Help Needed: Perfect Motion Vectors for Water – FLIP + Ocean Evaluate Blending Issue

[Luchino00]

Hi everyone,
I’m facing a blocking issue in my production and would really appreciate your insights.

I’m trying to get perfect motion vectors for a water simulation, but I’m stuck.
When I mesh my FLIP sim and pass it through Ocean Evaluate, I run into this problem:
    •    If I don’t enable Point Velocity, I only get the FLIP velocity, no ocean motion.
    •    If I do enable Point Velocity, I get the Ocean Evaluate velocity, but I lose the FLIP velocity.
    •    I’ve tried adding, blending, lerping, projecting the two velocities together in various ways, but I’ve never gotten a truly satisfying result.

To test this, I use a simple validation:
I switch between a Timeshift ($F+1) and an Attribute Wrangle that does:

@P += @v / 24.0;

If the motion vectors are correct, the resulting positions should match frame-to-frame, and there should be no visible jump when toggling between them.

Unfortunately, I haven’t found a way to properly merge the FLIP and Ocean Evaluate velocities to pass this test.

Context:
This is for a VR film using PresenZ technology, which heavily relies on motion vectors for temporal interpolation.
If the water motion vectors aren’t accurate, it causes stuttering and tearing in playback.
So I really need motion vectors that seamlessly combine the FLIP sim and the procedural ocean motion from the spectrum.

If anyone has solved this or has suggestions—whether it’s a different way to derive v, use a Trail SOP after the blend, or something else—I’d be extremely grateful.

Thanks in advance!

[haggi]

I do not understand what you mean by mesh my flip sim and pass it through the ocean evaulate. How do your flip looks like, do you have some screenshots? Usually there is no need to feed the flip sim into an ocean evaluate unless you simulate it flat and want to displace it later.

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