[Sidefx-houdini-list] pop previous velocity

Ben Schrijvers ben.schrijvers at guerrilla-games.com
Tue Apr 17 06:01:27 EDT 2007


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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