[ODE] Damping

[james at 6msoft.com]

I wanted the damping values to be independent of mass values, didn't want
people have to do F=MA when setting damping values, and if I was going to
go independent of mass, then I could just cut to the chase and apply to
the velocities directly. The result is both a tank and a ping pong ball
can use the same damping values for the same result.

the equation is something like...

a = damping * timeStep
a = clamp(a, 0, 1)
vel *= 1 - a

For disabling, right now setting the damping of a body to 0 disables it,
but currently no way across a world (but it can obviously be done.

-James

>
> Why did you implement it differently?
>
> I have been extremely happy with the behavior of the
> force/torque based dampening, and it's what my apps
> have been (and will keep on) doing. Behaviorally, it's
> well tested. If you have this particular piece of code,
> then I'll have to set it to 0, and do my own dampening
> anyway, and more useless work will be done by the
> library.
>
> But maybe you have a really good reason?
>
> Also, what is the unit? Is it scaled by the time step,
> or is that up to the user?
>
> If you want to have global dampening values, those
> should probably be dealt with the same way as the
> global body auto-sleep values; if you set them globally,
> then they are copied on creation, but changing them
> after a body is created has no effect on created bodies.
> That's the precedent, and it seems to work pretty well,
> so being consistent is a good thing.
>
> Another precedent is having a global enable. Something
> like dWorldSetAutoDampening() to enable/disable the
> behavior; possibly with a default of off.
>
> Cheers,
>
> 			/ h+
>
>
>
> -----Original Message-----
> From: ode-bounces at q12.org [mailto:ode-bounces at q12.org]On Behalf Of
> james at 6msoft.com
> Sent: Thursday, July 29, 2004 9:53 AM
> To: ode at q12.org
> Subject: [ODE] Damping
>
>
> Ok, I added linear and angular damping per body in ODE. I did it
> differently than Jon suggested and instead scaled velocities over time.
> This will be available to the community with the PSCL release (which also
> has Trimesh-plane collision).
>
> In the end, 4 functions were added:
>
> void dBodySetLinearDamping(dBodyID b, dReal d);
> void dBodySetAngularDamping(dBodyID b, dReal d);
> dReal dBodyGetLinearDamping(dBodyID b);
> dReal dBodyGetAngularDamping(dBodyID b);
>
> Does anyone think having global damping values might be worthwhile? ie,
> take the larger of the two damping values when applying damping to a body.
>
> -James
>
>> Here's something Jon posted a little while back.  If you do a Google
>> search with site:q12.org you can find answers to common questions like
>> this.
>>
>> 1) Add damping. For each object, for each step, add counter-
>>    force scaled by linear velocity, and counter-torque scaled
>>    by angular velocity. From memory, it looks something like:
>>
>>    /*  Apply damping to the body to get it to settle properly     */
>>    /*  over time. This also simulates "air drag" or "rolling      */
>>    /*  friction" to some extent.                                  */
>>    /*  Change "-0.01f" to some other negative number for tuning.  */
>>    dVector3 v;
>>    dBodyGetLinearVelocity( body, v );
>>    v[0] *= -0.01f;   v[1] *= -0.01f;   v[2] *= -0.01f;
>>    dBodyAddForcce( body, v );
>>    dBodyGetAngularVelocity( body, v );
>>    v[0] *= -0.01f;   v[1] *= -0.01f;   v[2] *= -0.01f;
>>    dBodyAddTorque( body, v );
>
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