[ODE] restraining rotation along axes

[Nate W]

On Mon, 13 May 2002, Glenn Watson wrote:

> Hi, what is the best way (if it's possible) to ensure
> that a rigid body (say falling under gravity and then
> colliding with static geo) can only rotate around the
> Y axis when it hits something?

Easy way:

Create an invisible body above the body in question.  Make the invisible
body immobile (attach it to the static environment), and attach the
falling body to the invisible body with a slider joint.

Hard way:

Check its orientation at each simulation step, and apply a small torque to
try to straighten it out if you notice it leaning along the X or Z axes.  
Calculating the proper torque could be tricky.  I'd start with something
like "torque_x = gain * (deviation from axis_x)" and the same for the z
axes, then plan to spend a bit of time tuning the gain parameter.

And there's probably other ways.  Of those two, the easy way will probably
work *much* better.

-- 

Nate Waddoups
Redmond WA USA
http://www.natew.com