436 lines
14 KiB
C#
436 lines
14 KiB
C#
/*
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* Farseer Physics Engine based on Box2D.XNA port:
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* Copyright (c) 2010 Ian Qvist
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*
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* Box2D.XNA port of Box2D:
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* Copyright (c) 2009 Brandon Furtwangler, Nathan Furtwangler
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*
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* Original source Box2D:
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* Copyright (c) 2006-2009 Erin Catto http://www.gphysics.com
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*
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* This software is provided 'as-is', without any express or implied
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* warranty. In no event will the authors be held liable for any damages
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* arising from the use of this software.
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* Permission is granted to anyone to use this software for any purpose,
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* including commercial applications, and to alter it and redistribute it
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* freely, subject to the following restrictions:
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* 1. The origin of this software must not be misrepresented; you must not
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* claim that you wrote the original software. If you use this software
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* in a product, an acknowledgment in the product documentation would be
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* appreciated but is not required.
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* 2. Altered source versions must be plainly marked as such, and must not be
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* misrepresented as being the original software.
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* 3. This notice may not be removed or altered from any source distribution.
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*/
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using System;
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using System.Diagnostics;
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using FarseerPhysics.Common;
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using Microsoft.Xna.Framework;
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namespace FarseerPhysics.Dynamics.Joints
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{
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public class LineJoint : Joint
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{
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private Vector2 _ax, _ay;
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private float _bias;
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private bool _enableMotor;
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private float _gamma;
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private float _impulse;
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private Vector2 _localXAxis;
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private Vector2 _localYAxisA;
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private float _mass;
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private float _maxMotorTorque;
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private float _motorImpulse;
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private float _motorMass;
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private float _motorSpeed;
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private float _sAx;
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private float _sAy;
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private float _sBx;
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private float _sBy;
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private float _springImpulse;
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private float _springMass;
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// Linear constraint (point-to-line)
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// d = pB - pA = xB + rB - xA - rA
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// C = dot(ay, d)
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// Cdot = dot(d, cross(wA, ay)) + dot(ay, vB + cross(wB, rB) - vA - cross(wA, rA))
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// = -dot(ay, vA) - dot(cross(d + rA, ay), wA) + dot(ay, vB) + dot(cross(rB, ay), vB)
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// J = [-ay, -cross(d + rA, ay), ay, cross(rB, ay)]
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// Spring linear constraint
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// C = dot(ax, d)
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// Cdot = = -dot(ax, vA) - dot(cross(d + rA, ax), wA) + dot(ax, vB) + dot(cross(rB, ax), vB)
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// J = [-ax -cross(d+rA, ax) ax cross(rB, ax)]
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// Motor rotational constraint
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// Cdot = wB - wA
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// J = [0 0 -1 0 0 1]
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internal LineJoint()
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{
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JointType = JointType.Line;
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}
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public LineJoint(Body bA, Body bB, Vector2 anchor, Vector2 axis)
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: base(bA, bB)
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{
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JointType = JointType.Line;
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LocalAnchorA = bA.GetLocalPoint(anchor);
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LocalAnchorB = bB.GetLocalPoint(anchor);
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LocalXAxis = bA.GetLocalVector(axis);
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}
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public Vector2 LocalAnchorA { get; set; }
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public Vector2 LocalAnchorB { get; set; }
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public override Vector2 WorldAnchorA
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{
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get { return BodyA.GetWorldPoint(LocalAnchorA); }
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}
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public override Vector2 WorldAnchorB
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{
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get { return BodyB.GetWorldPoint(LocalAnchorB); }
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set { Debug.Assert(false, "You can't set the world anchor on this joint type."); }
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}
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public float JointTranslation
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{
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get
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{
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Body bA = BodyA;
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Body bB = BodyB;
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Vector2 pA = bA.GetWorldPoint(LocalAnchorA);
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Vector2 pB = bB.GetWorldPoint(LocalAnchorB);
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Vector2 d = pB - pA;
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Vector2 axis = bA.GetWorldVector(LocalXAxis);
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float translation = Vector2.Dot(d, axis);
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return translation;
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}
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}
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public float JointSpeed
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{
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get
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{
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float wA = BodyA.AngularVelocityInternal;
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float wB = BodyB.AngularVelocityInternal;
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return wB - wA;
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}
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}
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public bool MotorEnabled
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{
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get { return _enableMotor; }
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set
