axiosengine/axios/Dynamics/Joints/FixedMouseJoint.cs

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2012-03-19 23:57:59 +00:00
/*
* 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
{
/// <summary>
/// A mouse joint is used to make a point on a body track a
/// specified world point. This a soft constraint with a maximum
/// force. This allows the constraint to stretch and without
/// applying huge forces.
/// NOTE: this joint is not documented in the manual because it was
/// developed to be used in the testbed. If you want to learn how to
/// use the mouse joint, look at the testbed.
/// </summary>
public class FixedMouseJoint : Joint
{
public Vector2 LocalAnchorA;
private Vector2 _C; // position error
private float _beta;
private float _gamma;
private Vector2 _impulse;
private Mat22 _mass; // effective mass for point-to-point constraint.
private Vector2 _worldAnchor;
/// <summary>
/// This requires a world target point,
/// tuning parameters, and the time step.
/// </summary>
/// <param name="body">The body.</param>
/// <param name="worldAnchor">The target.</param>
public FixedMouseJoint(Body body, Vector2 worldAnchor)
: base(body)
{
JointType = JointType.FixedMouse;
Frequency = 5.0f;
DampingRatio = 0.7f;
Debug.Assert(worldAnchor.IsValid());
Transform xf1;
BodyA.GetTransform(out xf1);
_worldAnchor = worldAnchor;
LocalAnchorA = BodyA.GetLocalPoint(worldAnchor);
}
public override Vector2 WorldAnchorA
{
get { return BodyA.GetWorldPoint(LocalAnchorA); }
}
public override Vector2 WorldAnchorB
{
get { return _worldAnchor; }
set
{
BodyA.Awake = true;
_worldAnchor = value;
}
}
/// <summary>
/// The maximum constraint force that can be exerted
/// to move the candidate body. Usually you will express
/// as some multiple of the weight (multiplier * mass * gravity).
/// </summary>
public float MaxForce { get; set; }
/// <summary>
/// The response speed.
/// </summary>
public float Frequency { get; set; }
/// <summary>
/// The damping ratio. 0 = no damping, 1 = critical damping.
/// </summary>
public float DampingRatio { get; set; }
public override Vector2 GetReactionForce(float inv_dt)
{
return inv_dt * _impulse;
}
public override float GetReactionTorque(float inv_dt)
{
return inv_dt * 0.0f;
}
internal override void InitVelocityConstraints(ref TimeStep step)
{
Body b = BodyA;
float mass = b.Mass;
// Frequency
float omega = 2.0f * Settings.Pi * Frequency;
// Damping coefficient
float d = 2.0f * mass * DampingRatio * omega;
// Spring stiffness
float k = mass * (omega * omega);
// magic formulas
// gamma has units of inverse mass.
// beta has units of inverse time.
Debug.Assert(d + step.dt * k > Settings.Epsilon);
_gamma = step.dt * (d + step.dt * k);
if (_gamma != 0.0f)
{
_gamma = 1.0f / _gamma;
}
_beta = step.dt * k * _gamma;
// Compute the effective mass matrix.
Transform xf1;
b.GetTransform(out xf1);
Vector2 r = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b.LocalCenter);
// K = [(1/m1 + 1/m2) * eye(2) - skew(r1) * invI1 * skew(r1) - skew(r2) * invI2 * skew(r2)]
// = [1/m1+1/m2 0 ] + invI1 * [r1.Y*r1.Y -r1.X*r1.Y] + invI2 * [r1.Y*r1.Y -r1.X*r1.Y]
// [ 0 1/m1+1/m2] [-r1.X*r1.Y r1.X*r1.X] [-r1.X*r1.Y r1.X*r1.X]
float invMass = b.InvMass;
float invI = b.InvI;
Mat22 K1 = new Mat22(new Vector2(invMass, 0.0f), new Vector2(0.0f, invMass));
Mat22 K2 = new Mat22(new Vector2(invI * r.Y * r.Y, -invI * r.X * r.Y),
new Vector2(-invI * r.X * r.Y, invI * r.X * r.X));
Mat22 K;
Mat22.Add(ref K1, ref K2, out K);
K.Col1.X += _gamma;
K.Col2.Y += _gamma;
_mass = K.Inverse;
_C = b.Sweep.C + r - _worldAnchor;
// Cheat with some damping
b.AngularVelocityInternal *= 0.98f;
// Warm starting.
_impulse *= step.dtRatio;
b.LinearVelocityInternal += invMass * _impulse;
b.AngularVelocityInternal += invI * MathUtils.Cross(r, _impulse);
}
internal override void SolveVelocityConstraints(ref TimeStep step)
{
Body b = BodyA;
Transform xf1;
b.GetTransform(out xf1);
Vector2 r = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b.LocalCenter);
// Cdot = v + cross(w, r)
Vector2 Cdot = b.LinearVelocityInternal + MathUtils.Cross(b.AngularVelocityInternal, r);
Vector2 impulse = MathUtils.Multiply(ref _mass, -(Cdot + _beta * _C + _gamma * _impulse));
Vector2 oldImpulse = _impulse;
_impulse += impulse;
float maxImpulse = step.dt * MaxForce;
if (_impulse.LengthSquared() > maxImpulse * maxImpulse)
{
_impulse *= maxImpulse / _impulse.Length();
}
impulse = _impulse - oldImpulse;
b.LinearVelocityInternal += b.InvMass * impulse;
b.AngularVelocityInternal += b.InvI * MathUtils.Cross(r, impulse);
}
internal override bool SolvePositionConstraints()
{
return true;
}
}
}