Retimed water drops chatter

[ttdww]

Running PFD 3.11. Did an ocean sim with correct output channels for splashes/mist/foam and rendering it at 50% speed (0.5). After reviewing frames, the drops are all chattering, plus the velocity channel makes it even worse (using post vector motion blur). The full speed (non slowed version) plays smooth. Was this something addressed in 3.12 or later, or a new issue? I have input blend set to 'interpolation' - maybe stop render and try Precise Tracing? Deadline soon - not sure if I should start render over again or if it wouldn't help. Thanks!

[Svetlin.Nikolov]

Hey,

Sorry but I can't follow you - which particle groups are chattering - the liquid mesh, or the foam or splash? Depending on your answer there are different things you could do. Do you mean you are using the velocity from a render element to add moblur in post, or is it visible in the render with motion blur enabled? If you can share all your settings plus a video of how the effect looks, it would be very helpful so I can help you with the settings.

Cheers!

[ttdww]

Sure thing - will put together a sample. We can save the velocity post mblur for another topic. The chatter shows for all the particles when I render at .5 input speed, set to interpolation. Was wondering if blend method set to either 'velocity' or 'precise tracing' might give better results for the particles. I do have those channels saved.

[ttdww]

(You'll need to right-click download these as they aren't browser compliant)

Here are 2 full res cropped renders showing left side and right side:

http://vfxfactory.com/movies/Ocean_S...pRawRender.mov
http://vfxfactory.com/movies/Ocean_S...pRawRender.mov

Here's a crop/blowup of flickering splash/mist. If you can't see it easily at full speed, step thru frame by frame and you'll see the it flicker from more of a black/white drop pattern, to almost solid white - back and forth:
http://vfxfactory.com/movies/Ocean_S...CropBlowup.mov

[ttdww]

#2 of this is that I'm also seeing a different chatter - when splash/foam drops go above/below the liquid (even happens are full speed 1.0). Should I raise the fluid surface meshsmooth? or do I need to go further and blur the liquid channel? You can see it happening at the far right of this clip, near the end of the clip: http://vfxfactory.com/movies/Ocean_S...pRawRender.mov

If I render all 3 features separately (fluid surface, splashes, foam), the chatter is not present

[Svetlin.Nikolov]

Hmm, first question - do you have any geometry under the ocean, serving as a bottom and do you have fog in your V-Ray material?

[ttdww]

I have a Vray plane set to dark blue and the ocean is basically a clear 100% refractive reflective (fresnel) material. I actually have it between the top of the water and the bottom of the sim box.

[ttdww]

I'm thinking of just going for Twixtor to get through this delivery so the tween-frames 0.5 speed isn't the pressing issue currently. Now my concern is the rippling flashing white seen at the far right. I've done tests today and it's happening when the foam is appearing above and below the water on random frames. I've tried high meshsmooth and it's still happening.

Here is a sample showing it on the far right - check out the last 10 frames:
http://vfxfactory.com/movies/Ocean_S...ideFlicker.mov
(this is retimed so it warps a little)

When I render only the water, foam, splashes, there's no chatter. My thought is maybe I can render the particles along with a falloff volume for the water mesh (like a volume grid of black fog), so I'm just gently fading the foam off as it goes underwater instead of the hard-edge cutoff caused by the refractive water material.

Thoughts?

[ttdww]

Wondering how to get this black fog idea to work - have tried volumetric, volu..geo, and heathaze and the water mesh just renders black.

Or my preference would be if you have any thoughts as to how I can change the refractive water shader so that the foam just under the surface is just as bright/visible as above water so the rays aren't diffused so quickly by the shader IOR. Almost like we'd want the deeper you go into the water, then the IOR can change from 1.0 to 1.5ish. That'd work great - then I can do this as and all-in-one render with no chatter.

I've messed with the above/below particle shader params and it has no effect when set to 'all'. It's the vray refractive shader that's chopping the foam off so drastically. It's a bummer too because I'm losing 90% of my foam underwater. If you look at that QT again, if that water mesh wasn't there, you'd see pretty much almost nothing but foam. That's how much of it is missing.

[Svetlin.Nikolov]

Hmm, my second suggestion would have been what you already figured out - foam is being born on exact frames, so when slowing down the animation, entire chunks of foam appear, and this is exacerbated by the IOR of the liquid surface. In case you DIDN'T have a V-Ray plane underneath, you'd hit another pitfall where V-Ray cannot shade particles underwater a mesh with a fog in the material, because V-Ray knows where the fog ends only when the mesh is closed by another geometry, and the Phoenix ocean is not. This is the reason why particles underwater might start to flicker or seemingly disappear altogether in animation. Seems like this might not be the issue in your scene though.

An alternative I can think of would be to render in two passes - foam only, and everything else. This way you won't have any refraction for the foam, and you can also use the Particle Shader's underwater tint option to gradually color the submerged particles. This might also be much faster than rendering the foam together with the liquid mesh, which gets very slow exactly because of the refractions the shader has to calculate all the time.

This is not going to sound good, but since 3.11 we did an improvement exactly to the particle blending when particles are being born or die, but since this might not be the central issue in your case, I can only advise you to try a latest nightly, but if time is very short and you think the main issue is because of refraction, you might skip this and try separate rendering and comping of the liquid + splash and the foam.

Cheers!

[ttdww]

Great - thanks so much for that info Svetlin! So what I'd really like to do is to fade off the foam as it goes down below the surface. How would I set up a volumetric for the water so it's like a controllable fog falloff? I can render this active with the foam. Thank you!

[Svetlin.Nikolov]

The underwater tint of the particle shader, combined with the liquid turbidity will make foam particles inside the Liquid Simulator's volume fade towards that color - they will not lose alpha, but for example become darker and darker or more blue the farther they are from the surface. However, using these settings would require you to render the foam in one pass and the liquid mesh + splashes in another. If this is not an option for you, the particle shader does not support variable alphas per particles at the moment - only the color can vary per particle.

[ttdww]

I could see that working well - even if the alpha doesn't fade. I can use it as a Luma pass and set the tint color to black and the IOR to 1.0 (or close to it) so the foam rays don't diffuse away. The only thing to figure out is how to keep foam within waves further away from being seen - the camera is very low to the water and it'll look like one giant mass of foam. Zdepth will help some - I'll run some tests. Thanks Svetlin!