291 lines
9.8 KiB
C#
291 lines
9.8 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 System;
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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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// Limit:
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// C = norm(pB - pA) - L
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// u = (pB - pA) / norm(pB - pA)
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// Cdot = dot(u, vB + cross(wB, rB) - vA - cross(wA, rA))
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// J = [-u -cross(rA, u) u cross(rB, u)]
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// K = J * invM * JT
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// = invMassA + invIA * cross(rA, u)^2 + invMassB + invIB * cross(rB, u)^2
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/// <summary>
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/// A rope joint enforces a maximum distance between two points on two bodies. It has no other effect.
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/// It can be used on ropes that are made up of several connected bodies, and if there is a need to support a heavy body.
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/// This joint is used for stabiliation of heavy objects on soft constraint joints.
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///
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/// Warning: if you attempt to change the maximum length during the simulation you will get some non-physical behavior.
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/// Use the DistanceJoint instead if you want to dynamically control the length.
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/// </summary>
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public class RopeJoint : Joint
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{
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// Solver shared
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private float _impulse;
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private float _length;
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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 _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 _mass;
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private Vector2 _rA, _rB;
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private Vector2 _u;
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internal RopeJoint()
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{
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JointType = JointType.Rope;
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}
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/// <summary>
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/// Constructor for RopeJoint.
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/// </summary>
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/// <param name="bodyA">The first body</param>
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/// <param name="bodyB">The second body</param>
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/// <param name="anchorA">The anchor on the first body</param>
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/// <param name="anchorB">The anchor on the second body</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 RopeJoint(Body bodyA, Body bodyB, Vector2 anchorA, Vector2 anchorB, bool useWorldCoordinates = false)
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: base(bodyA, bodyB)
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{
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JointType = JointType.Rope;
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if (useWorldCoordinates)
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{
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LocalAnchorA = bodyA.GetLocalPoint(anchorA);
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LocalAnchorB = bodyB.GetLocalPoint(anchorB);
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}
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else
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{
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LocalAnchorA = anchorA;
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LocalAnchorB = anchorB;
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}
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//FPE feature: Setting default MaxLength
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Vector2 d = WorldAnchorB - WorldAnchorA;
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MaxLength = d.Length();
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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 sealed 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 sealed 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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/// Get or set the maximum length of the rope.
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/// By default, it is the distance between the two anchor points.
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/// </summary>
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public float MaxLength { get; set; }
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/// <summary>
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/// Gets the state of the joint.
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/// </summary>
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public LimitState State { get; private set; }
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public override Vector2 GetReactionForce(float invDt)
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{
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return (invDt * _impulse) * _u;
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}
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public override float GetReactionTorque(float invDt)
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{
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return 0;
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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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Vector2 cA = data.positions[_indexA].c;
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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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Vector2 cB = data.positions[_indexB].c;
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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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_rA = MathUtils.Mul(qA, LocalAnchorA - _localCenterA);
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_rB = MathUtils.Mul(qB, LocalAnchorB - _localCenterB);
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_u = cB + _rB - cA - _rA;
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_length = _u.Length();
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float C = _length - MaxLength;
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if (C > 0.0f)
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{
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State = LimitState.AtUpper;
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}
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else
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{
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State = LimitState.Inactive;
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}
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if (_length > Settings.LinearSlop)
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{
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_u *= 1.0f / _length;
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}
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else
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{
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_u = Vector2.Zero;
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_mass = 0.0f;
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_impulse = 0.0f;
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return;
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}
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// Compute effective mass.
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float crA = MathUtils.Cross(_rA, _u);
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float crB = MathUtils.Cross(_rB, _u);
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float invMass = _invMassA + _invIA * crA * crA + _invMassB + _invIB * crB * crB;
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_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
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if (Settings.EnableWarmstarting)
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{
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// Scale the impulse to support a variable time step.
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_impulse *= data.step.dtRatio;
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Vector2 P = _impulse * _u;
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vA -= _invMassA * P;
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wA -= _invIA * MathUtils.Cross(_rA, P);
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vB += _invMassB * P;
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wB += _invIB * MathUtils.Cross(_rB, P);
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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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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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// Cdot = dot(u, v + cross(w, r))
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Vector2 vpA = vA + MathUtils.Cross(wA, _rA);
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Vector2 vpB = vB + MathUtils.Cross(wB, _rB);
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float C = _length - MaxLength;
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float Cdot = Vector2.Dot(_u, vpB - vpA);
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// Predictive constraint.
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if (C < 0.0f)
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{
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Cdot += data.step.inv_dt * C;
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}
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float impulse = -_mass * Cdot;
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float oldImpulse = _impulse;
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_impulse = Math.Min(0.0f, _impulse + impulse);
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impulse = _impulse - oldImpulse;
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Vector2 P = impulse * _u;
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vA -= _invMassA * P;
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wA -= _invIA * MathUtils.Cross(_rA, P);
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vB += _invMassB * P;
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wB += _invIB * MathUtils.Cross(_rB, P);
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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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Vector2 cA = data.positions[_indexA].c;
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float aA = data.positions[_indexA].a;
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Vector2 cB = data.positions[_indexB].c;
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float aB = data.positions[_indexB].a;
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Rot qA = new Rot(aA), qB = new Rot(aB);
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Vector2 rA = MathUtils.Mul(qA, LocalAnchorA - _localCenterA);
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Vector2 rB = MathUtils.Mul(qB, LocalAnchorB - _localCenterB);
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Vector2 u = cB + rB - cA - rA;
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float length = u.Length(); u.Normalize();
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float C = length - MaxLength;
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C = MathUtils.Clamp(C, 0.0f, Settings.MaxLinearCorrection);
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float impulse = -_mass * C;
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Vector2 P = impulse * u;
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cA -= _invMassA * P;
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aA -= _invIA * MathUtils.Cross(rA, P);
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cB += _invMassB * P;
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aB += _invIB * MathUtils.Cross(rB, P);
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data.positions[_indexA].c = cA;
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data.positions[_indexA].a = aA;
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data.positions[_indexB].c = cB;
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data.positions[_indexB].a = aB;
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return length - MaxLength < Settings.LinearSlop;
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}
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}
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} |