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Re: Door knocked off hinge constraint by collisions

Posted: Fri Jan 17, 2014 8:32 pm
by Erwin Coumans
I'd still like a solution that works in the non-axis aligned case though.
You can set two components of the the local inertia tensor to zero, instead of using the angular factor. This will give you a 'fixed' hinge axis in local space, so you just orient the body accordinaly (and use a btCompoundShape if necessary). I plan on adding a btTransformShape to avoid the cost of btCompoundShape when the only purpose is just to shift the center of mass.
You still need to set the linear factor to zero.

Code: Select all

localInertia[0] = 0;
localInertia[1] = 0;
body = new btRigidBody(..., localInertia);
//don't call body->setAngularFactor, because we already lock the angular motion using the inertia tensor
//don't need to create a btHingeConstraint either.
An alternative would be to use Featherstone btMultiBody. It combines well with btRigidBody, so you don't pay a penalty in performance.
With btMultiBody you can attach two free moving bodies in a way so that the hinge axis will never be violated.

Yet another alternative would be to use the btDanztigSolver or btLemkeSolver (see Bullet/Demos/ForkLiftDemo). Right now, the solver is used for all objects, so performance would drop.
It is possible to use the btDantzigSolver or btLemkeSolver only for specific constraints, as a 'block' solver, in combination with regular iterative constraints (sequential impulse/projected gauss seidel).
I am working on a demo in the github repository to show the various options.

Please let us know if that inertia tensor workaround does the trick for you.

Re: Door knocked off hinge constraint by collisions

Posted: Sat Jan 18, 2014 11:19 am
by bteitler
I didn't know about the inertia settings. Seems to be exactly what I needed. I tested it, and combined with the custom gravity per tick, this seems to perform pretty realistically. This should meet my requirements I think. Thanks for your help.

Re: Door knocked off hinge constraint by collisions

Posted: Sun Jan 19, 2014 9:45 am
by Flix
I know that this thread if almost officially closed. Anyway yesterday evening I've made a small Demo (and a couple of small videos) to show the benefits that can be derived from increasing the inertia tensor of a body inside a constaint. Hope it can help somebody :wink:
VIDEO - Door with local inertia multiplier = 1:
VIDEO - Door with local inertia multiplier = 100:
Please note that the "one axis trick" Erwin mentioned (i.e. only one local inertia component is not zero) has been applied to both cases: still, having a big multiplier improves stability at limits a lot (and this technique works for nearly every constraint type :D ).
(11.3 KiB) Downloaded 68 times
P.S. I've experienced the same issue with non-axis aligned btHingeConstraints bteitler faced. I had no time to fix it yesterday, but as a workaround, it's possible to use a btSliderConstraint (yes! a btSliderConstraint :!: ) instead (and that is what was used in the videos I've posted).

Hope this can be useful. Bye.

Re: Door knocked off hinge constraint by collisions

Posted: Wed Jan 22, 2014 5:55 am
by bteitler
Hmm, the hinged door solution with local inertia set to 0 for two of the axes doesn't seem to play nice with with static objects. The door simply rotates through the static objects instead of "blocking" the rotation as I'd expect.