cannon/src/equations/ContactEquation.js

A lightweight 3D physics engine written in JavaScript.

module.exports = ContactEquation;

var Equation = require('./Equation');
var Vec3 = require('../math/Vec3');
var Mat3 = require('../math/Mat3');

/**
 * Contact/non-penetration constraint equation
 * @class ContactEquation
 * @constructor
 * @author schteppe
 * @param {Body} bodyA
 * @param {Body} bodyB
 * @extends Equation
 */
function ContactEquation(bodyA, bodyB, maxForce){
    maxForce = typeof(maxForce) !== 'undefined' ? maxForce : 1e6;
    Equation.call(this, bodyA, bodyB, 0, maxForce);

    /**
     * @property restitution
     * @type {Number}
     */
    this.restitution = 0.0; // "bounciness": u1 = -e*u0

    /**
     * World-oriented vector that goes from the center of bi to the contact point.
     * @property {Vec3} ri
     */
    this.ri = new Vec3();

    /**
     * World-oriented vector that starts in body j position and goes to the contact point.
     * @property {Vec3} rj
     */
    this.rj = new Vec3();

    /**
     * Contact normal, pointing out of body i.
     * @property {Vec3} ni
     */
    this.ni = new Vec3();
}

ContactEquation.prototype = new Equation();
ContactEquation.prototype.constructor = ContactEquation;

var ContactEquation_computeB_temp1 = new Vec3(); // Temp vectors
var ContactEquation_computeB_temp2 = new Vec3();
var ContactEquation_computeB_temp3 = new Vec3();
ContactEquation.prototype.computeB = function(h){
    var a = this.a,
        b = this.b,
        bi = this.bi,
        bj = this.bj,
        ri = this.ri,
        rj = this.rj,
        rixn = ContactEquation_computeB_temp1,
        rjxn = ContactEquation_computeB_temp2,

        vi = bi.velocity,
        wi = bi.angularVelocity,
        fi = bi.force,
        taui = bi.torque,

        vj = bj.velocity,
        wj = bj.angularVelocity,
        fj = bj.force,
        tauj = bj.torque,

        penetrationVec = ContactEquation_computeB_temp3,

        GA = this.jacobianElementA,
        GB = this.jacobianElementB,

        n = this.ni;

    // Caluclate cross products
    ri.cross(n,rixn);
    rj.cross(n,rjxn);

    // g = xj+rj -(xi+ri)
    // G = [ -ni  -rixn  ni  rjxn ]
    n.negate(GA.spatial);
    rixn.negate(GA.rotational);
    GB.spatial.copy(n);
    GB.rotational.copy(rjxn);

    // Calculate the penetration vector
    penetrationVec.copy(bj.position);
    penetrationVec.vadd(rj,penetrationVec);
    penetrationVec.vsub(bi.position,penetrationVec);
    penetrationVec.vsub(ri,penetrationVec);

    var g = n.dot(penetrationVec);

    // Compute iteration
    var ePlusOne = this.restitution + 1;
    var GW = ePlusOne * vj.dot(n) - ePlusOne * vi.dot(n) + wj.dot(rjxn) - wi.dot(rixn);
    var GiMf = this.computeGiMf();

    var B = - g * a - GW * b - h*GiMf;

    return B;
};

var ContactEquation_getImpactVelocityAlongNormal_vi = new Vec3();
var ContactEquation_getImpactVelocityAlongNormal_vj = new Vec3();
var ContactEquation_getImpactVelocityAlongNormal_xi = new Vec3();
var ContactEquation_getImpactVelocityAlongNormal_xj = new Vec3();
var ContactEquation_getImpactVelocityAlongNormal_relVel = new Vec3();

/**
 * Get the current relative velocity in the contact point.
 * @method getImpactVelocityAlongNormal
 * @return {number}
 */
ContactEquation.prototype.getImpactVelocityAlongNormal = function(){
    var vi = ContactEquation_getImpactVelocityAlongNormal_vi;
    var vj = ContactEquation_getImpactVelocityAlongNormal_vj;
    var xi = ContactEquation_getImpactVelocityAlongNormal_xi;
    var xj = ContactEquation_getImpactVelocityAlongNormal_xj;
    var relVel = ContactEquation_getImpactVelocityAlongNormal_relVel;

    this.bi.position.vadd(this.ri, xi);
    this.bj.position.vadd(this.rj, xj);

    this.bi.getVelocityAtWorldPoint(xi, vi);
    this.bj.getVelocityAtWorldPoint(xj, vj);

    vi.vsub(vj, relVel);

    return this.ni.dot(relVel);
};