/*
* 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;
using System.Diagnostics;
using FarseerPhysics.Common;
using Microsoft.Xna.Framework;
namespace FarseerPhysics.Dynamics.Joints
{
/// <summary>
/// A revolute joint rains to bodies to share a common point while they
/// are free to rotate about the point. The relative rotation about the shared
/// point is the joint angle. You can limit the relative rotation with
/// a joint limit that specifies a lower and upper angle. You can use a motor
/// to drive the relative rotation about the shared point. A maximum motor torque
/// is provided so that infinite forces are not generated.
/// </summary>
public class FixedRevoluteJoint : Joint
{
private bool _enableLimit;
private bool _enableMotor;
private Vector3 _impulse;
private LimitState _limitState;
private float _lowerAngle;
private Mat33 _mass; // effective mass for point-to-point constraint.
private float _maxMotorTorque;
private float _motorImpulse;
private float _motorMass; // effective mass for motor/limit angular constraint.
private float _motorSpeed;
private float _upperAngle;
private Vector2 _worldAnchor;
/// <summary>
/// Initialize the bodies, anchors, and reference angle using the world
/// anchor.
/// This requires defining an
/// anchor point where the bodies are joined. The definition
/// uses local anchor points so that the initial configuration
/// can violate the constraint slightly. You also need to
/// specify the initial relative angle for joint limits. This
/// helps when saving and loading a game.
/// The local anchor points are measured from the body's origin
/// rather than the center of mass because:
/// 1. you might not know where the center of mass will be.
/// 2. if you add/remove shapes from a body and recompute the mass,
/// the joints will be broken.
/// </summary>
/// <param name="body">The body.</param>
/// <param name="bodyAnchor">The body anchor.</param>
/// <param name="worldAnchor">The world anchor.</param>
public FixedRevoluteJoint(Body body, Vector2 bodyAnchor, Vector2 worldAnchor)
: base(body)
{
JointType = JointType.FixedRevolute;
// Changed to local coordinates.
LocalAnchorA = bodyAnchor;
_worldAnchor = worldAnchor;
ReferenceAngle = -BodyA.Rotation;
_impulse = Vector3.Zero;
_limitState = LimitState.Inactive;
}
public override Vector2 WorldAnchorA
{
get { return BodyA.GetWorldPoint(LocalAnchorA); }
}
public override Vector2 WorldAnchorB
{
get { return _worldAnchor; }
set { _worldAnchor = value; }
}
public Vector2 LocalAnchorA { get; set; }
public float ReferenceAngle { get; set; }
/// <summary>
/// Get the current joint angle in radians.
/// </summary>
/// <value></value>
public float JointAngle
{
get { return BodyA.Sweep.A - ReferenceAngle; }
}
/// <summary>
/// Get the current joint angle speed in radians per second.
/// </summary>
/// <value></value>
public float JointSpeed
{
get { return BodyA.AngularVelocityInternal; }
}
/// <summary>
/// Is the joint limit enabled?
/// </summary>
/// <value><c>true</c> if [limit enabled]; otherwise, <c>false</c>.</value>
public bool LimitEnabled
{
get { return _enableLimit; }
set
{
WakeBodies();
_enableLimit = value;
}
}
/// <summary>
/// Get the lower joint limit in radians.
/// </summary>
/// <value></value>
public float LowerLimit
{
get { return _lowerAngle; }
set
{
WakeBodies();
_lowerAngle = value;
}
}
/// <summary>
/// Get the upper joint limit in radians.
/// </summary>
/// <value></value>
public float UpperLimit
{
get { return _upperAngle; }
set
{
WakeBodies();
_upperAngle = value;
}
}
/// <summary>
/// Is the joint motor enabled?
/// </summary>
/// <value><c>true</c> if [motor enabled]; otherwise, <c>false</c>.</value>
public bool MotorEnabled
{
get { return _enableMotor; }
set
{
WakeBodies();
_enableMotor = value;
}
}
/// <summary>
/// Set the motor speed in radians per second.
/// </summary>
/// <value>The speed.</value>
public float MotorSpeed
{
set
{
WakeBodies();
_motorSpeed = value;
}
get { return _motorSpeed; }
}
/// <summary>
/// Set the maximum motor torque, usually in N-m.
/// </summary>
/// <value>The torque.</value>
public float MaxMotorTorque
{
set
{
WakeBodies();
_maxMotorTorque = value;
}
get { return _maxMotorTorque; }
}
/// <summary>
/// Get the current motor torque, usually in N-m.
/// </summary>
/// <value></value>
public float MotorTorque
{
get { return _motorImpulse; }
set
{
WakeBodies();
_motorImpulse = value;
}
}
public override Vector2 GetReactionForce(float inv_dt)
{
return inv_dt * new Vector2(_impulse.X, _impulse.Y);
}
public override float GetReactionTorque(float inv_dt)
{
return inv_dt * _impulse.Z;
}
internal override void InitVelocityConstraints(ref TimeStep step)
{
Body b1 = BodyA;
if (_enableMotor || _enableLimit)
{
// You cannot create a rotation limit between bodies that
// both have fixed rotation.
