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Erin Catto
PostPosted: Thu Nov 20, 2008 5:23 pm 
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Posts: 2931
One thing to keep in mind is the hierarchy. The iterative solver has trouble with light objects supporting heavy objects. So stacking a heavy box on a light box will lead to instability (the lower box will pop out like a watermelon seed). The opposite scenario is perfectly stable, a light box on top of a heavy box is more stable than two boxes of equal mass.

So having a heavy helicopter supporting a light chain is no problem. The problem is using the light chain to lift a heavy object.

Certain trade-offs were made in the design of Box2D, and the solver design is at the core of this.

I haven't tried this, but if you know the configuration of the chain and the cargo, you might be able to deduce the weight felt by each linkage. You can use this to initialize the joint impulses. (this might require modifying Box2D a bit) This would get a bit tricky if there are contacts.

Another thing you can to to improve stability is to increase the rotational inertia of the links (without increasing the mass).
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makc
PostPosted: Mon Nov 24, 2008 5:05 pm 
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Joined: Tue Oct 14, 2008 1:30 pm
Posts: 65
I have somewhat similar problem with ragdoll revolute joints. When something happens to be inbetween the body and the arm, it gets ripped out of sockets. how come all the ragdoll parts penetrate each other with no problems, but this little circle has so much power? And how do I enforce joints "priority" over collisions?


Attachments:
doll.JPG
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makc
PostPosted: Mon Nov 24, 2008 5:24 pm 
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Joined: Tue Oct 14, 2008 1:30 pm
Posts: 65
a ha, I knew this was something about green color, getting that little thing some mass and holding it in place with SetLinearVelocity + SetXForm every frame did the trick, no sockets been hurt while simulating this :)
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micapolos
PostPosted: Wed Dec 03, 2008 1:03 pm 
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Joined: Wed Nov 19, 2008 2:04 pm
Posts: 4
Hi,

I'd like to share with you the results of my experiments with implementing a "STRONG" rope.
Below you'll find a code snippet in Java.
You can see the results here: http://www.dustedpixels.com/download/webstart/RopeDemo.jnlp
The rope on the left is the "strong" one, the rope on the right is the "ordinary" one.
As a point of reference, the box connected to the rope is 500x heavier than every of the links.

Code:
    private static final int COUNT = 16;
    private static final boolean STRONG = false;

    // Create links. Last link will be a very heavy box.
    List<Body> links = new ArrayList<Body>();
    for (int i = 1; i <= COUNT; i++) {
      BodyDef bodyDef = new BodyDef();
      bodyDef.position = anchor.add(new Vec2(0, i * 0.25f));
      Body link = world.createBody(bodyDef);
     
      if (i != COUNT) {
        // A link
        CircleDef circleDef = new CircleDef();
        circleDef.radius = 0.025f;
        circleDef.density = 1f;
        link.createShape(circleDef);
      } else {
        // A box attached at the end of the rope
        PolygonDef polygonDef = new PolygonDef();
        polygonDef.setAsBox(0.5f, 0.3f, new Vec2(0, 0.3f), 0);
        polygonDef.density = 1f;
        link.createShape(polygonDef);
      }

      link.setMassFromShapes();
      links.add(link);
    }
   
    // Connects links together. First link is connected directly to the "ceiling".
    Body lastLink = null;
    for (Body link : links) {
      DistanceJointDef distanceJointDef = new DistanceJointDef();
      if (lastLink == null) {
        distanceJointDef.initialize(link, world.getGroundBody(), link.getPosition(), anchor);
      } else {
        distanceJointDef.initialize(link, lastLink, link.getPosition(), lastLink.getPosition());
      }
      world.createJoint(distanceJointDef);     
      lastLink = link;
    }

    if (STRONG) {
      // To make rope stronger, connect each link directly to the ceiling.
      for (Body link : links) {
        BodyDef bodyDef = new BodyDef();
        bodyDef.position = anchor;
        Body attachement = world.createBody(bodyDef);
         
        MassData massData = new MassData();
        massData.mass = link.getMass();
        massData.I = link.getInertia();
        attachement.setMass(massData);
               
        RevoluteJointDef revoluteJointDef = new RevoluteJointDef();
        revoluteJointDef.initialize(attachement, world.getGroundBody(), attachement.getPosition());
        world.createJoint(revoluteJointDef);
         
        PrismaticJointDef prismaticJointDef = new PrismaticJointDef();
        prismaticJointDef.initialize(attachement, link, link.getPosition(), new Vec2(0, -1));
        prismaticJointDef.enableLimit = true;
        prismaticJointDef.lowerTranslation = 0;
        prismaticJointDef.upperTranslation = (link.getPosition().y - anchor.y);
        world.createJoint(prismaticJointDef);
      }
    }


