272 lines
9.0 KiB
C#
272 lines
9.0 KiB
C#
/*
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* Farseer Physics Engine:
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* Copyright (c) 2012 Ian Qvist
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*
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* Original source Box2D:
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* Copyright (c) 2006-2011 Erin Catto http://www.box2d.org
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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 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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// Solver shared
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private Vector2 _linearImpulse;
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private float _angularImpulse;
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// Solver temp
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private int _indexA;
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private int _indexB;
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private Vector2 _rA;
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private Vector2 _rB;
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private Vector2 _localCenterA;
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private Vector2 _localCenterB;
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private float _invMassA;
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private float _invMassB;
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private float _invIA;
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private float _invIB;
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private float _angularMass;
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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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/// <summary>
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/// Constructor for FrictionJoint.
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/// </summary>
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/// <param name="bodyA"></param>
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/// <param name="bodyB"></param>
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/// <param name="anchor"></param>
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/// <param name="useWorldCoordinates">Set to true if you are using world coordinates as anchors.</param>
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public FrictionJoint(Body bodyA, Body bodyB, Vector2 anchor, bool useWorldCoordinates = false)
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: base(bodyA, bodyB)
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{
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JointType = JointType.Friction;
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if (useWorldCoordinates)
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{
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LocalAnchorA = BodyA.GetLocalPoint(anchor);
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LocalAnchorB = BodyB.GetLocalPoint(anchor);
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}
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else
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{
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LocalAnchorA = anchor;
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LocalAnchorB = anchor;
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}
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}
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/// <summary>
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/// The local anchor point on BodyA
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/// </summary>
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public Vector2 LocalAnchorA { get; set; }
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/// <summary>
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/// The local anchor point on BodyB
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/// </summary>
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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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set { LocalAnchorA = BodyA.GetLocalPoint(value); }
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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 { LocalAnchorB = BodyB.GetLocalPoint(value); }
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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 invDt)
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{
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return invDt * _linearImpulse;
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}
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public override float GetReactionTorque(float invDt)
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{
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return invDt * _angularImpulse;
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}
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internal override void InitVelocityConstraints(ref SolverData data)
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{
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_indexA = BodyA.IslandIndex;
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_indexB = BodyB.IslandIndex;
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_localCenterA = BodyA._sweep.LocalCenter;
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_localCenterB = BodyB._sweep.LocalCenter;
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_invMassA = BodyA._invMass;
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_invMassB = BodyB._invMass;
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_invIA = BodyA._invI;
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_invIB = BodyB._invI;
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float aA = data.positions[_indexA].a;
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Vector2 vA = data.velocities[_indexA].v;
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float wA = data.velocities[_indexA].w;
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float aB = data.positions[_indexB].a;
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Vector2 vB = data.velocities[_indexB].v;
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float wB = data.velocities[_indexB].w;
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Rot qA = new Rot(aA), qB = new Rot(aB);
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// Compute the effective mass matrix.
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_rA = MathUtils.Mul(qA, LocalAnchorA - _localCenterA);
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_rB = MathUtils.Mul(qB, LocalAnchorB - _localCenterB);
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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 = _invMassA, mB = _invMassB;
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float iA = _invIA, iB = _invIB;
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Mat22 K = new Mat22();
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K.ex.X = mA + mB + iA * _rA.Y * _rA.Y + iB * _rB.Y * _rB.Y;
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K.ex.Y = -iA * _rA.X * _rA.Y - iB * _rB.X * _rB.Y;
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K.ey.X = K.ex.Y;
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K.ey.Y = mA + mB + iA * _rA.X * _rA.X + iB * _rB.X * _rB.X;
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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 *= data.step.dtRatio;
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_angularImpulse *= data.step.dtRatio;
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Vector2 P = new Vector2(_linearImpulse.X, _linearImpulse.Y);
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vA -= mA * P;
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wA -= iA * (MathUtils.Cross(_rA, P) + _angularImpulse);
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vB += mB * P;
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wB += 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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data.velocities[_indexA].v = vA;
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data.velocities[_indexA].w = wA;
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data.velocities[_indexB].v = vB;
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data.velocities[_indexB].w = wB;
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}
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internal override void SolveVelocityConstraints(ref SolverData data)
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{
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Vector2 vA = data.velocities[_indexA].v;
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float wA = data.velocities[_indexA].w;
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Vector2 vB = data.velocities[_indexB].v;
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float wB = data.velocities[_indexB].w;
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float mA = _invMassA, mB = _invMassB;
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float iA = _invIA, iB = _invIB;
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float h = data.step.dt;
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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 = h * 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.Mul(ref _linearMass, Cdot);
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Vector2 oldImpulse = _linearImpulse;
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_linearImpulse += impulse;
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float maxImpulse = h * 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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data.velocities[_indexA].v = vA;
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data.velocities[_indexA].w = wA;
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data.velocities[_indexB].v = vB;
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data.velocities[_indexB].w = wB;
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}
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internal override bool SolvePositionConstraints(ref SolverData data)
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{
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return true;
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}
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}
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} |