Contact constraint derivation question

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Joined: Wed Mar 28, 2012 6:12 pm

Contact constraint derivation question

Postby Himura78 » Mon Apr 03, 2017 4:03 am

This question is not specifically about Box2D, but it's related to the derivation of the contact constraint used in Box2D.
From Erin Catto's paper "Iterative Dynamics With Temporal Coherence" he states that when we calculate Cdot for the contact (normal) constraint, we use the product rule and get two terms, one which is essentially the velocity dotted with the normal, and the other which is the penetration dotted with the derivative of the normal. His paper goes on to say that the second term can be neglected when we make the assumption that the penetration is small. What I'm wondering is, why is this a valid assumption? Particularly when (as in the paper) discrete collision detection is being used, doesn't that mean that the penetration can be arbitrarily deep, depending on the velocities and sizes of the interacting bodies?
The other question I have is, since Cdot is always going to be linear in the velocity (and will be equal to JV) doesn't that mean that we should still be able to rearrange the equation for Cdot into JV, with both of these terms (ie without neglecting the second term)? Would it not be more accurate to do so?
Thanks :)

Louis Langholtz
Posts: 22
Joined: Tue Dec 20, 2016 10:53 am

Re: Contact constraint derivation question

Postby Louis Langholtz » Fri May 05, 2017 6:45 am

Hi Himura78,

I like your topic and questions. Any chance you can reference particular functions or methods in Box2D that relate to these?

Sounds like you have theoretically thought about these things but perhaps haven't tried them out on Box2D code. Is that a fair assessment?

I ask because I'd love to know what results we'd have if we modified the code and tried changing these aspects. Happy to help with that.


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