p2s/src/equations/ContactEquation.js

A JavaScript 2D physics engine.

var Equation = require("./Equation"),
    vec2 = require('../math/vec2');

module.exports = ContactEquation;

/**
 * Non-penetration constraint equation. Tries to make the contactPointA and contactPointB vectors coincide, while keeping the applied force repulsive.
 *
 * @class ContactEquation
 * @constructor
 * @extends Equation
 * @param {Body} bodyA
 * @param {Body} bodyB
 */
function ContactEquation(bodyA, bodyB){
    Equation.call(this, bodyA, bodyB, 0, Number.MAX_VALUE);

    /**
     * Vector from body i center of mass to the contact point.
     * @property contactPointA
     * @type {Array}
     */
    this.contactPointA = vec2.create();
    this.penetrationVec = vec2.create();

    /**
     * World-oriented vector from body A center of mass to the contact point.
     * @property contactPointB
     * @type {Array}
     */
    this.contactPointB = vec2.create();

    /**
     * The normal vector, pointing out of body i
     * @property normalA
     * @type {Array}
     */
    this.normalA = vec2.create();

    /**
     * The restitution to use (0=no bounciness, 1=max bounciness).
     * @property restitution
     * @type {Number}
     */
    this.restitution = 0;

    /**
     * This property is set to true if this is the first impact between the bodies (not persistant contact).
     * @property firstImpact
     * @type {Boolean}
     * @readOnly
     */
    this.firstImpact = false;

    /**
     * The shape in body i that triggered this contact.
     * @property shapeA
     * @type {Shape}
     */
    this.shapeA = null;

    /**
     * The shape in body j that triggered this contact.
     * @property shapeB
     * @type {Shape}
     */
    this.shapeB = null;
}
ContactEquation.prototype = new Equation();
ContactEquation.prototype.constructor = ContactEquation;
ContactEquation.prototype.computeB = function(a,b,h){
    var bi = this.bodyA,
        bj = this.bodyB,
        ri = this.contactPointA,
        rj = this.contactPointB,
        xi = bi.position,
        xj = bj.position;

    var n = this.normalA,
        G = this.G;

    // Caluclate cross products
    var rixn = vec2.crossLength(ri,n),
        rjxn = vec2.crossLength(rj,n);

    // G = [-n -rixn n rjxn]
    G[0] = -n[0];
    G[1] = -n[1];
    G[2] = -rixn;
    G[3] = n[0];
    G[4] = n[1];
    G[5] = rjxn;


    // Compute iteration
    var GW, Gq;
    if(this.firstImpact && this.restitution !== 0){
        Gq = 0;
        GW = (1/b)*(1+this.restitution) * this.computeGW();
    } else {
        // Calculate q = xj+rj -(xi+ri) i.e. the penetration vector
        var penetrationVec = this.penetrationVec;
        addSubSub(penetrationVec,xj,rj,xi,ri);
        Gq = vec2.dot(n,penetrationVec) + this.offset;
        GW = this.computeGW();
    }

    var GiMf = this.computeGiMf();
    var B = - Gq * a - GW * b - h*GiMf;

    return B;
};

function addSubSub(out, a, b, c, d){
    out[0] = a[0] + b[0] - c[0] - d[0];
    out[1] = a[1] + b[1] - c[1] - d[1];
}

var vi = vec2.create();
var vj = vec2.create();
var relVel = vec2.create();

/**
 * Get the relative velocity along the normal vector.
 * @method getVelocityAlongNormal
 * @return {number}
 */
ContactEquation.prototype.getVelocityAlongNormal = function(){

    this.bodyA.getVelocityAtPoint(vi, this.contactPointA);
    this.bodyB.getVelocityAtPoint(vj, this.contactPointB);

    vec2.subtract(relVel, vi, vj);

    return vec2.dot(this.normalA, relVel);
};