[Sidefx-houdini-list] nonLinear fluid reTime

Olex P hoknamahn at gmail.com
Fri Jul 8 06:15:13 EDT 2011


I agree in some situations it works (clouds in the sky?). While in
others like let's say a plume of steam from a steam train (initial
velocity is about 50-75 m/s) it wouldn't work.
Plus particles have other benefits like additional details, rest positions etc.

On Fri, Jul 8, 2011 at 11:06 AM, Szymon Kapeniak
<szymon.kapeniak at gmail.com> wrote:
> 2011/7/8 Olex P <hoknamahn at gmail.com>:
>> I don't think that blending of two volumes in substep makes any sense
>> especially if a difference in state of the fluid in two frames is high
>> enough.
>
> Well, people are using this technique to retime their smoke sims,
> Olex. I understand why blending volumes works worse than particles,
> but still it seems to be enough for most retiming cases, since frames
> tend to be similar.
>
> Agree though, particles from volume might work better for high displacements.
>
>  But we can jump from Eulerian coordinate system (grids with
>> static voxels and quantities going through this grid) to Lagrangian
>> (particles where every particle is a local coordinate system with some
>> quantity, let's say density, on it).
>> In other words we can advect particles using velocity field,
>> interpolate particle positions in subframes and put density and other
>> needed attributes back into a grid.
>>
>> On Fri, Jul 8, 2011 at 10:33 AM, Szymon Kapeniak
>> <szymon.kapeniak at gmail.com> wrote:
>>> Hey Ben,
>>> fluids are volumes (SDF), so they could be interpolated by Timeblend,
>>> Sequenceblend and Timeshift Sops, similarly to plain geometries. I've
>>> never tried it on fluids, but theoretically this should work the same
>>> as on smoke (unless interpolating SDF is more computational
>>> problematic, than density fields).
>>>
>>> So I would try to increase a little su- stepping to provide good
>>> samples for extreme interpolation, and retime my SDFs (taken straight
>>> from DOPS, without meshing), after which meshing should be possible as
>>> usual.
>>>
>>>
>>> hth,
>>> skk.
>>>
>>> 2011/7/8 Ben Frost <benfrost3d at gmail.com>:
>>>> hi all..
>>>>
>>>> I'm looking for anyones opinion regarding a non-linear reTime of a fluid
>>>> sim.. and possible approaches to take.. the reTime is quite severe.. going
>>>> from 24fps to around 432fps.. so 18 times slower.. and is based on a
>>>> particle sim..
>>>>
>>>> at the moment I'm using a brute force method for having the subSteps at 18,
>>>> writing out the subFrame for each step.. and then read it back in, and
>>>> reTiming.. this works.. but as you can imagine the sim times goes through
>>>> the roof a bit.. and change the sim some what.. although it's more accurate
>>>> it does create the problem of previewing the simulation at 18 subSteps..
>>>> ideally I'd like to simulate at as fewer subSteps as possible.. until i'm
>>>> happy with the sim.. and then upRess and bake out subFrames/subSteps..
>>>>
>>>> we've reTimed the particles using the velocities to interpolate and get us
>>>> the subFrames needed.. not sure there is a way to interpret this to fluid
>>>> though..
>>>>
>>>> and to make things worse.. I've been shown an animatable "sim scale" option
>>>> in maya.. which works all too well.. I noticed houdini has the same option
>>>> of sim scale, but this is only able to be set at sim start.. just wondering
>>>> if this is possible to be manipulated and animate..?
>>>>
>>>> anyway I'd be great to hear your opinions or possible more efficient less
>>>> brute force approach..
>>>>
>>>> cheers
>>>>
>>>> ben..
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>>>
>>>
>>>
>>> --
>>> skk.
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>
>
>
> --
> skk.
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