p2s/src/objects/LinearSpring.js

A JavaScript 2D physics engine.

var vec2 = require('../math/vec2');
var Spring = require('./Spring');

module.exports = LinearSpring;

/**
 * A spring, connecting two bodies.
 *
 * The Spring explicitly adds force and angularForce to the bodies.
 *
 * @class LinearSpring
 * @extends Spring
 * @constructor
 * @param {Body} bodyA
 * @param {Body} bodyB
 * @param {Object} [options]
 * @param {number} [options.restLength]   A number > 0. Default is the current distance between the world anchor points.
 * @param {number} [options.stiffness=100]  Spring constant (see Hookes Law). A number >= 0.
 * @param {number} [options.damping=1]      A number >= 0. Default: 1
 * @param {Array}  [options.worldAnchorA]   Where to hook the spring to body A, in world coordinates. Overrides the option "localAnchorA" if given.
 * @param {Array}  [options.worldAnchorB]
 * @param {Array}  [options.localAnchorA]   Where to hook the spring to body A, in local body coordinates. Defaults to the body center.
 * @param {Array}  [options.localAnchorB]
 *
 * @example
 *     var spring = new LinearSpring(bodyA, bodyB, {
 *         stiffness: 100,
 *         damping: 1,
 *         localAnchorA: [0,0], // center of bodyA
 *         localAnchorB: [0,0] // center of bodyB
 *     });
 *     world.addSpring(spring);
 */
function LinearSpring(bodyA,bodyB,options){
    options = options || {};

    Spring.call(this, bodyA, bodyB, options);

    /**
     * Anchor for bodyA in local bodyA coordinates.
     * @property localAnchorA
     * @type {Array}
     */
    this.localAnchorA = vec2.create();

    /**
     * Anchor for bodyB in local bodyB coordinates.
     * @property localAnchorB
     * @type {Array}
     */
    this.localAnchorB = vec2.create();

    if(options.localAnchorA){ vec2.copy(this.localAnchorA, options.localAnchorA); }
    if(options.localAnchorB){ vec2.copy(this.localAnchorB, options.localAnchorB); }
    if(options.worldAnchorA){ this.setWorldAnchorA(options.worldAnchorA); }
    if(options.worldAnchorB){ this.setWorldAnchorB(options.worldAnchorB); }

    var worldAnchorA = vec2.create();
    var worldAnchorB = vec2.create();
    this.getWorldAnchorA(worldAnchorA);
    this.getWorldAnchorB(worldAnchorB);
    var worldDistance = vec2.distance(worldAnchorA, worldAnchorB);

    /**
     * Rest length of the spring. Can be set dynamically.
     * @property restLength
     * @type {number}
     */
    this.restLength = options.restLength !== undefined ? options.restLength : worldDistance;
}
LinearSpring.prototype = new Spring();
LinearSpring.prototype.constructor = LinearSpring;

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

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

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

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

var applyForce_r =              vec2.create(),
    applyForce_r_unit =         vec2.create(),
    applyForce_u =              vec2.create(),
    applyForce_f =              vec2.create(),
    applyForce_worldAnchorA =   vec2.create(),
    applyForce_worldAnchorB =   vec2.create(),
    applyForce_ri =             vec2.create(),
    applyForce_rj =             vec2.create(),
    applyForce_tmp =            vec2.create();

/**
 * Apply the spring force to the connected bodies.
 * @private
 * @method applyForce
 */
LinearSpring.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;

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

    // Get offset points
    vec2.subtract(ri, worldAnchorA, bodyA.position);
    vec2.subtract(rj, worldAnchorB, bodyB.position);

    // Compute distance vector between world anchor points
    vec2.subtract(r, worldAnchorB, worldAnchorA);
    var rlen = vec2.length(r);
    vec2.normalize(r_unit,r);

    // Compute relative velocity of the anchor points, u
    vec2.subtract(u, bodyB.velocity, bodyA.velocity);
    vec2.crossZV(tmp, bodyB.angularVelocity, rj);
    vec2.add(u, u, tmp);
    vec2.crossZV(tmp, bodyA.angularVelocity, ri);
    vec2.subtract(u, u, tmp);

    // F = - k * ( x - L ) - D * ( u )
    vec2.scale(f, r_unit, -k*(rlen-l) - d*vec2.dot(u,r_unit));

    // Add forces to bodies
    vec2.subtract( bodyA.force, bodyA.force, f);
    vec2.add( bodyB.force, bodyB.force, f);

    // Angular force
    var ri_x_f = vec2.crossLength(ri, f);
    var rj_x_f = vec2.crossLength(rj, f);
    bodyA.angularForce -= ri_x_f;
    bodyB.angularForce += rj_x_f;
};