249 lines
8.4 KiB
C#
249 lines
8.4 KiB
C#
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/*
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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.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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// Point-to-point constraint
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// Cdot = v2 - v1
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// = v2 + cross(w2, r2) - v1 - cross(w1, r1)
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// J = [-I -r1_skew I r2_skew ]
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// Identity used:
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// w k % (rx i + ry j) = w * (-ry i + rx j)
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// Angle constraint
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// Cdot = w2 - w1
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// J = [0 0 -1 0 0 1]
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// K = invI1 + invI2
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/// <summary>
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/// Friction joint. This is used for top-down friction.
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/// It provides 2D translational friction and angular friction.
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/// </summary>
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public class FrictionJoint : Joint
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{
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public Vector2 LocalAnchorA;
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public Vector2 LocalAnchorB;
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private float _angularImpulse;
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private float _angularMass;
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private Vector2 _linearImpulse;
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private Mat22 _linearMass;
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internal FrictionJoint()
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{
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JointType = JointType.Friction;
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}
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public FrictionJoint(Body bodyA, Body bodyB, Vector2 localAnchorA, Vector2 localAnchorB)
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: base(bodyA, bodyB)
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{
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JointType = JointType.Friction;
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LocalAnchorA = localAnchorA;
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LocalAnchorB = localAnchorB;
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}
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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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/// <summary>
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/// The maximum friction force in N.
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/// </summary>
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public float MaxForce { get; set; }
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/// <summary>
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/// The maximum friction torque in N-m.
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/// </summary>
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public float MaxTorque { get; set; }
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public override Vector2 GetReactionForce(float inv_dt)
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{
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return inv_dt * _linearImpulse;
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}
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public override float GetReactionTorque(float inv_dt)
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{
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return inv_dt * _angularImpulse;
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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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Transform xfA, xfB;
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bA.GetTransform(out xfA);
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bB.GetTransform(out xfB);
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// Compute the effective mass matrix.
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Vector2 rA = MathUtils.Multiply(ref xfA.R, LocalAnchorA - bA.LocalCenter);
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Vector2 rB = MathUtils.Multiply(ref xfB.R, LocalAnchorB - bB.LocalCenter);
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// J = [-I -r1_skew I r2_skew]
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// [ 0 -1 0 1]
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// r_skew = [-ry; rx]
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// Matlab
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// K = [ mA+r1y^2*iA+mB+r2y^2*iB, -r1y*iA*r1x-r2y*iB*r2x, -r1y*iA-r2y*iB]
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// [ -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB, r1x*iA+r2x*iB]
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// [ -r1y*iA-r2y*iB, r1x*iA+r2x*iB, iA+iB]
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float mA = bA.InvMass, mB = bB.InvMass;
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float iA = bA.InvI, iB = bB.InvI;
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Mat22 K1 = new Mat22();
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K1.Col1.X = mA + mB;
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K1.Col2.X = 0.0f;
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K1.Col1.Y = 0.0f;
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K1.Col2.Y = mA + mB;
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Mat22 K2 = new Mat22();
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K2.Col1.X = iA * rA.Y * rA.Y;
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K2.Col2.X = -iA * rA.X * rA.Y;
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K2.Col1.Y = -iA * rA.X * rA.Y;
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K2.Col2.Y = iA * rA.X * rA.X;
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Mat22 K3 = new Mat22();
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K3.Col1.X = iB * rB.Y * rB.Y;
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K3.Col2.X = -iB * rB.X * rB.Y;
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K3.Col1.Y = -iB * rB.X * rB.Y;
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K3.Col2.Y = iB * rB.X * rB.X;
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Mat22 K12;
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Mat22.Add(ref K1, ref K2, out K12);
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Mat22 K;
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Mat22.Add(ref K12, ref K3, out K);
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_linearMass = K.Inverse;
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_angularMass = iA + iB;
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if (_angularMass > 0.0f)
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{
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_angularMass = 1.0f / _angularMass;
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}
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if (Settings.EnableWarmstarting)
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{
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// Scale impulses to support a variable time step.
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_linearImpulse *= step.dtRatio;
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_angularImpulse *= step.dtRatio;
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Vector2 P = new Vector2(_linearImpulse.X, _linearImpulse.Y);
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bA.LinearVelocityInternal -= mA * P;
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bA.AngularVelocityInternal -= iA * (MathUtils.Cross(rA, P) + _angularImpulse);
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bB.LinearVelocityInternal += mB * P;
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bB.AngularVelocityInternal += iB * (MathUtils.Cross(rB, P) + _angularImpulse);
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}
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else
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{
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_linearImpulse = Vector2.Zero;
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_angularImpulse = 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.LinearVelocityInternal;
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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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float mA = bA.InvMass, mB = bB.InvMass;
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float iA = bA.InvI, iB = bB.InvI;
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Transform xfA, xfB;
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bA.GetTransform(out xfA);
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bB.GetTransform(out xfB);
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Vector2 rA = MathUtils.Multiply(ref xfA.R, LocalAnchorA - bA.LocalCenter);
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Vector2 rB = MathUtils.Multiply(ref xfB.R, LocalAnchorB - bB.LocalCenter);
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// Solve angular friction
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{
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float Cdot = wB - wA;
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float impulse = -_angularMass * Cdot;
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float oldImpulse = _angularImpulse;
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float maxImpulse = step.dt * MaxTorque;
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_angularImpulse = MathUtils.Clamp(_angularImpulse + impulse, -maxImpulse, maxImpulse);
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impulse = _angularImpulse - oldImpulse;
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wA -= iA * impulse;
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wB += iB * impulse;
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}
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// Solve linear friction
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{
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Vector2 Cdot = vB + MathUtils.Cross(wB, rB) - vA - MathUtils.Cross(wA, rA);
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Vector2 impulse = -MathUtils.Multiply(ref _linearMass, Cdot);
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Vector2 oldImpulse = _linearImpulse;
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_linearImpulse += impulse;
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float maxImpulse = step.dt * MaxForce;
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if (_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
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{
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_linearImpulse.Normalize();
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_linearImpulse *= maxImpulse;
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}
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impulse = _linearImpulse - oldImpulse;
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vA -= mA * impulse;
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wA -= iA * MathUtils.Cross(rA, impulse);
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vB += mB * impulse;
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wB += iB * MathUtils.Cross(rB, impulse);
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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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return true;
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}
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}
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}
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