Liquid Motion Blur

[Indigo_1]

Please see the attached images on what my render looks like when I enable motion blur with Phoenix liquid. I can't figure out what's going on. Can anyone help?

[Joelaff]

The motion blur essentially draws streaks based on the velocity stored with each particle. This looks like the motion blur is much too high, try adjusting the Motion blur multiplier to 0.1 or something like that. This is in the Rendering rollout of Phoenix, IIRC.

[Indigo_1]

I don't think that's the problem. I created the grid and simulated. Rendered a test frame and it worked as expected. Closed the file, opened it back up, did another test render and it's all over the place again. Makes no sense. I made no changes between the 2 renders.

I'm using Phoenix 4, V-Ray 5, and max 2021. I saved down to max 2020 that has Phoenix 3 and V-Ray 3 and did another test render. Same results...

[Joelaff]

So the SAME frame rendered differently between restarts of Max? That really looks like vector motion blur gone haywire, which it does quite a bit. I have seen issues sometimes where indeed ONE frame will have motion vectors that are way too big.

If you export the particles to PRT, and have Krakatoa you could set a maximum velocity for the particles pretty easily with a Magma Modifier.

So you tried lowering the Motion Blur Multiplier in the Rendering Rollout? No effect?

[Indigo_1]

I tried rendering multiple different frames including the same one. I don't have Krakatoa. I did lower the Motion Blur Multiplier in the Rendering Rollout and it decreased the streaking but it was still awful. Even if I lowered it to .001

[Joelaff]

OK. So it is motion blur related.

This is all liquid voxels, right? Not particles? Do you have Foam or Splashes or anything?

Perhaps try more steps per frame in the Dynamics rollout to see if the simulation can work itself out without these flyers. Turn off foam and splashes, etc. and just try the voxels and see if they work right.

Otherwise you might look at the new Voxel Tuner and or Particle Tuner to limit the velocity of particles or voxels during the simulation. (Test if speed greater than X then set speed to X).

https://docs.chaosgroup.com/display/...Particle+Tuner

If you can post a scene that would help. I am sure the Phoenix crew will comment soon as well.

[georgi.zhekov]

Hey,

Are you using the latest Phoenix build? It is Phoenix 4.20 and you can get it from the download section at chaosgroup.com

Can you show us your settings for the motion blur - are you using the V-Ray settings or the camera ones?

If you render the same frame a few times do you still get a different result? What if you restart Max?

[Svetlin.Nikolov]

Just some more info - older Phoenix 3 had motion blur length issues, but we don't know of any with Phoenix 4.

As Joe said, it could be either a rendering problem, or you could be getting too high velocities from the simulation - have you boosted the Motion Velocity multiplier for any Source or from any Right-click Phoenix properties?

A couple more things you should check out:
- Make sure Grid Velocity is exported in the Output rollout when you simulate. Otherwise you'll get velocity only each 15th frame by default - this is the default for the Backup Interval when velocity is written to cache files so you can Restore the simulation from these backup frames after you have stopped it.
- The voxel Velocity preview is your friend - enable Velocity under the Preview rollout and see if there aren't any weird velocities, much larger than the rest.

Cheers!

[Indigo_1]

Georgi

I am using Phoenix 4.20. If I render the same frame multiple times I get the same result. No difference if I restart max.

Svetlin

Haven't boosted the Motion Velocity multiplier. The Grid Velocity is exported. When I first created the grid and ran the sim, everything looked correct. Once I closed max and reopened the file is when everything is messed up even though I made no changes. I just changed my shutter speed from 200 to 800 and it started to look correct. If I leave the shutter speed at 200 and change the Phoenix motion blur multiplier in the render section even as low as .00001, it's still messed up. It doesn't make sense though that before I closed the file it worked with my original camera settings, but not when I reopen the file. So, I feel like something else is going on.

Here's the file if you don't mind taking a look. I was rendering frame 10.

https://we.tl/t-96VpWXe0lp

Thanks guys for any help you can provide!

[georgi.zhekov]

Thanks for the scene! Indeed looks like there is something fishy with it.

We'll update the thread once we know something more.

Cheers!

[Indigo_1]

Hi Georgi,

Just wanted to check in and see if you have any new info on this?

Thanks!

[Svetlin.Nikolov]

Hey,

I'm gonna need a couple more days to get there. As promised, we won't forget to update you when there is progress

Thank you!

[Indigo_1]

Cool. Thanks!

[Svetlin.Nikolov]

Hey, back with some news:

Looks to me like there are a few things going on in the scene, maybe separate.

First off, I wanted to use the velocity preview in order to figure out what's up with the velocities and if anything abnormal shows up in the viewport. However, turned out that the minimum range for both the voxel velocity preview and the streamlines velocity preview were capped to at most 10000, while in this setup the velocity goes up to 30+ K. In tomorrow's nightlies this will be fixed.

So after this I found some strangely symmetrical velocity streamlines going between the two wheels. The reason for those could be that due to the huge velocity range, the default AUR cache compression loses some data, so my first suggestion would be to try out simulating with higher Storage Quality - like 17 or 18, just as a test before tomorrow's nightly comes out. If this helps, then we are looking at the right thing. Another thing you could try would be to enable Grid Channel Smoothing under the Input rollout for the Velocity channel and set all 3 smoothing options to 0. This should smooth out the velocity field and should remove some of the velocity spikes too. You can combine both the Storage Quality solution and the Velocity Smoothing.

And a final thing that I'm also gonna change for tomorrow's nightlies - in the container, the velocity is made up of two factors - the velocity of the fluid inside the container, and the inertial velocity of the moving container in the scene. The Motion Blur Multiplier was affecting only the fluid velocity, but the inertial velocity was always at full strength. In tomorrow's nightlies the Motion Blur Multiplier will be able to scale both to 0, so you'd be able to dial down these velocity spikes even if my above suggestions fail.

Thanks very much for reporting this and please ping us when you've had the chance to test it.
Cheers!

[Indigo_1]

Thanks for the info! I'm about to try your suggestions and I'll report back as soon as I can.