[ODE] "Upright" body

[Colin Bonstead]

Thanks Alen, that's the kind of solution I was looking for.  I tried 
hacking in a quick implementation and it seemed to work.  Hopefully it 
will stand up to some more complex test cases.

Colin

-----Original Message-----
From: Alen Ladavac <alenl-ml at croteam.com>
Sent: Wed, 7 Apr 2004 09:50:43 -0000
To: <ode at q12.org>
Subject: Re: [ODE] "Upright" body

We implemented the standup avatars as a special case. We have 3
different types of bodies: dynamic, kinematic and hybrid. For
dynamic, forces can influence all 6 D0Fs of of it, for kinematic they
cannot influence it at all (similar as static geometry, but still has
velocity), and for hybrid they can influence only the first 3 DOFs
(translation). Adds a few if-s to the joint evaluation, but gives you
perfectly stiff upwards standing objects.

If you don't want those extra DOFs on some bodies, it is better to
not have them in the first place, than to allow them and then add
joints that remove them. It is both faster and more stable.

HTH, Alen

----- Original Message -----
From: "Colin Bonstead" <colin at cyan.com>
To: <ode at q12.org>
Sent: Tuesday, April 06, 2004 23:59
Subject: [ODE] "Upright" body

> I'd like to have a physical that always stands upright on one of
> it's axis, for the avatar.  In our previous physics engine this was
> done by setting the inverse inertial tensor to all zeroes, except
> for element 2,2 which was set to DBL_EPSILON.  Frankly, I have no
> idea how this works.  However, that matrix is not invertable, so it
> can only work if you can set the inverse matrix directly.
>
> I actually hacked in a backdoor so I could set the inverse tensor
> but
it
> didn't work right in ODE.  The avatar stayed upright, but I
> couldn't move him.  I'd prefer to move away from that method
> anyway, since it kind of seems like a hack.
>
> Another thing I tried is just zeroing out any rotation in the X and
> Y after every step (Z is up in my sim).  I know that is a crappy
> way to
do
> it, but I just wanted to see if it would work.  It did, but not
> very well.  From searching through the ODE archives the only other
suggestion
> I could find was to apply a counter force after every step.  That
> doesn't sound very viable to me since you'll always be one step
behind,
> so your avatar will sort of vibrate.
>
> Anyone have any other ideas?  The angular motor seems like it could
> help, but everything I've tried with it hasn't worked.  I've read
> the docs on it at least 10 times and I still don't think I really
understand
> what it does.
>
> Thanks, Colin
>
>
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