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gravity does not apply when I manually set the fixed delta time - lufydad - 10-01-2022

Hello,

When I set the fixed delta time from script to 1/60 in an empty scene with just a rope, the rope just stay still, the gravity does not seem to apply on it. If I change the fixed delta time to another value the rope falls down normally.

I use Obi 6.2, Unity 2021.2.4f, and DX11.

Is there something that I can do to fixed this ?


RE: gravity does not apply when I manually set the fixed delta time - josemendez - 10-01-2022

(10-01-2022, 11:17 AM)lufydad Wrote: Hello,

When I set the fixed delta time from script to 1/60 in an empty scene with just a rope, the rope just stay still, the gravity does not seem to apply on it. If I change the fixed delta time to another value the rope falls down normally.

I use Obi 6.2, Unity 2021.2.4f, and DX11.

Is there something that I can do to fixed this ?
Hi!

Just set the solver's sleep threshold accordingly. This is the minimum kinetic energy above which particles are allowed to move. If you reduce the timestep, the kinetic energy during a single timestep will also be reduced, preventing any particle whose energy is below the threshold to move. See:
http://obi.virtualmethodstudio.com/manual/6.3/obisolver.html

You can set it to zero if you want particles to move regardless of how much energy they have. Note this may result in positional drifting/jittering.


RE: gravity does not apply when I manually set the fixed delta time - lufydad - 10-01-2022

(10-01-2022, 11:30 AM)josemendez Wrote: Hi!

Just set the solver's sleep threshold accordingly. This is the minimum kinetic energy above which particles are allowed to move. If you reduce the timestep, the kinetic energy during a single timestep will also be reduced, preventing any particle whose energy is below the threshold to move. See:
http://obi.virtualmethodstudio.com/manual/6.3/obisolver.html

You can set it to zero if you want particles to move regardless of how much energy they have. Note this may result in positional drifting/jittering.

Thank you a lot it seems to work, when I reduce the sleep threshold  Sonrisa
In my application the fixed delta time is entered by user, which sleep threshold should I use to be sure to not have this problem anymore and avoid drifting and jittering ? dt/10 is enough ?


RE: gravity does not apply when I manually set the fixed delta time - josemendez - 10-01-2022

(10-01-2022, 11:35 AM)lufydad Wrote: Thank you a lot it seems to work, when I reduce the sleep threshold  Sonrisa
In my application the fixed delta time is entered by user, which sleep threshold should I use to be sure to not have this problem anymore and avoid drifting and jittering ? dt/10 is enough ?


Allowing your user to set an arbitrary fixed delta time is a recipe for terrible performance, but assuming you're not concerned by this: You could use the smallest value that works with the smallest delta time you allow.

Or instead of using a very small threshold (which may not be enough for very small timesteps), you can calculate the value you need for a given timestep.
Kinetic energy is defined as 1/2 * sqr(velocity) * mass. In Obi, the threshold is mass-normalized, so not multiplied by mass.

Velocity is defined as change in position over time. In Unity, this "time" is the timestep (aka fixed delta time) value. So:

Code:
velocity = (position at the end of the timestep - position at the start of the timestep) / timestep.

Hence, kinetic energy is:

Code:
1/2 * sqr((end position - start position) / timestep).

This is the value compared to your sleep threshold to determine if a particle can move or not.
To make the threshold independent of timestep, you can do:

Code:
timestepIndependentThreshold = sqrt(threshold * 2) * timestep;

And then feed this value to Obi instead of a fixed threshold.


RE: gravity does not apply when I manually set the fixed delta time - lufydad - 10-01-2022

(10-01-2022, 11:48 AM)josemendez Wrote: Allowing your user to set an arbitrary fixed delta time is a recipe for terrible performance, but assuming you're not concerned by this: You could use the smallest value that works with the smallest delta time you allow.

Or instead of using a very small threshold (which may not be enough for very small timesteps), you can calculate the value you need for a given timestep.
Kinetic energy is defined as 1/2 * sqr(velocity) * mass. In Obi, the threshold is mass-normalized, so not multiplied by mass.

Velocity is defined as change in position over time. In Unity, this "time" is the timestep (aka fixed delta time) value. So:

Code:
velocity = (position at the end of the timestep - position at the start of the timestep) / timestep.

Hence, kinetic energy is:

Code:
1/2 * sqr((end position - start position) / timestep).

This is the value compared to your sleep threshold to determine if a particle can move or not.
To make the threshold independent of timestep, you can do:

Code:
timestepIndependentThreshold = sqrt(threshold * 2) * timestep;

And then feed this value to Obi instead of a fixed threshold.

Thank you for taking the time to explain.
If I may ask one last question, I don't understand the last equation of your post. Is "threshold" in the equation the member sleepThreshold of obi solver ?


RE: gravity does not apply when I manually set the fixed delta time - josemendez - 10-01-2022

(10-01-2022, 02:43 PM)lufydad Wrote: Thank you for taking the time to explain.
If I may ask one last question, I don't understand the last equation of your post. Is "threshold" in the equation the member sleepThreshold of obi solver ?

Hi!

"Theshold" is the (time-dependent) input threshold you want to use, this is a value you set. The equation outputs a time-independent threshold, which is the one you feed to the solver's sleepThreshold.

So the equation sits in between your hand-picked threshold value, and the one fed to Obi.


RE: gravity does not apply when I manually set the fixed delta time - lufydad - 10-01-2022

(10-01-2022, 02:44 PM)josemendez Wrote: Hi!

"Theshold" is the (time-dependent) input threshold you want to use, this is a value you set. The equation outputs a time-independent threshold, which is the one you feed to the solver's sleepThreshold.

So the equation sits in between your hand-picked threshold value, and the one fed to Obi.

Ok, then suppose I input 0.001 for the sleepThreshold at the beginning of the simulation and the user input 1/60 for the fixedDeltaFrame, then using your equation :
Code:
timestepIndependentThreshold = sqrt(0.001 * 2) * 1/60 = 3.33*1e-5

I should put the sleepThreshold to this value, is this correct ?


RE: gravity does not apply when I manually set the fixed delta time - josemendez - 10-01-2022

(10-01-2022, 02:56 PM)lufydad Wrote: Ok, then suppose I input 0.001 for the sleepThreshold at the beginning of the simulation and the user input 1/60 for the fixedDeltaFrame, then using your equation :
Code:
timestepIndependentThreshold = sqrt(0.001 * 2) * 1/60 = 3.33*1e-5

I should put the sleepThreshold to this value, is this correct ?

Yes, that should work.


RE: gravity does not apply when I manually set the fixed delta time - lufydad - 10-01-2022

(10-01-2022, 02:59 PM)josemendez Wrote: Yes, that should work.

Great !! Once again, thanks a lot Gran sonrisa


RE: gravity does not apply when I manually set the fixed delta time - josemendez - 10-01-2022

(10-01-2022, 03:06 PM)lufydad Wrote: Great !! Once again, thanks a lot Gran sonrisa
You're welcome! Gran sonrisa