Create Joint
The joint is created by createDistanceJoint function. This function returns an instance of b2DistanceJoint, which provides all other operations with the joint. This createDistanceJoint function has a little more parameters, which specifies the joint‘s behavior. The first two paramenters are connected bodies followed by x and y coordinates of anchor point on both bodies.
Anchor point
The bodies do not have to be connected in their center. The points where the bodies are connected in the joint are called anchor points. Different position of anchor point causes different behavior. If the anchor point is placed in the center, its movement does not affect body rotation. However, if the anchor point is outside the center, its movement affects the body rotation. The rotation is affected more, if the anchor point is further away from the center.
The createDistanceJoint function has some additional options like frequency, damping ratio and distanceLength. If the distanceLength is not set, the joint used the length which is between the bodies, when the joint was created.
In this sample, two bodies are connected with distance joint between them.
Example: create bodies and distance joint between them
// create two bodies and connect them with the distance joint
function _createBodies()
{
// the body side
var bodySide = 80;
// first rectangle
this._rect1 = this.addPolygonBody(Bitmap.fromFile("app://rectangle.png"), #dynamic, 1.0, 0.1, 0.7, bodySide, bodySide);
// set body damping
this._rect1.setLinearDamping(0.5);
this._rect1.setAngularDamping(0.5);
// place the body
this._rect1.setPosition(System.width / 2, System.height / 2);
// secod rectangle
this._rect2 = this.addPolygonBody(Bitmap.fromFile("app://rectangle.png"), #dynamic, 1.0, 0.1, 0.7, bodySide, bodySide);
// set body damping
this._rect2.setLinearDamping(0.5);
this._rect2.setAngularDamping(0.5);
// place the body
this._rect2.setPosition(System.width / 2, 3*System.height / 4);
// create the distance joint
var jointDef = {
frequencyHz : 100,
dampingRatio : 0.0,
// distanceLength : use default distance (distance which was between the bodies when they were created
}
this._distanceJoint = this.createDistanceJoint(this._rect1, this._rect2, System.width / 2,System.height / 2,System.width / 2,3*System.height / 4, jointDef, false)
}
Mouse joint
To allow users manipulate with the boxes, a mouse joint has to be implemented. The joint is created when user taps onto one of the boxes. To check whether the user tapped onto the box, intersectsPoint function is used. This function checks x and y coordinates and see whether they lie in the object or not.
// reaction to pointer pressed event
function pointerPressed(x, y)
{
var mouseJointDef;
// check if user clicked on the body
if (this._rect1.intersectsPoint(x, y)) {
// create joint to move the first rectangle
mouseJointDef = {
maxForce : 500 * this._rect1.getMass(), // max force, which attracts the body to the target point
frequencyHz : 100,
dampingRatio : 0.0, // damping ratio - prevent oscillation of the body around the target point
targetX : x / this.scale, // X position of anchor point on the body (circle)
targetY : (System.height - y) / this.scale // Y position of anchor point on the body (circle)
};
this._mouseJoint = this.createMouseJoint(this._ground, this._rect1, mouseJointDef, true);
} else if (this._rect2.intersectsPoint(x, y)) {
// create joint to move the first rectangle
mouseJointDef = {
maxForce : 500 * this._rect2.getMass(), // max force, which attracts the body to the target point
frequencyHz : 100,
dampingRatio : 0.0, // damping ratio - prevent oscillation of the body around the target point
targetX : x / this.scale, // X position of anchor point on the body (circle)
targetY : (System.height - y) / this.scale // Y position of anchor point on the body (circle)
};
this._mouseJoint = this.createMouseJoint(this._ground, this._rect2, mouseJointDef, true);
} else {
// do nothing
return;
}
}
To ensure the box movement is used, we need to implement setTarget function in pointerDragged function.
For more information about Mouse joint see mouse joint sample.
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