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Chipmunk Physics • View topic - Simulating a skeleton system

Simulating a skeleton system

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Simulating a skeleton system

Postby Mephisto » Mon May 17, 2010 4:08 pm

Hi,

I would like to simulate a small skeletal creature in Chipmunk, but I'm not sure about how I could best model it.
Say for example that it is just basically two lines that should have pin joint in the middle.

My first thought was to create three balls and connect them using Pin Joints connected to axles. The only problem with that is that the balls roll away so the creature can't stay up because it falls down flat. So I solved that by setting the inertia of the circles to infinity and setting frictions, and that seems to work ok, but I'm not completely satisfied with the solution.

Is there any other way I could try to model this system? Maybe with Line segments? I would eventually like to have the creatures move around in various ways.

And second, how could I go about applying a torque around the middle joint for both segments? I see that I can apply impulses and forces, but not directly torques... I would like to simulate a "muscle" in there at least approximately.

Thank you for your help
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Re: Simulating a skeleton system

Postby Tam Toucan » Tue May 18, 2010 4:23 am

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Re: Simulating a skeleton system

Postby slembcke » Tue May 18, 2010 8:36 am

Yup, pivot and rotary limit joints are a good start.

To give your skeleton muscles you just need more constraints. I need to document this properly at some point, but joints have some adjustable parameters that you can use to make them behave very differently from their intended uses. The most powerful are maxBias and maxForce.

maxForce is fairly obvious, it limits the amount of force (or torque) a joint can use to correct itself. If it requires more force to hold it together, it will stretch. If you want to make a muscle for your skeleton, add a gear joint with a gear radius of 1.0 to the two bodies. That will cause the bodies to hold a specific angle in relation to each other, specifically the "phase" angle that you can change at any time using cpGearJointSetPhase(). If you also set the maxForce for the gear joint, it will work to pull the elbow or knee or whatever to the angle you specify, but won't use an unbounded amount of force to do it. I've found that this works very well.

The other parameter I mentioned is maxBias, this sets the rate (velocity or angular velocity) at which the joint can correct itself. Play with maxForce first, but you can use maxBias to prevent fast or unnatural looking movement.

The docs on the wiki mostly cover this: http://code.google.com/p/chipmunk-physi ... Constraint
I do need to add some recipes though.
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