AxiosEngine-old 

AxiosEngine-old Mercurial Source Tree


Root/axios/Dynamics/Joints/FixedFrictionJoint.cs

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

Archive Download this file

Page rendered in 0.87885s using 11 queries.