Rope made from Box2D bodies

Here is a video of how a rope looks like made from box2d bodies. Earlier i has given a tutorial of how to make a rope using Verlets. This one, on the contrary, has bodies attached to it. Verlets can be used where only a visual simulation is required and the rope is not to interact with other bodies. That ways you can save up on performance. This will however take up more resources.

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Some word on how it works

So here is how it goes. The objective of the demonstration was to create a rope out of box2d bodies. The rope would respond to collisions with other bodies and the walls of the world. Alongside, just for demonstration sakes, i also added a mouse joint so that you can drag the rope and see how it works.

Start off by defining some common properties for creating bodies:

//body and fixture defs are common to all chain links
b2BodyDef bodyDef;
bodyDef.type = b2_dynamicBody;
bodyDef.position = startPos;
b2FixtureDef fixtureDef;
fixtureDef.density = 0.1;
b2PolygonShape polygonShape;
polygonShape.SetAsBox(linkWidth,linkHeight);
fixtureDef.shape = &polygonShape;

and some others for a revolute joint

//set up the common properties of the joint before entering the loop
b2RevoluteJointDef revoluteJointDef;
revoluteJointDef.localAnchorA.Set( 0, linkHeight);
revoluteJointDef.localAnchorB.Set( 0, -linkHeight);

then we create the links in the chain

//use same definitions to create multiple bodies
for (int i = 0; i < 20; i++) { b2Body* newLink = world->CreateBody( &bodyDef );
newLink->CreateFixture( &fixtureDef );
PhysicsSprite* segmentSprite = [PhysicsSprite spriteWithFile:@"rope_seg_new2.png"];
[self addChild:segmentSprite];
[segmentSprite setPhysicsBody:link];

revoluteJointDef.bodyA = link;
revoluteJointDef.bodyB = newLink;
world->CreateJoint( &revoluteJointDef );

link = newLink;//prepare for next iteration
}

Then to create the circle that is attached to the rope. We do the following:

PhysicsSprite* circleBodySprite = [PhysicsSprite spriteWithFile:@"circle2x_bounce.png"];
[self addChild:circleBodySprite z:1];

//body with circle fixture
b2CircleShape circleShape;
circleShape.m_radius = circleBodySprite.contentSize.width/2 / PTM_RATIO;
fixtureDef.shape = &circleShape;
b2Body* chainBase =world->CreateBody( &bodyDef );
chainBase->CreateFixture( &fixtureDef );
[circleBodySprite setPhysicsBody:chainBase];

Then we define two revolute joints. One for the circle and the other one between the cirlce and the chain.

//a revolute joint to connect the circle to the ground
revoluteJointDef.bodyA = referenceBody;//provided by testbed
revoluteJointDef.bodyB = chainBase;
revoluteJointDef.localAnchorA = startPos;//world coords, because m_groundBody is at (0,0)
revoluteJointDef.localAnchorB.Set(0,0);//center of circle
world->CreateJoint( &revoluteJointDef );

//another revolute joint to connect the chain to the circle
revoluteJointDef.bodyA = link;//the last added link of the chain
revoluteJointDef.bodyB = chainBase;

//the regular position for chain link joints, as above
revoluteJointDef.localAnchorA.Set(0,linkWidth);

//a little in from the edge of the circle
revoluteJointDef.localAnchorB.Set(0,linkWidth);
world->CreateJoint( &revoluteJointDef );


Now that we have our chain ready. We will add mouse joints upon tap. For that i created this function that will fetch the body that was under the tap location:

-(b2Body *) getBodyAtLocation:(b2Vec2) aLocation {
for (b2Body* b = world->GetBodyList(); b; b = b->GetNext()) {
b2Fixture* bodyFixture = b->GetFixtureList();
if (bodyFixture->TestPoint(aLocation)){
return b;
}
}
return NULL;
}

and this is how the mouse joint is installed

if (mouseJoint != NULL) return;

UITouch *myTouch = [touches anyObject];
CGPoint location = [myTouch locationInView:[myTouch view]];
location = [[CCDirector sharedDirector] convertToGL:location];
b2Vec2 locationWorld = [self toMeters:location];
freeBody = [self getBodyAtLocation:locationWorld];

//hit test point in box2d
if (freeBody) {
b2MouseJointDef md;
md.bodyA = referenceBody;
md.bodyB = freeBody;
md.target = locationWorld;
md.collideConnected = true;
md.maxForce = 100.0f;

mouseJoint = (b2MouseJoint *)world->CreateJoint(&md);
freeBody->SetAwake(true);
}

Here is a link to the source code and here is a link to the video of how it works. Here is a link to another source code that doesn’t use mouse joint and rather translates swipe variables (angle and distance) to move the body.