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{
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BodyA.Awake = true;
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BodyB.Awake = true;
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_enableMotor = value;
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}
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}
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public float MotorSpeed
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{
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set
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{
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BodyA.Awake = true;
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BodyB.Awake = true;
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_motorSpeed = value;
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}
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get { return _motorSpeed; }
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}
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public float MaxMotorTorque
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{
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set
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{
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BodyA.Awake = true;
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BodyB.Awake = true;
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_maxMotorTorque = value;
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}
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get { return _maxMotorTorque; }
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}
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public float Frequency { get; set; }
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public float DampingRatio { get; set; }
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public Vector2 LocalXAxis
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{
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get { return _localXAxis; }
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set
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{
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_localXAxis = value;
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_localYAxisA = MathUtils.Cross(1.0f, _localXAxis);
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}
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}
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public override Vector2 GetReactionForce(float invDt)
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{
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return invDt * (_impulse * _ay + _springImpulse * _ax);
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}
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public override float GetReactionTorque(float invDt)
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{
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return invDt * _motorImpulse;
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}
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internal override void InitVelocityConstraints(ref TimeStep step)
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{
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Body bA = BodyA;
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Body bB = BodyB;
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LocalCenterA = bA.LocalCenter;
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LocalCenterB = bB.LocalCenter;
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Transform xfA;
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bA.GetTransform(out xfA);
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Transform xfB;
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bB.GetTransform(out xfB);
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// Compute the effective masses.
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Vector2 rA = MathUtils.Multiply(ref xfA.R, LocalAnchorA - LocalCenterA);
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Vector2 rB = MathUtils.Multiply(ref xfB.R, LocalAnchorB - LocalCenterB);
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Vector2 d = bB.Sweep.C + rB - bA.Sweep.C - rA;
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InvMassA = bA.InvMass;
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InvIA = bA.InvI;
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InvMassB = bB.InvMass;
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InvIB = bB.InvI;
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// Point to line constraint
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{
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_ay = MathUtils.Multiply(ref xfA.R, _localYAxisA);
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_sAy = MathUtils.Cross(d + rA, _ay);
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_sBy = MathUtils.Cross(rB, _ay);
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_mass = InvMassA + InvMassB + InvIA * _sAy * _sAy + InvIB * _sBy * _sBy;
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if (_mass > 0.0f)
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{
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_mass = 1.0f / _mass;
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}
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}
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// Spring constraint
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_springMass = 0.0f;
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if (Frequency > 0.0f)
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{
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_ax = MathUtils.Multiply(ref xfA.R, LocalXAxis);
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_sAx = MathUtils.Cross(d + rA, _ax);
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_sBx = MathUtils.Cross(rB, _ax);
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float invMass = InvMassA + InvMassB + InvIA * _sAx * _sAx + InvIB * _sBx * _sBx;
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if (invMass > 0.0f)
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{
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_springMass = 1.0f / invMass;
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float C = Vector2.Dot(d, _ax);
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// Frequency
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float omega = 2.0f * Settings.Pi * Frequency;
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// Damping coefficient
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float da = 2.0f * _springMass * DampingRatio * omega;
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// Spring stiffness
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float k = _springMass * omega * omega;
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// magic formulas
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_gamma = step.dt * (da + step.dt * k);
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if (_gamma > 0.0f)
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{
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_gamma = 1.0f / _gamma;
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}
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_bias = C * step.dt * k * _gamma;
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_springMass = invMass + _gamma;
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if (_springMass > 0.0f)
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{
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_springMass = 1.0f / _springMass;
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}
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}
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}
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else
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{
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_springImpulse = 0.0f;
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_springMass = 0.0f;
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}
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// Rotational motor
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if (_enableMotor)
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{
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_motorMass = InvIA + InvIB;
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if (_motorMass > 0.0f)
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{
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_motorMass = 1.0f / _motorMass;
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}
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}
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else
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{
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_motorMass = 0.0f;
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_motorImpulse = 0.0f;
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}
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if (Settings.EnableWarmstarting)
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{
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// Account for variable time step.
