Hello, looking for physics friends

[Dan Royer (Marginally Clever Robots)]

Greetings!

I make robots as a hobby and have been building Unity style simulations in an open source Java app called Robot Overlord.  Recently I've begun adding ODE4J 0.5.2 (see one gif example below).  I'm also running the ODE4J Discord where I hope people who like playing with physics engines can help each other, maybe with less-than-mailing-list level urgency.

I've begun adding hinges and I'm wondering about the "anchor points".  I assumed that, when satisfied, this would be a world space point shared between two DBody, probably on the axis of rotation.  In practice when I look at the data the anchor point is centered on one body or the other.  What am I missing?  Is this a Java-version specific issue?

Thanks!

[rocco martino]

I haven't used ODE in a while, so I hope someone will correct me if I'm wrong.

If I understand your problem correctly, you expect the anchor point to remain fixed at a point in world space, but the result is not as expected.

When you set a constraint, axes and points are transformed into the reference frames of the respective bodies. The solver tries to minimize the error between the two constraints by imposing a velocity, so the anchor point is not a point in world space, but moves with the bodies.

Perhaps in some constraints you can set the respective anchor points, for example in dJointDoubleBall. However, I cannot recall whether this joint is in the master branch or from a patch.

Hope this helps,
Have a nice day

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