Debug.Assert(b1.InvI > 0.0f /* || b2._invI > 0.0f*/);
}
// Compute the effective mass matrix.
Transform xf1;
b1.GetTransform(out xf1);
Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
Vector2 r2 = _worldAnchor; // MathUtils.Multiply(ref xf2.R, LocalAnchorB - b2.LocalCenter);
// J = [-I -r1_skew I r2_skew]
// [ 0 -1 0 1]
// r_skew = [-ry; rx]
// Matlab
// K = [ m1+r1y^2*i1+m2+r2y^2*i2, -r1y*i1*r1x-r2y*i2*r2x, -r1y*i1-r2y*i2]
// [ -r1y*i1*r1x-r2y*i2*r2x, m1+r1x^2*i1+m2+r2x^2*i2, r1x*i1+r2x*i2]
// [ -r1y*i1-r2y*i2, r1x*i1+r2x*i2, i1+i2]
float m1 = b1.InvMass;
const float m2 = 0;
float i1 = b1.InvI;
const float i2 = 0;
_mass.Col1.X = m1 + m2 + r1.Y * r1.Y * i1 + r2.Y * r2.Y * i2;
_mass.Col2.X = -r1.Y * r1.X * i1 - r2.Y * r2.X * i2;
_mass.Col3.X = -r1.Y * i1 - r2.Y * i2;
_mass.Col1.Y = _mass.Col2.X;
_mass.Col2.Y = m1 + m2 + r1.X * r1.X * i1 + r2.X * r2.X * i2;
_mass.Col3.Y = r1.X * i1 + r2.X * i2;
_mass.Col1.Z = _mass.Col3.X;
_mass.Col2.Z = _mass.Col3.Y;
_mass.Col3.Z = i1 + i2;
_motorMass = i1 + i2;
if (_motorMass > 0.0f)
{
_motorMass = 1.0f / _motorMass;
}
if (_enableMotor == false)
{
_motorImpulse = 0.0f;
}
if (_enableLimit)
{
float jointAngle = 0 - b1.Sweep.A - ReferenceAngle;
if (Math.Abs(_upperAngle - _lowerAngle) < 2.0f * Settings.AngularSlop)
{
_limitState = LimitState.Equal;
}
else if (jointAngle <= _lowerAngle)
{
if (_limitState != LimitState.AtLower)
{
_impulse.Z = 0.0f;
}
_limitState = LimitState.AtLower;
}
else if (jointAngle >= _upperAngle)
{
if (_limitState != LimitState.AtUpper)
{
_impulse.Z = 0.0f;
}
_limitState = LimitState.AtUpper;
}
else
{
_limitState = LimitState.Inactive;
_impulse.Z = 0.0f;
}
}
else
{
_limitState = LimitState.Inactive;
}
if (Settings.EnableWarmstarting)
{
// Scale impulses to support a variable time step.
_impulse *= step.dtRatio;
_motorImpulse *= step.dtRatio;
Vector2 P = new Vector2(_impulse.X, _impulse.Y);
b1.LinearVelocityInternal -= m1 * P;
b1.AngularVelocityInternal -= i1 * (MathUtils.Cross(r1, P) + _motorImpulse + _impulse.Z);
}
else
{
_impulse = Vector3.Zero;
_motorImpulse = 0.0f;
}
}
internal override void SolveVelocityConstraints(ref TimeStep step)
{
Body b1 = BodyA;
Vector2 v1 = b1.LinearVelocityInternal;
float w1 = b1.AngularVelocityInternal;
Vector2 v2 = Vector2.Zero;
const float w2 = 0;
float m1 = b1.InvMass;
float i1 = b1.InvI;
// Solve motor constraint.
if (_enableMotor && _limitState != LimitState.Equal)
{
float Cdot = w2 - w1 - _motorSpeed;
float impulse = _motorMass * (-Cdot);
float oldImpulse = _motorImpulse;
float maxImpulse = step.dt * _maxMotorTorque;
_motorImpulse = MathUtils.Clamp(_motorImpulse + impulse, -maxImpulse, maxImpulse);
impulse = _motorImpulse - oldImpulse;
w1 -= i1 * impulse;
}
// Solve limit constraint.
