Re: [Pyode-user] holding an object at a certain level with a chain

[nick k. <nk...@ze...>]

(I thought I should send my response to the entire list as  
well...sorry for the duplicate e-mail Ethan)

On Jun 19, 2007, at 2:24 PM, Ethan Glasser-Camp wrote:
> How did you build chains using ODE? (This is personal curiosity; the
> most effective chains I've been able to build have been hinge joint +
> slider, but that's only in 2D.)

Thanks Ethan.  I basically based my chains on the examples that come  
with the ODE distribution (specifically "ode/test/test_chain1.c").   
I've included my (not-very-well commented) code below.  I model each  
chain as a GeomCapsule and attach them to each other using ball  
joints.  I'm able to get 20 links in the chain without a problem, and  
with expected bending behavior (screenshot: http://zeitkunst.org/ 
media/images/chains.png).  Applying all types of forces to the chain  
gives reasonably expected movement and bending of the chains.

> I'm not sure but this looks like you should be using some kind of
> motor instead of computing forces like this.

Does anyone have good references for using motors?  I've looked a bit  
at the examples on the PyODE website, but would appreciate other  
suggestions.

Thanks,

nick


xPosition, yPosition, zPosition = initialPosition
         for linkNumber in range(self.numLinks):
             link = ode.Body(self.world)
             M = ode.Mass()
             M.setBox(density, size[0], size[1], size[2])
             M.adjust(0.5)
             link.setMass(M)

             # offset the start position by half of the y dimension
             k = (linkNumber * size[1]) + (initialPosition[1] + size 
[1]/2.0)

             link.setPosition((initialPosition[0], k, initialPosition 
[2]))
             link.currentPosition = (initialPosition[0], k,  
initialPosition[2])
             yPosition += size[1] - 0.1

             self.bodies.append(link)

             geom = ode.GeomCapsule(self.space, radius = self.size[1]/ 
2.0, length=self.size[0])
             geom.setBody(link)
             self.geom.append(geom)

         self.bodyHoldPosition = array((initialPosition[0], k,  
initialPosition[2]))

         # create joint with creature
         joint = ode.BallJoint(self.world)
         joint.attach(self.bodies[0], self.creatureBody)
         joint.setAnchor((initialPosition[0], initialPosition[1] -  
size[1]/2.0, initialPosition[2]))
         self.joints.append(joint)

         for linkNumber in range(self.numLinks - 1):
             joint = ode.BallJoint(self.world)
             joint.attach(self.bodies[linkNumber], self.bodies 
[linkNumber + 1])
             k = (linkNumber + 1) * size[1] + initialPosition[1] -  
size[1]/2.0
             joint.setAnchor((initialPosition[0], k, initialPosition 
[2]))
             self.joints.append(joint)