planck-js/lib/joint/RopeJoint.js

2D physics engine for JavaScript/HTML5 game development

/*
 * Copyright (c) 2016      Ali Shakiba http://shakiba.me/planck.js
 * Copyright (c) 2006-2011 Erin Catto  http://www.box2d.org
 *
 * This software is provided 'as-is', without any express or implied
 * warranty.  In no event will the authors be held liable for any damages
 * arising from the use of this software.
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 * 1. The origin of this software must not be misrepresented; you must not
 * claim that you wrote the original software. If you use this software
 * in a product, an acknowledgment in the product documentation would be
 * appreciated but is not required.
 * 2. Altered source versions must be plainly marked as such, and must not be
 * misrepresented as being the original software.
 * 3. This notice may not be removed or altered from any source distribution.
 */

module.exports = RopeJoint;

var options = require('../util/options');
var create = require('../util/create');
var Settings = require('../Settings');

var Math = require('../common/Math');
var Vec2 = require('../common/Vec2');
var Vec3 = require('../common/Vec3');
var Mat22 = require('../common/Mat22');
var Mat33 = require('../common/Mat33');
var Rot = require('../common/Rot');
var Sweep = require('../common/Sweep');
var Transform = require('../common/Transform');
var Velocity = require('../common/Velocity');
var Position = require('../common/Position');

var Joint = require('../Joint');

var inactiveLimit = 0;
var atLowerLimit = 1;
var atUpperLimit = 2;
var equalLimits = 3;

RopeJoint.TYPE = 'rope-joint';

RopeJoint._super = Joint;
RopeJoint.prototype = create(RopeJoint._super.prototype);

/**
 * Rope joint definition. This requires two body anchor points and a maximum
 * lengths. Note: by default the connected objects will not collide. see
 * collideConnected in JointDef.
 * 
 * @prop {float} maxLength The maximum length of the rope. Warning: this must be
 *       larger than b2_linearSlop or the joint will have no effect.
 */
var RopeJointDef = {
  maxLength : 0.0
};

/**
 * A rope joint enforces a maximum distance between two points on two bodies. It
 * has no other effect.
 * 
 * Warning: if you attempt to change the maximum length during the simulation
 * you will get some non-physical behavior.
 * 
 * A model that would allow you to dynamically modify the length would have some
 * sponginess, so I chose not to implement it that way. See DistanceJoint if you
 * want to dynamically control length.
 * 
 * @prop {Vec2} localAnchorA The local anchor point relative to bodyA's origin.
 * @prop {Vec2} localAnchorB The local anchor point relative to bodyB's origin.
 */
function RopeJoint(def, bodyA, bodyB, anchor) {
  if (!(this instanceof RopeJoint)) {
    return new RopeJoint(def, bodyA, bodyB, anchor);
  }

  def = options(def, RopeJointDef);
  Joint.call(this, def, bodyA, bodyB);
  this.m_type = RopeJoint.TYPE;

  this.m_localAnchorA = def.localAnchorA || bodyA.GetLocalPoint(anchor);
  this.m_localAnchorB = def.localAnchorB || bodyB.GetLocalPoint(anchor);

  this.m_maxLength = def.maxLength;

  this.m_mass = 0.0;
  this.m_impulse = 0.0;
  this.m_length = 0.0;
  this.m_state = inactiveLimit;

  // Solver temp
  this.m_u; // Vec2
  this.m_rA; // Vec2
  this.m_rB; // Vec2
  this.m_localCenterA; // Vec2
  this.m_localCenterB; // Vec2
  this.m_invMassA; // float
  this.m_invMassB; // float
  this.m_invIA; // float
  this.m_invIB; // float
  this.m_mass; // float

  // Limit:
  // C = norm(pB - pA) - L
  // u = (pB - pA) / norm(pB - pA)
  // Cdot = dot(u, vB + cross(wB, rB) - vA - cross(wA, rA))
  // J = [-u -cross(rA, u) u cross(rB, u)]
  // K = J * invM * JT
  // = invMassA + invIA * cross(rA, u)^2 + invMassB + invIB * cross(rB, u)^2
};

/**
 * The local anchor point relative to bodyA's origin.
 */
RopeJoint.prototype.GetLocalAnchorA = function() {
  return this.m_localAnchorA;
}

/**
 * The local anchor point relative to bodyB's origin.
 */
RopeJoint.prototype.GetLocalAnchorB = function() {
  return this.m_localAnchorB;
}

/**
 * Set/Get the maximum length of the rope.
 */
RopeJoint.prototype.SetMaxLength = function(length) {
  this.m_maxLength = length;
}

RopeJoint.prototype.GetMaxLength = function() {
  return this.m_maxLength;
}

RopeJoint.prototype.GetLimitState = function() {
  // TODO LimitState
  return this.m_state;
}

RopeJoint.prototype.GetAnchorA = function() {
  return this.m_bodyA.GetWorldPoint(this.m_localAnchorA);
}

RopeJoint.prototype.GetAnchorB = function() {
  return this.m_bodyB.GetWorldPoint(this.m_localAnchorB);
}

RopeJoint.prototype.GetReactionForce = function(inv_dt) {
  var F = (inv_dt * this.m_impulse) * this.m_u; // Vec2
  return F;
}

RopeJoint.prototype.GetReactionTorque = function(inv_dt) {
  B2_NOT_USED(inv_dt);
  return 0.0;
}

