[ODE] Forcing bodies to specified "non-physic-calculated" positions

[Marc.Schwellinger at Dornier.eads.net]

Martin C. Martin wrote:

> Marc, see if you can't change the "world" half of the attachment point
> every frame, and see if that works.

What did you mean with the "world" half? I change the crane tip position
every frame on which the chain is hanging. It works but the top body of the
chain jiggles around that point. dWorldImpulseToForce() calculates the
forces needed to move the body to the desired position. But thats seems to
be imprecise. dWorldImpulseToForce() wants a stepsize which means "Say me
the time in which this movement should be done...". I use the duration of
the last frame for this. Because frame time isn't absolutely constant the
calculated forces are too low or too high.
One method that reduces that effect is to divide the world steps in smaller
parts. For example....draw graphics every 16 ms and do physics calculations
16 times with 1 ms stepsize each all in one frame. Thats the method I'm
using now but it's very CPU expensive. But it works with not too much
objects on a P4 2,8 GHz.
I think about building the crane out of hinge joints and/or motor joints.
But I think it would'nt be stiff enough if I hang heavy loads on it.

Another method I've tried is to use a ball joint connect it to the static
environment on the one side and on the other side to the chain top body.
Then setting the joint position every frame. Doesn't work. ERP lets me
define something between joint breaking and jiggling.

Cheers,
Marc

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