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_impulse *= step.dtRatio;
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_springImpulse *= step.dtRatio;
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_motorImpulse *= step.dtRatio;
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Vector2 P = _impulse * _ay + _springImpulse * _ax;
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float LA = _impulse * _sAy + _springImpulse * _sAx + _motorImpulse;
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float LB = _impulse * _sBy + _springImpulse * _sBx + _motorImpulse;
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bA.LinearVelocityInternal -= InvMassA * P;
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bA.AngularVelocityInternal -= InvIA * LA;
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bB.LinearVelocityInternal += InvMassB * P;
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bB.AngularVelocityInternal += InvIB * LB;
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}
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else
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{
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_impulse = 0.0f;
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_springImpulse = 0.0f;
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_motorImpulse = 0.0f;
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}
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}
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internal override void SolveVelocityConstraints(ref TimeStep step)
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{
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Body bA = BodyA;
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Body bB = BodyB;
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Vector2 vA = bA.LinearVelocity;
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float wA = bA.AngularVelocityInternal;
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Vector2 vB = bB.LinearVelocityInternal;
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float wB = bB.AngularVelocityInternal;
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// Solve spring constraint
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{
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float Cdot = Vector2.Dot(_ax, vB - vA) + _sBx * wB - _sAx * wA;
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float impulse = -_springMass * (Cdot + _bias + _gamma * _springImpulse);
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_springImpulse += impulse;
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Vector2 P = impulse * _ax;
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float LA = impulse * _sAx;
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float LB = impulse * _sBx;
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vA -= InvMassA * P;
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wA -= InvIA * LA;
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vB += InvMassB * P;
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wB += InvIB * LB;
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}
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// Solve rotational motor constraint
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{
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float Cdot = wB - wA - _motorSpeed;
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float impulse = -_motorMass * Cdot;
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float oldImpulse = _motorImpulse;
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float maxImpulse = step.dt * _maxMotorTorque;
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_motorImpulse = MathUtils.Clamp(_motorImpulse + impulse, -maxImpulse, maxImpulse);
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impulse = _motorImpulse - oldImpulse;
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wA -= InvIA * impulse;
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wB += InvIB * impulse;
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}
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// Solve point to line constraint
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{
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float Cdot = Vector2.Dot(_ay, vB - vA) + _sBy * wB - _sAy * wA;
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float impulse = _mass * (-Cdot);
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_impulse += impulse;
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Vector2 P = impulse * _ay;
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float LA = impulse * _sAy;
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float LB = impulse * _sBy;
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vA -= InvMassA * P;
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wA -= InvIA * LA;
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vB += InvMassB * P;
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wB += InvIB * LB;
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}
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bA.LinearVelocityInternal = vA;
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bA.AngularVelocityInternal = wA;
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bB.LinearVelocityInternal = vB;
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bB.AngularVelocityInternal = wB;
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}
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internal override bool SolvePositionConstraints()
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{
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Body bA = BodyA;
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Body bB = BodyB;
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Vector2 xA = bA.Sweep.C;
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float angleA = bA.Sweep.A;
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Vector2 xB = bB.Sweep.C;
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float angleB = bB.Sweep.A;
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Mat22 RA = new Mat22(angleA);
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Mat22 RB = new Mat22(angleB);
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Vector2 rA = MathUtils.Multiply(ref RA, LocalAnchorA - LocalCenterA);
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Vector2 rB = MathUtils.Multiply(ref RB, LocalAnchorB - LocalCenterB);
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Vector2 d = xB + rB - xA - rA;
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Vector2 ay = MathUtils.Multiply(ref RA, _localYAxisA);
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float sAy = MathUtils.Cross(d + rA, ay);
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float sBy = MathUtils.Cross(rB, ay);
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float C = Vector2.Dot(d, ay);
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float k = InvMassA + InvMassB + InvIA * _sAy * _sAy + InvIB * _sBy * _sBy;
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float impulse;
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if (k != 0.0f)
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{
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impulse = -C / k;
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}
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else
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{
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impulse = 0.0f;
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}
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Vector2 P = impulse * ay;
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float LA = impulse * sAy;
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float LB = impulse * sBy;
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xA -= InvMassA * P;
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angleA -= InvIA * LA;
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xB += InvMassB * P;
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angleB += InvIB * LB;
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// TODO_ERIN remove need for this.
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bA.Sweep.C = xA;
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bA.Sweep.A = angleA;
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bB.Sweep.C = xB;
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bB.Sweep.A = angleB;
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bA.SynchronizeTransform();
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bB.SynchronizeTransform();
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return Math.Abs(C) <= Settings.LinearSlop;
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}
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public float GetMotorTorque(float invDt)
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{
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return invDt * _motorImpulse;
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}
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}
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} |