RopeJoint.prototype.InitVelocityConstraints = function(step) {
  this.m_localCenterA = this.m_bodyA.m_sweep.localCenter;
  this.m_localCenterB = this.m_bodyB.m_sweep.localCenter;
  this.m_invMassA = this.m_bodyA.m_invMass;
  this.m_invMassB = this.m_bodyB.m_invMass;
  this.m_invIA = this.m_bodyA.m_invI;
  this.m_invIB = this.m_bodyB.m_invI;

  var cA = this.m_bodyA.c_position.c;
  var aA = this.m_bodyA.c_position.a;
  var vA = this.m_bodyA.c_velocity.v;
  var wA = this.m_bodyA.c_velocity.w;

  var cB = this.m_bodyB.c_position.c;
  var aB = this.m_bodyB.c_position.a;
  var vB = this.m_bodyB.c_velocity.v;
  var wB = this.m_bodyB.c_velocity.w;

  var qA = Rot(aA);
  var qB = Rot(aB);

  this.m_rA = Rot.MulSub(qA, this.m_localAnchorA, this.m_localCenterA);
  this.m_rB = Rot.MulSub(qB, this.m_localAnchorB, this.m_localCenterB);
  this.m_u = Vec2();
  this.m_u.WAdd(1, cB, 1, this.m_rB);
  this.m_u.WSub(1, cA, 1, this.m_rA); // Vec2

  this.m_length = this.m_u.Length();

  var C = this.m_length - this.m_maxLength; // float
  if (C > 0.0) {
    this.m_state = atUpperLimit;
  } else {
    this.m_state = inactiveLimit;
  }

  if (this.m_length > Settings.linearSlop) {
    this.m_u.Mul(1.0 / this.m_length);
  } else {
    this.m_u.SetZero();
    this.m_mass = 0.0;
    this.m_impulse = 0.0;
    return;
  }

  // Compute effective mass.
  var crA = Vec2.Cross(this.m_rA, this.m_u); // float
  var crB = Vec2.Cross(this.m_rB, this.m_u); // float
  var invMass = this.m_invMassA + this.m_invIA * crA * crA + this.m_invMassB
      + this.m_invIB * crB * crB; // float

  this.m_mass = invMass != 0.0 ? 1.0 / invMass : 0.0;

  if (step.warmStarting) {
    // Scale the impulse to support a variable time step.
    this.m_impulse *= step.dtRatio;

    var P = Vec2.WGet(this.m_impulse, this.m_u); // Vec2
    vA.WSub(this.m_invMassA, P);
    wA -= this.m_invIA * Cross(this.m_rA, P);
    vB.WAdd(this.m_invMassB, P);
    wB += this.m_invIB * Cross(this.m_rB, P);
  } else {
    this.m_impulse = 0.0;
  }

  this.m_bodyA.c_velocity.v.Set(vA);
  this.m_bodyA.c_velocity.w = wA;
  this.m_bodyB.c_velocity.v.Set(vB);
  this.m_bodyB.c_velocity.w = wB;
}

RopeJoint.prototype.SolveVelocityConstraints = function(step) {
  var vA = this.m_bodyA.c_velocity.v;
  var wA = this.m_bodyA.c_velocity.w;
  var vB = this.m_bodyB.c_velocity.v;
  var wB = this.m_bodyB.c_velocity.w;

  // Cdot = dot(u, v + cross(w, r))
  var vpA = Vec2.AddCross(vA, wA, this.m_rA); // Vec2
  var vpB = Vec2.AddCross(vB, wB, this.m_rB); // Vec2
  var C = this.m_length - this.m_maxLength; // float
  var Cdot = Vec2.Dot(this.m_u, Vec2.Sub(vpB, vpA)); // float

  // Predictive constraint.
  if (C < 0.0) {
    Cdot += step.inv_dt * C;
  }

  var impulse = -this.m_mass * Cdot; // float
  var oldImpulse = this.m_impulse; // float
  this.m_impulse = Math.min(0.0, this.m_impulse + impulse);
  impulse = this.m_impulse - oldImpulse;

  var P = Vec2.WGet(impulse, this.m_u); // Vec2
  vA.WSub(this.m_invMassA, P);
  wA -= this.m_invIA * Vec2.Cross(this.m_rA, P);
  vB.WAdd(this.m_invMassB, P);
  wB += this.m_invIB * Vec2.Cross(this.m_rB, P);

  this.m_bodyA.c_velocity.v = vA;
  this.m_bodyA.c_velocity.w = wA;
  this.m_bodyB.c_velocity.v = vB;
  this.m_bodyB.c_velocity.w = wB;
}

RopeJoint.prototype.SolvePositionConstraints = function(step) {
  var cA = this.m_bodyA.c_position.c; // Vec2
  var aA = this.m_bodyA.c_position.a; // float
  var cB = this.m_bodyB.c_position.c; // Vec2
  var aB = this.m_bodyB.c_position.a; // float

  var qA = Rot(aA);
  var qB = Rot(aB);

  var rA = Rot.MulSub(qA, this.m_localAnchorA, this.m_localCenterA);
  var rB = Rot.MulSub(qB, this.m_localAnchorB, this.m_localCenterB);
  var u = Vec2();
  u.WAdd(1, cB, 1, rB);
  u.WSub(1, cA, 1, rA); // Vec2

  var length = u.Normalize(); // float
  var C = length - this.m_maxLength; // float

  C = Math.clamp(C, 0.0, Settings.maxLinearCorrection);

  var impulse = -this.m_mass * C; // float
  var P = Vec2.WGet(impulse, u); // Vec2

  cA.WSub(this.m_invMassA, P);
  aA -= this.m_invIA * Vec2.Cross(rA, P);
  cB.WAdd(this.m_invMassB, P);
  aB += this.m_invIB * Vec2.Cross(rB, P);

  this.m_bodyA.c_position.c.Set(cA);
  this.m_bodyA.c_position.a = aA;
  this.m_bodyB.c_position.c.Set(cB);
  this.m_bodyB.c_position.a = aB;

  return length - this.m_maxLength < Settings.linearSlop;
}