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Can someone explain to me why the motion blur behaves so strangely when using simulation nodes? Even if I give the individual points their own ID, a motion blur is strangely rendered, but at the same time the instance is overlaid without a motion blur. What is this? Only blurred lines should be visible, not sharp cubes. Does anyone understand this?

My Node Setup:

enter image description here

The Result: enter image description here

Thanks for help!

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  • $\begingroup$ For every frame you add new cubes to the simulation zone, via the Join Geometry node. These new cubes has no movement until the next frame. $\endgroup$ Commented Apr 12, 2024 at 10:53
  • $\begingroup$ You're right, but that doesn't matter because the cubes are added at the bottom. All the cubes shown in the picture are part of the simulation node loop and were not just added. $\endgroup$ Commented Apr 15, 2024 at 6:55

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Can't tell from your screenshots but if your playhead is on the last cached (or baked) frame, this result is to be expected. The next frame needs to be cached for motion blur to work properly (unless it is set to "End on Frame").

Things to bare in mind:

  1. Regardless of whether you have a simulation or not:

    • Be wary of integers, they can affect sub-frame interpolation: enter image description here

      enter image description here

      enter image description here

      enter image description here

    • Make sure the relevant IDs (Points, instances or vertices) are unique and stable. For that, you can display the ID in the Viewport using the Attribute Text Overlay: Attribute Text Overlay screenshot then look for either:

      • IDs that change from one frame to the next when they should stay unchanged, in which case you may want to reorder the inputs in the Join Geometry node (or it could be something else in your node tree)
      • IDs that are the same in multiple different places, in which case you need to use the Set ID Node with some operations that ensure each element has a unique ID. E.g. if you have a nested sim like exploding sparks, you may want the child particle IDs to be "prefixed" by their parent ID by multiplying the parent ID by a round number to have enough zeroes to add the child ID in.

      There's an open issue about the subject.

  2. Motion blur with simulation nodes is currently limited and buggy If the same motion can be achieved without a simulation zone and without lowering performance, replacing it by other nodes may prevent some motion blur limitations and yield better results. If however, a simulation is the best/only way:
    • Before rendering, either cache the next frame or bake your simulation. Otherwise the cache may break.
    • Don't expect sub-frame steps to be evaluated, they're not. Whether in Eevee or Cycles. The motion is linearly interpolated between frames.

There's also a long in-depth tutorial about the broader subject of motion blur troubleshooting with Geometry Nodes.

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