiding behind this: stock blueprint exposure for niagara variables is incomplete, and several types simply have no set node, vector4 and its arrays included, plus everything outside the user namespace is unreachable. if the goal is setting those arrays from blueprint or c++ every frame, the Expose Niagara Variables C++ & Blueprints plugin covers exactly that gap, 18 variable types across all 8 niagara namespaces including the ones not exposed by default, with the getters too, in blueprint and c++: Expose Niagara Variables C++ & Blueprints | Fab