/*
* Farseer Physics Engine based on Box2D.XNA port:
* Copyright (c) 2010 Ian Qvist
*
* Box2D.XNA port of Box2D:
* Copyright (c) 2009 Brandon Furtwangler, Nathan Furtwangler
*
* Original source Box2D:
* Copyright (c) 2006-2009 Erin Catto http://www.gphysics.com
*
* 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.
*/
using System.Diagnostics;
using FarseerPhysics.Common;
using Microsoft.Xna.Framework;
namespace FarseerPhysics.Dynamics.Joints
{
// Point-to-point constraint
// Cdot = v2 - v1
// = v2 + cross(w2, r2) - v1 - cross(w1, r1)
// J = [-I -r1_skew I r2_skew ]
// Identity used:
// w k % (rx i + ry j) = w * (-ry i + rx j)
// Angle constraint
// Cdot = w2 - w1
// J = [0 0 -1 0 0 1]
// K = invI1 + invI2
///
/// Friction joint. This is used for top-down friction.
/// It provides 2D translational friction and angular friction.
///
public class FrictionJoint : Joint
{
public Vector2 LocalAnchorA;
public Vector2 LocalAnchorB;
private float _angularImpulse;
private float _angularMass;
private Vector2 _linearImpulse;
private Mat22 _linearMass;
internal FrictionJoint()
{
JointType = JointType.Friction;
}
public FrictionJoint(Body bodyA, Body bodyB, Vector2 localAnchorA, Vector2 localAnchorB)
: base(bodyA, bodyB)
{
JointType = JointType.Friction;
LocalAnchorA = localAnchorA;
LocalAnchorB = localAnchorB;
}
public override Vector2 WorldAnchorA
{
get { return BodyA.GetWorldPoint(LocalAnchorA); }
}
public override Vector2 WorldAnchorB
{
get { return BodyB.GetWorldPoint(LocalAnchorB); }
set { Debug.Assert(false, "You can't set the world anchor on this joint type."); }
}
///
/// The maximum friction force in N.
///
public float MaxForce { get; set; }
///
/// The maximum friction torque in N-m.
///
public float MaxTorque { get; set; }
public override Vector2 GetReactionForce(float inv_dt)
{
return inv_dt * _linearImpulse;
}
public override float GetReactionTorque(float inv_dt)
{
return inv_dt * _angularImpulse;
}
internal override void InitVelocityConstraints(ref TimeStep step)
{
Body bA = BodyA;
Body bB = BodyB;
Transform xfA, xfB;
bA.GetTransform(out xfA);
bB.GetTransform(out xfB);
// Compute the effective mass matrix.
Vector2 rA = MathUtils.Multiply(ref xfA.R, LocalAnchorA - bA.LocalCenter);
Vector2 rB = MathUtils.Multiply(ref xfB.R, LocalAnchorB - bB.LocalCenter);
// J = [-I -r1_skew I r2_skew]
// [ 0 -1 0 1]
// r_skew = [-ry; rx]
// Matlab
// K = [ mA+r1y^2*iA+mB+r2y^2*iB, -r1y*iA*r1x-r2y*iB*r2x, -r1y*iA-r2y*iB]
// [ -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB, r1x*iA+r2x*iB]
// [ -r1y*iA-r2y*iB, r1x*iA+r2x*iB, iA+iB]
float mA = bA.InvMass, mB = bB.InvMass;
float iA = bA.InvI, iB = bB.InvI;
Mat22 K1 = new Mat22();
K1.Col1.X = mA + mB;
K1.Col2.X = 0.0f;
K1.Col1.Y = 0.0f;
K1.Col2.Y = mA + mB;
Mat22 K2 = new Mat22();
K2.Col1.X = iA * rA.Y * rA.Y;
K2.Col2.X = -iA * rA.X * rA.Y;
K2.Col1.Y = -iA * rA.X * rA.Y;
K2.Col2.Y = iA * rA.X * rA.X;
Mat22 K3 = new Mat22();
K3.Col1.X = iB * rB.Y * rB.Y;
K3.Col2.X = -iB * rB.X * rB.Y;
K3.Col1.Y = -iB * rB.X * rB.Y;
K3.Col2.Y = iB * rB.X * rB.X;
Mat22 K12;
Mat22.Add(ref K1, ref K2, out K12);
Mat22 K;
Mat22.Add(ref K12, ref K3, out K);
_linearMass = K.Inverse;
_angularMass = iA + iB;
if (_angularMass > 0.0f)
{
_angularMass = 1.0f / _angularMass;
}
if (Settings.EnableWarmstarting)
{
// Scale impulses to support a variable time step.
_linearImpulse *= step.dtRatio;
_angularImpulse *= step.dtRatio;
Vector2 P = new Vector2(_linearImpulse.X, _linearImpulse.Y);
bA.LinearVelocityInternal -= mA * P;
bA.AngularVelocityInternal -= iA * (MathUtils.Cross(rA, P) + _angularImpulse);
bB.LinearVelocityInternal += mB * P;
bB.AngularVelocityInternal += iB * (MathUtils.Cross(rB, P) + _angularImpulse);
}
else
{
_linearImpulse = Vector2.Zero;
_angularImpulse = 0.0f;
}
}
internal override void SolveVelocityConstraints(ref TimeStep step)
{
Body bA = BodyA;
Body bB = BodyB;
Vector2 vA = bA.LinearVelocityInternal;
float wA = bA.AngularVelocityInternal;
Vector2 vB = bB.LinearVelocityInternal;
float wB = bB.AngularVelocityInternal;
float mA = bA.InvMass, mB = bB.InvMass;
float iA = bA.InvI, iB = bB.InvI;
Transform xfA, xfB;
bA.GetTransform(out xfA);
bB.GetTransform(out xfB);
Vector2 rA = MathUtils.Multiply(ref xfA.R, LocalAnchorA - bA.LocalCenter);
Vector2 rB = MathUtils.Multiply(ref xfB.R, LocalAnchorB - bB.LocalCenter);
// Solve angular friction
{
float Cdot = wB - wA;
float impulse = -_angularMass * Cdot;
float oldImpulse = _angularImpulse;
float maxImpulse = step.dt * MaxTorque;
_angularImpulse = MathUtils.Clamp(_angularImpulse + impulse, -maxImpulse, maxImpulse);
impulse = _angularImpulse - oldImpulse;
wA -= iA * impulse;
wB += iB * impulse;
}
// Solve linear friction
{
Vector2 Cdot = vB + MathUtils.Cross(wB, rB) - vA - MathUtils.Cross(wA, rA);
Vector2 impulse = -MathUtils.Multiply(ref _linearMass, Cdot);
Vector2 oldImpulse = _linearImpulse;
_linearImpulse += impulse;
float maxImpulse = step.dt * MaxForce;
if (_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
{
_linearImpulse.Normalize();
_linearImpulse *= maxImpulse;
}
impulse = _linearImpulse - oldImpulse;
vA -= mA * impulse;
wA -= iA * MathUtils.Cross(rA, impulse);
vB += mB * impulse;
wB += iB * MathUtils.Cross(rB, impulse);
}
bA.LinearVelocityInternal = vA;
bA.AngularVelocityInternal = wA;
bB.LinearVelocityInternal = vB;
bB.AngularVelocityInternal = wB;
}
internal override bool SolvePositionConstraints()
{
return true;
}
}
}