Help Zero-g

[innovine]

I would just like a quick confirmation before I buy, since I didn't see this answered anywhere...  Obi Rope will work with zero-g environments, right?  Currently I am disabling the Gravity option on individual Rigidbodies, but if necessary I can also do Physics.gravity = Vector3.zero;

[josemendez]

I would just like a quick confirmation before I buy, since I didn't see this answered anywhere...  Obi Rope will work with zero-g environments, right?  Currently I am disabling the Gravity option on individual Rigidbodies, but if necessary I can also do Physics.gravity = Vector3.zero;

Hi!

Obi solvers have their own gravity settings, and you can of course set it to zero for zero-g simulation.

[innovine]

I have two small mass (1kg) rigidbodies, one on each end of the rope (particle mass 0.05. It seems to always come to a rest with the rope being straight, but with some angular velocity remaining. Why is the rope straight? The masses do not have any drag set on their Rigidbodies. I expect the system to bounce and twist indefinitely. Damping in the solver is set to 0. Is there some kind of internal elasticity in the rope causing it to straighten out?

If i increase the mass of the particles, the rope spawns in knots and looks very agitated, spiraling like crazy. Why is this?

[josemendez]

I have two small mass (1kg) rigidbodies, one on each end of the rope (particle mass 0.05. It seems to always come to a rest with the rope being straight, but with some angular velocity remaining. Why is the rope straight? The masses do not have any drag set on their Rigidbodies. I expect the system to bounce and twist indefinitely. Damping in the solver is set to 0. Is there some kind of internal elasticity in the rope causing it to straighten out?

If i increase the mass of the particles, the rope spawns in knots and looks very agitated, spiraling like crazy. Why is this?

The rope has internal bending resistance, so it always strives to be completely straight. Disable bend constraints or set their stiffness to zero.

As with any physics engine, large mass ratios reduce stability. Unity recommends keeping the mass ratio under 1/10 at all times. You can either increase the mass of the rope particles, or decrease the mass of the bodies.

[innovine]

Ahh, removing the bend constraints is a lot nicer now, thanks.

Unity recommends keeping the mass ratio under 1/10 at all times. 

And where do they recommend that?

[josemendez]

Ahh, removing the bend constraints is a lot nicer now, thanks.


And where do they recommend that?

In the official docs:

https://forum.unity.com/threads/rigidbody-mass.343761/

https://docs.unity3d.com/ScriptReference...-mass.html

Initially they mentioned that "You should strive to keep mass close to 0.1 and never more than 10. Large masses make physics simulation unstable.". Later they replaced it with the more correct "Different Rigidbodies with large differences in mass can make the physics simulation unstable.", as it is the mass ratio, not the mass itself what you need to keep an eye on. the mass of objects can be any value you want, as long as the ratio between two connected or touching objects is not larger than 10: 100 and 1000, 1 and 10, 1000 and 10000, etc.

By the way this holds true for all iterative solvers (PhysX, Havok, ODE, pretty much all game physics engines in existence). It's a basic consequence of how solvers work, which is turn a consequence of how time is represented in computers (in a discrete -non continuous- way).

[innovine]

In the official docs:

https://forum.unity.com/threads/rigidbody-mass.343761/

https://docs.unity3d.com/ScriptReference...-mass.html

Initially they mentioned that "You should strive to keep mass close to 0.1 and never more than 10. Large masses make physics simulation unstable.". Later they replaced it with the more correct "Different Rigidbodies with large differences in mass can make the physics simulation unstable.", as it is the mass ratio, not the mass itself what you need to keep an eye on. the mass of objects can be any value you want, as long as the ratio between two connected objects is not larger than 10: 100 and 1000, 1 and 10, 1000 and 10000, etc.

By the way this holds true for all iterative solvers (PhysX, Havok, ODE, pretty much all game physics engines in existence). It's a basic consequence of how solvers work, which is turn a consequence of how time is represented in computers (in a discrete -non continuous- way).

Isn't 0.1 - 10 a ratio of 100 though, not 10 as you keep saying? Where did you get 10 from? 
How is one supposed to implement any realistic looking physics if the masses are all wrong? Say I have a 1000kg car and it crashes into a table of small objects of 1kg. What is recommended, make apples 100kg, or make my car 10kg?

[josemendez]

Isn't 0.1 - 10 a ratio of 100 though, not 10 as you keep saying? Where did you get 10 from? 
How is one supposed to implement any realistic looking physics if the masses are all wrong? Say I have a 1000kg car and it crashes into a table of small objects of 1kg. What is recommended, make apples 100kg, or make my car 10kg?

This is a widely known limitation of realtime physics in general.

Yes, 0.1 - 10 is a ratio of 100, however things begin to break sooner if you have multiple bodies (and not just 2).

Excerpt from the official PhysX (UNity's physics engine) docs:
https://docs.nvidia.com/gameworks/conten...oints.html

The solver can have difficulty converging well when a light object is constrained between two heavy objects. Mass ratios of higher than 10 are best avoided in such scenarios.

How is one supposed to implement any realistic looking physics if the masses are all wrong? Say I have a 1000kg car and it crashes into a table of small objects of 1kg. What is recommended, make apples 100kg, or make my car 10kg?

Short answer: you don't. Just set their masses closer together and/or decrease the fixed timestep to increase solver accuracy (this allows you to use larger mass ratios). Depending on what you're after that means making the car lighter or the apple heavier. Some engines give you the option of scaling the masses upon contact, so that the mass ratio is kept in a sane range. The resulting behavior is obviously non-physical, but at least it does not explode -as often-. Either way, it is not generally possible to simulate very heavy and very light objects together in realtime.

Solutions to this: use a direct solver (which are much more expensive than iterative ones both financially and performance-wise, so realtime applications avoid them), or use all kind of hacks like the above mentioned.

Fun experiment you can do in Unity: place a 100 kg box on top of a 1 kg box, then watch them dance . The heavier the top box, the wilder the result.

[innovine]

This is a widely known limitation of realtime physics in general.

Yes, 0.1 - 10 is a ratio of 100, however things begin to break sooner if you have multiple bodies (and not just 2).

Excerpt from the official PhysX (UNity's physics engine) docs:
https://docs.nvidia.com/gameworks/conten...oints.html



Short answer: you don't. Just set their masses closer together and/or decrease the fixed timestep to increase solver accuracy (this allows you to use larger mass ratios). Depending on what you're after that means making the car lighter or the apple heavier. Some engines give you the option of scaling the masses upon contact, so that the mass ratio is kept in a sane range. The resulting behavior is obviously non-physical, but at least it does not explode -as often-. Either way, it is not generally possible to simulate very heavy and very light objects together in realtime.

Solutions to this: use a direct solver (which are much more expensive than iterative ones both financially and performance-wise, so realtime applications avoid them), or use all kind of hacks like the above mentioned.

Fun experiment you can do in Unity: place a 100 kg box on top of a 1 kg box, then watch them dance . The heavier the top box, the wilder the result.

Thanks! I can see this is going to cause me no end of problems

[innovine]

Increasing the mass of the particles definitely causes the rope to become more twisty and wobbly. It spawns and wiggles and goes in a spiral around itself rather energetically even if there are no forces applied. This is not related to mass ratio.