[Sidefx-houdini-list] pop previous velocity

Pablo Giménez pablogipi at gmail.com
Tue Apr 17 13:15:15 EDT 2007


2007/4/17, Luca Zappala <luca at cinesite.co.uk>:
>
> @Pablo
> It would most definitely work but it wouldn't be very efficient
> (increased disk access times). I guess the trail sop would be an overall
> faster solution.


I think not, cacho SOP works on RAM not on disk.
If you use a very little cache, about three frames it is not so slow.

@Ben
> I tested velocity minus acceleration but it gives me wrong oldVel values
> (in the range of 4 times what I expected).
> I've also tried to create an attribute oldVel at the end of the popNet
> (using current velocity as values) and to cache the popnet outside by
> using the rop driver node; but oldVel and vel seem to still be the same.
> Any ideas ?
>
> Thanks a lot for your help
> Luca
>
>
>
> Ben Schrijvers wrote:
> > I would say that when you add some Attribute POP's at the end of the
> > chain to create an oldVel or prevAge attribute.
> > Just set it to $VX,$VY,$VZ.
> > Then next frame that attribute should be the previous frame velocity
> > or age.
> >
> > In fact I believe that while at the current frame you always access
> > the previous frame attributes.
> > So really you are talking about the one before the previous... but
> > let's forget about that as it makes thinking more complicated.
> >
> > Another way to get previous velocity is to subtract the acceleration
> > from the velocity. oldVel = velocity - acceleration
> > Acceleration is the change in velocity and so is the difference
> > between the velocity and previous velocity.
> >
> > In the same way you could calculate the previous age as you know the
> > duration of a frame.
> > oldAge = Age - ( (1/$FPS) / ch("../oversample") )
> >
> > succes,
> > benS
> >
> >
> > Luca Zappala wrote:
> >> Ok, I found a decent solution by storing the v to n, use the trail (1
> >> frame) and compute velocity, copy v to oldVel, copy n back to v. This
> >> way I get correct oldVel except for the last frame before the
> >> particle dies, but I can live with that.
> >>
> >> Now I'm getting stuck with the particle Age (life[0]): I would also
> >> like to store the previous frame's particle Age.  :)
> >> Any ideas?
> >> I guess this would be an interested topic on how to store simulation
> >> variables at a different time from current time.
> >> thanks
> >> Luca
> >>
> >> Luca Zappala wrote:
> >>> Hello there,
> >>>
> >>> I'm trying to cache the particle oldVel (velocity at the previous
> >>> frame).
> >>> I read the upvector node should create a previousVelocity local
> >>> variable (PV) but this doesn't seem to be working properly, in fact
> >>> PV is the same as V.
> >>> Do you have any other idea on how to store the oldVel as well?
> >>>
> >>> thanks a lot
> >>> Luca
> >>>
> >>>
> >>>
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-- 

Un saludo
Best Regards
Pablo Giménez



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