Box2d Elastic Rope Knot

Question

I am writing a game and I need to assemble a rope. I put it together b2RopeJoint, but there I did not find an opportunity to make it elastic. Then I searched for b2DistanceJoint, everything is cool with elasticity, but I can not find a way to set the restriction only on the maximum distance (without minimum).

How can i do this?

Answer 1

Try it.

-(void) CreateElasticRope { //=======Params // Position and size b2Vec2 lastPos = b2Vec2(4,4); //set position first body float widthBody = 0.35; float heightBody = 0.1; // Body params float density = 0.05; float restitution = 0.5; float friction = 0.5; // Distance joint float dampingRatio = 0.0; float frequencyHz = 0; // Rope joint float kMaxWidth = 1.1; // Bodies int countBodyInChain = 15; b2Body* prevBody; //========Create bodies and joints for (int k = 0; k < countBodyInChain; k++) { b2BodyDef bodyDef; if(k==0 ) bodyDef.type = b2_staticBody; //first body is static else bodyDef.type = b2_dynamicBody; bodyDef.position = lastPos; lastPos += b2Vec2(2*widthBody, 0); //modify b2Vect for next body bodyDef.fixedRotation = YES; b2Body* body = world->CreateBody(&bodyDef); b2PolygonShape distBodyBox; distBodyBox.SetAsBox(widthBody, heightBody); b2FixtureDef fixDef; fixDef.density = density; fixDef.restitution = restitution; fixDef.friction = friction; fixDef.shape = &distBodyBox; body->CreateFixture(&fixDef); body->SetHealth(9999999); body->SetLinearDamping(0.0005f); if(k>0) { //Create distance joint b2DistanceJointDef distJDef; b2Vec2 anchor1 = prevBody->GetWorldCenter(); b2Vec2 anchor2 = body->GetWorldCenter(); distJDef.Initialize(prevBody, body, anchor1, anchor2); distJDef.collideConnected = false; distJDef.dampingRatio = dampingRatio; distJDef.frequencyHz = frequencyHz; world->CreateJoint(&distJDef); //Create rope joint b2RopeJointDef rDef; rDef.maxLength = (body->GetPosition() - prevBody->GetPosition()).Length() * kMaxWidth; rDef.localAnchorA = rDef.localAnchorB = b2Vec2_zero; rDef.bodyA = prevBody; rDef.bodyB = body; world->CreateJoint(&rDef); } //if k>0 prevBody = body; } //for } 

Answer 2

An implementation of ropejoint style version 2.2 is implemented here: https://github.com/aaronfarr/box2Dweb

Answer 3

You can use both a rope connection and a remote connection. Make the distance from the cable joint a little longer than the distance.

Answer 4

In the Box2D sample example, I modified the "Web" example as follows:

 #ifndef ELASTICROPE_H #define ELASTICROPE_H #define NUM_JOINTS 7 #define NUM_LINKS 8 class ElasticRope : public Test { public: ElasticRope() { b2Body* ground = NULL; { b2BodyDef bd; ground = m_world->CreateBody(&bd); b2EdgeShape shape; shape.Set(b2Vec2(-40.0f, 0.0f), b2Vec2(40.0f, 0.0f)); ground->CreateFixture(&shape, 0.0f); } { b2CircleShape shape; shape.m_radius = 0.8f; // create bodies for (int b = 0; b < NUM_LINKS; b++) { b2BodyDef bd; bd.type = b2_dynamicBody; bd.position.Set(5.0f, NUM_LINKS-b); m_bodies[b] = m_world->CreateBody(&bd); m_bodies[b]->CreateFixture(&shape, 5.0f); } for (int j = 0; j < NUM_JOINTS; j++) { b2DistanceJointDef jd; b2Vec2 p1, p2, d; jd.frequencyHz = 5.0f; jd.dampingRatio = 1.0f; jd.bodyA = m_bodies[j]; jd.bodyB = m_bodies[j+1]; m_joints[j] = m_world->CreateJoint(&jd); } } } void Step(Settings* settings) { Test::Step(settings); m_debugDraw.DrawString(5, m_textLine, "This demonstrates an elastic rope."); m_textLine += DRAW_STRING_NEW_LINE; } void JointDestroyed(b2Joint* joint) { for (int32 i = 0; i < 8; ++i) { if (m_joints[i] == joint) { m_joints[i] = NULL; break; } } } static Test* Create() { return new ElasticRope; } b2Body* m_bodies[NUM_LINKS]; b2Joint* m_joints[NUM_JOINTS]; }; #endif // ELASTICROPE_H 

This is far from ideal, but may be the starting point.