Regards,
Michal.
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makc
PostPosted: Wed Dec 03, 2008 2:27 pm 
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Joined: Tue Oct 14, 2008 1:30 pm
Posts: 65
Michal, this code ROCKS! As a free service for everyone, here's HelloWorld.as -compatible translation:
Code:
    var COUNT:int = 16;
    var STRONG:Boolean = !false;

   var anchor:b2Vec2 = new b2Vec2 (5, 0);

    // Create links. Last link will be a very heavy box.
    var links:Array = [];
    for (var i:int = 1; i <= COUNT; i++) {
      var bodyDef:b2BodyDef = new b2BodyDef();
      bodyDef.position = anchor.Copy(); bodyDef.position.Add(new b2Vec2(0, i * 0.5));
      var link:b2Body = m_world.CreateBody(bodyDef);
     
      if (i != COUNT) {
        // A link
        var circleDef:b2CircleDef = new b2CircleDef();
        circleDef.radius = 0.025;
        circleDef.density = 1.0;
        link.CreateShape(circleDef);
      } else {
        // A box attached at the end of the rope
        var polygonDef:b2PolygonDef = new b2PolygonDef();
        polygonDef.SetAsBox(0.5, 0.3)//, new b2Vec2(0, 0.3), 0);
        polygonDef.density = 1.0;
        link.CreateShape(polygonDef);
      }

      link.SetMassFromShapes();
      links.push (link);
    }
   
    // Connects links together. First link is connected directly to the "ceiling".
    var lastLink:b2Body = null;
    for each (link in links) {
      var distanceJointDef:b2DistanceJointDef = new b2DistanceJointDef();
      if (lastLink == null) {
        distanceJointDef.Initialize(link, m_world.GetGroundBody(), link.GetPosition(), anchor);
      } else {
        distanceJointDef.Initialize(link, lastLink, link.GetPosition(), lastLink.GetPosition());
      }
      m_world.CreateJoint(distanceJointDef);     
      lastLink = link;
    }

    if (STRONG) {
      // To make rope stronger, connect each link directly to the ceiling.
      for each (link in links) {
        bodyDef = new b2BodyDef();
        bodyDef.position = anchor;
        var attachement:b2Body = m_world.CreateBody(bodyDef);
         
        var massData:b2MassData = new b2MassData();
        massData.mass = link.GetMass();
        massData.I = link.GetInertia();
        attachement.SetMass(massData);
               
        var revoluteJointDef:b2RevoluteJointDef = new b2RevoluteJointDef();
        revoluteJointDef.Initialize(attachement, m_world.GetGroundBody(), attachement.GetPosition());
        m_world.CreateJoint(revoluteJointDef);
         
        var prismaticJointDef:b2PrismaticJointDef = new b2PrismaticJointDef();
        prismaticJointDef.Initialize(attachement, link, link.GetPosition(), new b2Vec2(0, -1));
        prismaticJointDef.enableLimit = true;
        prismaticJointDef.lowerTranslation = 0;
        prismaticJointDef.upperTranslation = (link.GetPosition().y - anchor.y);
        m_world.CreateJoint(prismaticJointDef);
      }
    }   


Attachments:
HelloWorld.zip [51.42 KiB]
Downloaded 724 times
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szataniol
PostPosted: Fri Dec 12, 2008 6:03 am 
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Joined: Wed Jun 25, 2008 3:08 am
Posts: 4
The problem is when you remove joints drawings and actually try to make it look like a real rope. If you move it it would never stop and move slightly all the time. :(
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makc
PostPosted: Fri Dec 12, 2008 11:58 am 
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Joined: Tue Oct 14, 2008 1:30 pm
Posts: 65
szataniol wrote:
If you move it it would never stop and move slightly all the time. :(

Manual wrote:
You can use a joint motor to simulate joint friction. Just set the joint speed to zero, and set the maximum torque to some small, but significant value. The motor will try to prevent the joint from rotating, but will yield to a significant load.
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darkzerox
PostPosted: Fri Dec 12, 2008 11:17 pm 
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Joined: Fri Dec 07, 2007 3:09 pm
Posts: 241
That looks awesome guys! Thanks for sharing.
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micapolos
PostPosted: Tue Dec 16, 2008 7:14 pm 
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Joined: Wed Nov 19, 2008 2:04 pm
Posts: 4
szataniol wrote:
The problem is when you remove joints drawings and actually try to make it look like a real rope. If you move it it would never stop and move slightly all the time. :(


I'm not sure if I'm thinking about the same "slights movements" as you.
If yes, then the solution to that problem is simple.
You need to slightly "loose" the prismatic joint which attaches link to the ceiling,
by modifying 3-rd but last line in the Java code snippet like that:

Code:
prismaticJointDef.lowerTranslation = -0.01f * (link.getPosition().y - anchor.y);


Did it help?

Regards,
Michal.
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clipdepelicula
PostPosted: Thu Jun 11, 2009 10:01 am 
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Joined: Tue Feb 24, 2009 3:06 am
Posts: 3
Please can you send me the class?? Im beginer and I dont know how to initialize it. Sorry for my english. :roll:

Thanks

administraor@gmail.com
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