p2s/src/constraints/LockConstraint.js

A JavaScript 2D physics engine.

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

module.exports = LockConstraint;

/**
 * Locks the relative position and rotation between two bodies.
 *
 * @class LockConstraint
 * @constructor
 * @author schteppe
 * @param {Body} bodyA
 * @param {Body} bodyB
 * @param {Object} [options]
 * @param {Array}  [options.localOffsetB] The offset of bodyB in bodyA's frame. If not given the offset is computed from current positions.
 * @param {number} [options.localAngleB] The angle of bodyB in bodyA's frame. If not given, the angle is computed from current angles.
 * @param {number} [options.maxForce]
 * @extends Constraint
 *
 * @example
 *     // Locks the relative position and rotation between bodyA and bodyB
 *     var constraint = new LockConstraint(bodyA, bodyB);
 *     world.addConstraint(constraint);
 */
function LockConstraint(bodyA, bodyB, options){
    options = options || {};

    Constraint.call(this,bodyA,bodyB,Constraint.LOCK,options);

    var maxForce = ( typeof(options.maxForce)==="undefined" ? Number.MAX_VALUE : options.maxForce );

    // Use 3 equations:
    // gx =   (xj - xi - l) * xhat = 0
    // gy =   (xj - xi - l) * yhat = 0
    // gr =   (xi - xj + r) * that = 0
    //
    // ...where:
    //   l is the localOffsetB vector rotated to world in bodyA frame
    //   r is the same vector but reversed and rotated from bodyB frame
    //   xhat, yhat are world axis vectors
    //   that is the tangent of r
    //
    // For the first two constraints, we get
    // G*W = (vj - vi - ldot  ) * xhat
    //     = (vj - vi - wi x l) * xhat
    //
    // Since (wi x l) * xhat = (l x xhat) * wi, we get
    // G*W = [ -1   0   (-l x xhat)  1   0   0] * [vi wi vj wj]
    //
    // The last constraint gives
    // GW = (vi - vj + wj x r) * that
    //    = [  that   0  -that  (r x t) ]

    var x =     new Equation(bodyA,bodyB,-maxForce,maxForce),
        y =     new Equation(bodyA,bodyB,-maxForce,maxForce),
        rot =   new Equation(bodyA,bodyB,-maxForce,maxForce);

    var l = vec2.create(),
        g = vec2.create(),
        that = this;
    x.computeGq = function(){
        vec2.rotate(l, that.localOffsetB, bodyA.angle);
        vec2.subtract(g, bodyB.position, bodyA.position);
        vec2.subtract(g, g, l);
        return g[0];
    };
    y.computeGq = function(){
        vec2.rotate(l, that.localOffsetB, bodyA.angle);
        vec2.subtract(g, bodyB.position, bodyA.position);
        vec2.subtract(g, g, l);
        return g[1];
    };
    var r = vec2.create(),
        t = vec2.create();
    rot.computeGq = function(){
        vec2.rotate(r, that.localOffsetB, bodyB.angle - that.localAngleB);
        vec2.scale(r,r,-1);
        vec2.subtract(g,bodyA.position,bodyB.position);
        vec2.add(g,g,r);
        vec2.rotate(t,r,-Math.PI/2);
        vec2.normalize(t,t);
        return vec2.dot(g,t);
    };

    /**
     * The offset of bodyB in bodyA's frame.
     * @property {Array} localOffsetB
     */
    this.localOffsetB = vec2.create();
    if(options.localOffsetB){
        vec2.copy(this.localOffsetB, options.localOffsetB);
    } else {
        // Construct from current positions
        vec2.subtract(this.localOffsetB, bodyB.position, bodyA.position);
        vec2.rotate(this.localOffsetB, this.localOffsetB, -bodyA.angle);
    }

    /**
     * The offset angle of bodyB in bodyA's frame.
     * @property {Number} localAngleB
     */
    this.localAngleB = 0;
    if(typeof(options.localAngleB) === 'number'){
        this.localAngleB = options.localAngleB;
    } else {
        // Construct
        this.localAngleB = bodyB.angle - bodyA.angle;
    }

    this.equations.push(x, y, rot);
    this.setMaxForce(maxForce);
}
LockConstraint.prototype = new Constraint();
LockConstraint.prototype.constructor = LockConstraint;

/**
 * Set the maximum force to be applied.
 * @method setMaxForce
 * @param {Number} force
 */
LockConstraint.prototype.setMaxForce = function(force){
    var eqs = this.equations;
    for(var i=0; i<this.equations.length; i++){
        eqs[i].maxForce =  force;
        eqs[i].minForce = -force;
    }
};

/**
 * Get the max force.
 * @method getMaxForce
 * @return {Number}
 */
LockConstraint.prototype.getMaxForce = function(){
    return this.equations[0].maxForce;
};

var l = vec2.create();
var r = vec2.create();
var t = vec2.create();
var xAxis = vec2.fromValues(1,0);
var yAxis = vec2.fromValues(0,1);
LockConstraint.prototype.update = function(){
    var x =   this.equations[0],
        y =   this.equations[1],
        rot = this.equations[2],
        bodyA = this.bodyA,
        bodyB = this.bodyB;

    vec2.rotate(l,this.localOffsetB,bodyA.angle);
    vec2.rotate(r,this.localOffsetB,bodyB.angle - this.localAngleB);
    vec2.scale(r,r,-1);

    vec2.rotate(t,r,Math.PI/2);
    vec2.normalize(t,t);

    x.G[0] = -1;
    x.G[1] =  0;
    x.G[2] = -vec2.crossLength(l,xAxis);
    x.G[3] =  1;

    y.G[0] =  0;
    y.G[1] = -1;
    y.G[2] = -vec2.crossLength(l,yAxis);
    y.G[4] =  1;

    rot.G[0] =  -t[0];
    rot.G[1] =  -t[1];
    rot.G[3] =  t[0];
    rot.G[4] =  t[1];
    rot.G[5] =  vec2.crossLength(r,t);
};