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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 | /* * 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 { // 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 /// <summary> /// Friction joint. This is used for top-down friction. /// It provides 2D translational friction and angular friction. /// </summary> public class FixedFrictionJoint : Joint { public Vector2 LocalAnchorA; /// <summary> /// The maximum friction force in N. /// </summary> public float MaxForce; /// <summary> /// The maximum friction torque in N-m. /// </summary> public float MaxTorque; private float _angularImpulse; private float _angularMass; private Vector2 _linearImpulse; private Mat22 _linearMass; public FixedFrictionJoint(Body body, Vector2 localAnchorA) : base (body) { JointType = JointType.FixedFriction; LocalAnchorA = localAnchorA; //Setting default max force and max torque const float gravity = 10.0f; // For a circle: I = 0.5 * m * r * r ==> r = sqrt(2 * I / m) float radius = ( float )Math.Sqrt(2.0 * (body.Inertia / body.Mass)); MaxForce = body.Mass * gravity; MaxTorque = body.Mass * radius * gravity; } public override Vector2 WorldAnchorA { get { return BodyA.GetWorldPoint(LocalAnchorA); } } public override Vector2 WorldAnchorB { get { return Vector2.Zero; } set { Debug.Assert( false , "You can't set the world anchor on this joint type." ); } } public override Vector2 GetReactionForce( float invDT) { return invDT * _linearImpulse; } public override float GetReactionTorque( float invDT) { return invDT * _angularImpulse; } internal override void InitVelocityConstraints( ref TimeStep step) { Body bA = BodyA; Transform xfA; bA.GetTransform( out xfA); // Compute the effective mass matrix. Vector2 rA = MathUtils.Multiply( ref xfA.R, LocalAnchorA - bA.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; float iA = bA.InvI; Mat22 K1 = new Mat22(); K1.Col1.X = mA; K1.Col2.X = 0.0f; K1.Col1.Y = 0.0f; K1.Col2.Y = mA; 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 K12; Mat22.Add( ref K1, ref K2, out K12); _linearMass = K12.Inverse; _angularMass = iA; 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); } else { _linearImpulse = Vector2.Zero; _angularImpulse = 0.0f; } } internal override void SolveVelocityConstraints( ref TimeStep step) { Body bA = BodyA; Vector2 vA = bA.LinearVelocityInternal; float wA = bA.AngularVelocityInternal; float mA = bA.InvMass; float iA = bA.InvI; Transform xfA; bA.GetTransform( out xfA); Vector2 rA = MathUtils.Multiply( ref xfA.R, LocalAnchorA - bA.LocalCenter); // Solve angular friction { float Cdot = -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; } // Solve linear friction { Vector2 Cdot = -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); } bA.LinearVelocityInternal = vA; bA.AngularVelocityInternal = wA; } internal override bool SolvePositionConstraints() { return true ; } } } |
Source at commit 5a9d1b17f228 created 12 years 6 months ago. By Nathan Adams, Adding double extension |
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