Rendertime artefacts on backup frames

[JuhaHo]

Has anyone else seen this? After a month on simulation I was able to move the simulation to my render scene. I noticed that every 15th frame there's some artefacts on rendered frames. I noticed the affected frames are the backup frames, while the other frames are rendering correctly. I'm using v3.14.01 nightly. build 2019081729430.

[Svetlin.Nikolov]

Yup, check it here: https://docs.chaosgroup.com/display/...Phoenix+FD+FAQ

[georgi.zhekov]

Hey,

Probably you have motion blur enabled for the render, but you have not simulated with the velocity channel enabled in the Output rollout.
As every backup frame contains velocity data so you could restore the simulation from there - the backup frames will render with motion blur while the rest of the frames won't.

Cheers,

[JuhaHo]

Thanks, I saw that in the document linked by Svetlin. I just noticed I might have enabled the wrong velocity of the two. I seem to have the grid velocity not enabled. Should I enable that too?

Anyway the artefacts are quite far from the liquid. I see no liquid nowhere near there where this behavior occurs. I wonder if it's fixed too if I simulate the scene again.

[Svetlin.Nikolov]

You need grid velocity for the liquid mesh, and particle velocity for the particles rendered by the particle shader.

How do the artifacts look like? Do you have any count multipliers set up in the particle shaders or play speed different than 1.0?

[JuhaHo]

OK, I started the simulation again, now with Grid velocity enabled. I should see in a month or so if it helped.

I remember that earlier, I had to render some animations without motion blur, because of tearing effect on liquid. It happened, when there was quickly moving liquids. There was nothing to see on viewport meshed liquid; they existed only on rendering. Is their velocities in sub frames, while motion blur made it visible?

[Svetlin.Nikolov]

Huh, why a month? :O

The motion blur of fluids has no subframes - instead, vertex velocities are used. This is due to the changing topology of the mesh - at each new frame new faces appear and disappear.