#

API Docs for: 0.6.1
Show: Inherited Protected Private Deprecated

# File: src/objects/Spring.js

    var Vec3 = require('../math/Vec3');

    module.exports = Spring;

    /**
     * A spring, connecting two bodies.
     *
     * @class Spring
     * @constructor
     * @param {Body} bodyA
     * @param {Body} bodyB
     * @param {Object} [options]
     * @param {number} [options.restLength]   A number > 0. Default: 1
     * @param {number} [options.stiffness]    A number >= 0. Default: 100
     * @param {number} [options.damping]      A number >= 0. Default: 1
     * @param {Vec3}  [options.worldAnchorA] Where to hook the spring to body A, in world coordinates.
     * @param {Vec3}  [options.worldAnchorB]
     * @param {Vec3}  [options.localAnchorA] Where to hook the spring to body A, in local body coordinates.
     * @param {Vec3}  [options.localAnchorB]
     */
    function Spring(bodyA,bodyB,options){
        options = options || {};

        /**
         * Rest length of the spring.
         * @property restLength
         * @type {number}
         */
        this.restLength = typeof(options.restLength) === "number" ? options.restLength : 1;

        /**
         * Stiffness of the spring.
         * @property stiffness
         * @type {number}
         */
        this.stiffness = options.stiffness || 100;

        /**
         * Damping of the spring.
         * @property damping
         * @type {number}
         */
        this.damping = options.damping || 1;

        /**
         * First connected body.
         * @property bodyA
         * @type {Body}
         */
        this.bodyA = bodyA;

        /**
         * Second connected body.
         * @property bodyB
         * @type {Body}
         */
        this.bodyB = bodyB;

        /**
         * Anchor for bodyA in local bodyA coordinates.
         * @property localAnchorA
         * @type {Vec3}
         */
        this.localAnchorA = new Vec3();

        /**
         * Anchor for bodyB in local bodyB coordinates.
         * @property localAnchorB
         * @type {Vec3}
         */
        this.localAnchorB = new Vec3();

        if(options.localAnchorA){
            this.localAnchorA.copy(options.localAnchorA);
        }
        if(options.localAnchorB){
            this.localAnchorB.copy(options.localAnchorB);
        }
        if(options.worldAnchorA){
            this.setWorldAnchorA(options.worldAnchorA);
        }
        if(options.worldAnchorB){
            this.setWorldAnchorB(options.worldAnchorB);
        }
    }

    /**
     * Set the anchor point on body A, using world coordinates.
     * @method setWorldAnchorA
     * @param {Vec3} worldAnchorA
     */
    Spring.prototype.setWorldAnchorA = function(worldAnchorA){
        this.bodyA.pointToLocalFrame(worldAnchorA,this.localAnchorA);
    };

    /**
     * Set the anchor point on body B, using world coordinates.
     * @method setWorldAnchorB
     * @param {Vec3} worldAnchorB
     */
    Spring.prototype.setWorldAnchorB = function(worldAnchorB){
        this.bodyB.pointToLocalFrame(worldAnchorB,this.localAnchorB);
    };

    /**
     * Get the anchor point on body A, in world coordinates.
     * @method getWorldAnchorA
     * @param {Vec3} result The vector to store the result in.
     */
    Spring.prototype.getWorldAnchorA = function(result){
        this.bodyA.pointToWorldFrame(this.localAnchorA,result);
    };

    /**
     * Get the anchor point on body B, in world coordinates.
     * @method getWorldAnchorB
     * @param {Vec3} result The vector to store the result in.
     */
    Spring.prototype.getWorldAnchorB = function(result){
        this.bodyB.pointToWorldFrame(this.localAnchorB,result);
    };

    var applyForce_r =              new Vec3(),
        applyForce_r_unit =         new Vec3(),
        applyForce_u =              new Vec3(),
        applyForce_f =              new Vec3(),
        applyForce_worldAnchorA =   new Vec3(),
        applyForce_worldAnchorB =   new Vec3(),
        applyForce_ri =             new Vec3(),
        applyForce_rj =             new Vec3(),
        applyForce_ri_x_f =         new Vec3(),
        applyForce_rj_x_f =         new Vec3(),
        applyForce_tmp =            new Vec3();

    /**
     * Apply the spring force to the connected bodies.
     * @method applyForce
     */
    Spring.prototype.applyForce = function(){
        var k = this.stiffness,
            d = this.damping,
            l = this.restLength,
            bodyA = this.bodyA,
            bodyB = this.bodyB,
            r = applyForce_r,
            r_unit = applyForce_r_unit,
            u = applyForce_u,
            f = applyForce_f,
            tmp = applyForce_tmp;

        var worldAnchorA = applyForce_worldAnchorA,
            worldAnchorB = applyForce_worldAnchorB,
            ri = applyForce_ri,
            rj = applyForce_rj,
            ri_x_f = applyForce_ri_x_f,
            rj_x_f = applyForce_rj_x_f;

        // Get world anchors
        this.getWorldAnchorA(worldAnchorA);
        this.getWorldAnchorB(worldAnchorB);

        // Get offset points
        worldAnchorA.vsub(bodyA.position,ri);
        worldAnchorB.vsub(bodyB.position,rj);

        // Compute distance vector between world anchor points
        worldAnchorB.vsub(worldAnchorA,r);
        var rlen = r.norm();
        r_unit.copy(r);
        r_unit.normalize();

        // Compute relative velocity of the anchor points, u
        bodyB.velocity.vsub(bodyA.velocity,u);
        // Add rotational velocity

        bodyB.angularVelocity.cross(rj,tmp);
        u.vadd(tmp,u);
        bodyA.angularVelocity.cross(ri,tmp);
        u.vsub(tmp,u);

        // F = - k * ( x - L ) - D * ( u )
        r_unit.mult(-k*(rlen-l) - d*u.dot(r_unit), f);

        // Add forces to bodies
        bodyA.force.vsub(f,bodyA.force);
        bodyB.force.vadd(f,bodyB.force);

        // Angular force
        ri.cross(f,ri_x_f);
        rj.cross(f,rj_x_f);
        bodyA.torque.vsub(ri_x_f,bodyA.torque);
        bodyB.torque.vadd(rj_x_f,bodyB.torque);
    };