if (_enableLimit && _limitState != LimitState.Inactive)
{
Transform xf1;
b1.GetTransform(out xf1);
Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
Vector2 r2 = _worldAnchor;
// Solve point-to-point constraint
Vector2 Cdot1 = v2 + MathUtils.Cross(w2, r2) - v1 - MathUtils.Cross(w1, r1);
float Cdot2 = w2 - w1;
Vector3 Cdot = new Vector3(Cdot1.X, Cdot1.Y, Cdot2);
Vector3 impulse = _mass.Solve33(-Cdot);
if (_limitState == LimitState.Equal)
{
_impulse += impulse;
}
else if (_limitState == LimitState.AtLower)
{
float newImpulse = _impulse.Z + impulse.Z;
if (newImpulse < 0.0f)
{
Vector2 reduced = _mass.Solve22(-Cdot1);
impulse.X = reduced.X;
impulse.Y = reduced.Y;
impulse.Z = -_impulse.Z;
_impulse.X += reduced.X;
_impulse.Y += reduced.Y;
_impulse.Z = 0.0f;
}
}
else if (_limitState == LimitState.AtUpper)
{
float newImpulse = _impulse.Z + impulse.Z;
if (newImpulse > 0.0f)
{
Vector2 reduced = _mass.Solve22(-Cdot1);
impulse.X = reduced.X;
impulse.Y = reduced.Y;
impulse.Z = -_impulse.Z;
_impulse.X += reduced.X;
_impulse.Y += reduced.Y;
_impulse.Z = 0.0f;
}
}
Vector2 P = new Vector2(impulse.X, impulse.Y);
v1 -= m1 * P;
w1 -= i1 * (MathUtils.Cross(r1, P) + impulse.Z);
}
else
{
Transform xf1;
b1.GetTransform(out xf1);
Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
Vector2 r2 = _worldAnchor;
// Solve point-to-point constraint
Vector2 Cdot = v2 + MathUtils.Cross(w2, r2) - v1 - MathUtils.Cross(w1, r1);
Vector2 impulse = _mass.Solve22(-Cdot);
_impulse.X += impulse.X;
_impulse.Y += impulse.Y;
v1 -= m1 * impulse;
w1 -= i1 * MathUtils.Cross(r1, impulse);
}
b1.LinearVelocityInternal = v1;
b1.AngularVelocityInternal = w1;
}
internal override bool SolvePositionConstraints()
{
// TODO_ERIN block solve with limit. COME ON ERIN
Body b1 = BodyA;
float angularError = 0.0f;
float positionError;
// Solve angular limit constraint.
if (_enableLimit && _limitState != LimitState.Inactive)
{
float angle = 0 - b1.Sweep.A - ReferenceAngle;
float limitImpulse = 0.0f;
if (_limitState == LimitState.Equal)
{
// Prevent large angular corrections
float C = MathUtils.Clamp(angle - _lowerAngle, -Settings.MaxAngularCorrection,
Settings.MaxAngularCorrection);
limitImpulse = -_motorMass * C;
angularError = Math.Abs(C);
}
else if (_limitState == LimitState.AtLower)
{
float C = angle - _lowerAngle;
angularError = -C;
// Prevent large angular corrections and allow some slop.
C = MathUtils.Clamp(C + Settings.AngularSlop, -Settings.MaxAngularCorrection, 0.0f);
limitImpulse = -_motorMass * C;
}
else if (_limitState == LimitState.AtUpper)
{
float C = angle - _upperAngle;
angularError = C;
// Prevent large angular corrections and allow some slop.
C = MathUtils.Clamp(C - Settings.AngularSlop, 0.0f, Settings.MaxAngularCorrection);
limitImpulse = -_motorMass * C;
}
b1.Sweep.A -= b1.InvI * limitImpulse;
b1.SynchronizeTransform();
}
// Solve point-to-point constraint.
{
Transform xf1;
b1.GetTransform(out xf1);
Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
Vector2 r2 = _worldAnchor;
Vector2 C = Vector2.Zero + r2 - b1.Sweep.C - r1;
positionError = C.Length();
float invMass1 = b1.InvMass;
const float invMass2 = 0;
float invI1 = b1.InvI;
const float invI2 = 0;
// Handle large detachment.
const float k_allowedStretch = 10.0f * Settings.LinearSlop;
if (C.LengthSquared() > k_allowedStretch * k_allowedStretch)
{
// Use a particle solution (no rotation).
Vector2 u = C;
u.Normalize();
float k = invMass1 + invMass2;
Debug.Assert(k > Settings.Epsilon);
float m = 1.0f / k;
Vector2 impulse2 = m * (-C);
const float k_beta = 0.5f;
b1.Sweep.C -= k_beta * invMass1 * impulse2;
C = Vector2.Zero + r2 - b1.Sweep.C - r1;
}
Mat22 K1 = new Mat22(new Vector2(invMass1 + invMass2, 0.0f), new Vector2(0.0f, invMass1 + invMass2));
Mat22 K2 = new Mat22(new Vector2(invI1 * r1.Y * r1.Y, -invI1 * r1.X * r1.Y),
new Vector2(-invI1 * r1.X * r1.Y, invI1 * r1.X * r1.X));
Mat22 K3 = new Mat22(new Vector2(invI2 * r2.Y * r2.Y, -invI2 * r2.X * r2.Y),
new Vector2(-invI2 * r2.X * r2.Y, invI2 * r2.X * r2.X));
Mat22 Ka;
Mat22.Add(ref K1, ref K2, out Ka);
Mat22 K;
Mat22.Add(ref Ka, ref K3, out K);
Vector2 impulse = K.Solve(-C);
b1.Sweep.C -= b1.InvMass * impulse;
b1.Sweep.A -= b1.InvI * MathUtils.Cross(r1, impulse);
b1.SynchronizeTransform();
}
return positionError <= Settings.LinearSlop && angularError <= Settings.AngularSlop;
}
}
}
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