541 lines
19 KiB
C#
541 lines
19 KiB
C#
/*
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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;
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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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/// <summary>
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/// A revolute joint rains to bodies to share a common point while they
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/// are free to rotate about the point. The relative rotation about the shared
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/// point is the joint angle. You can limit the relative rotation with
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/// a joint limit that specifies a lower and upper angle. You can use a motor
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/// to drive the relative rotation about the shared point. A maximum motor torque
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/// is provided so that infinite forces are not generated.
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/// </summary>
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public class FixedRevoluteJoint : Joint
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{
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private bool _enableLimit;
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private bool _enableMotor;
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private Vector3 _impulse;
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private LimitState _limitState;
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private float _lowerAngle;
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private Mat33 _mass; // effective mass for point-to-point constraint.
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private float _maxMotorTorque;
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private float _motorImpulse;
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private float _motorMass; // effective mass for motor/limit angular constraint.
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private float _motorSpeed;
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private float _upperAngle;
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private Vector2 _worldAnchor;
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/// <summary>
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/// Initialize the bodies, anchors, and reference angle using the world
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/// anchor.
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/// This requires defining an
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/// anchor point where the bodies are joined. The definition
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/// uses local anchor points so that the initial configuration
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/// can violate the constraint slightly. You also need to
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/// specify the initial relative angle for joint limits. This
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/// helps when saving and loading a game.
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/// The local anchor points are measured from the body's origin
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/// rather than the center of mass because:
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/// 1. you might not know where the center of mass will be.
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/// 2. if you add/remove shapes from a body and recompute the mass,
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/// the joints will be broken.
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/// </summary>
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/// <param name="body">The body.</param>
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/// <param name="bodyAnchor">The body anchor.</param>
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/// <param name="worldAnchor">The world anchor.</param>
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public FixedRevoluteJoint(Body body, Vector2 bodyAnchor, Vector2 worldAnchor)
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: base(body)
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{
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JointType = JointType.FixedRevolute;
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// Changed to local coordinates.
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LocalAnchorA = bodyAnchor;
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_worldAnchor = worldAnchor;
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ReferenceAngle = -BodyA.Rotation;
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_impulse = Vector3.Zero;
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_limitState = LimitState.Inactive;
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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 _worldAnchor; }
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set { _worldAnchor = value; }
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}
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public Vector2 LocalAnchorA { get; set; }
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public float ReferenceAngle { get; set; }
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/// <summary>
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/// Get the current joint angle in radians.
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/// </summary>
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/// <value></value>
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public float JointAngle
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{
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get { return BodyA.Sweep.A - ReferenceAngle; }
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}
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/// <summary>
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/// Get the current joint angle speed in radians per second.
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/// </summary>
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/// <value></value>
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public float JointSpeed
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{
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get { return BodyA.AngularVelocityInternal; }
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}
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/// <summary>
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/// Is the joint limit enabled?
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/// </summary>
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/// <value><c>true</c> if [limit enabled]; otherwise, <c>false</c>.</value>
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public bool LimitEnabled
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{
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get { return _enableLimit; }
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set
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{
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WakeBodies();
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_enableLimit = value;
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}
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}
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/// <summary>
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/// Get the lower joint limit in radians.
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/// </summary>
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/// <value></value>
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public float LowerLimit
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{
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get { return _lowerAngle; }
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set
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{
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WakeBodies();
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_lowerAngle = value;
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}
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}
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/// <summary>
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/// Get the upper joint limit in radians.
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/// </summary>
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/// <value></value>
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public float UpperLimit
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{
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get { return _upperAngle; }
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set
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{
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WakeBodies();
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_upperAngle = value;
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}
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}
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/// <summary>
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/// Is the joint motor enabled?
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/// </summary>
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/// <value><c>true</c> if [motor enabled]; otherwise, <c>false</c>.</value>
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public bool MotorEnabled
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{
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get { return _enableMotor; }
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set
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{
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WakeBodies();
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_enableMotor = value;
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}
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}
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/// <summary>
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/// Set the motor speed in radians per second.
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/// </summary>
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/// <value>The speed.</value>
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public float MotorSpeed
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{
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set
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{
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WakeBodies();
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_motorSpeed = value;
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}
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get { return _motorSpeed; }
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}
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/// <summary>
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/// Set the maximum motor torque, usually in N-m.
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/// </summary>
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/// <value>The torque.</value>
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public float MaxMotorTorque
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{
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set
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{
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WakeBodies();
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_maxMotorTorque = value;
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}
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get { return _maxMotorTorque; }
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}
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/// <summary>
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/// Get the current motor torque, usually in N-m.
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/// </summary>
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/// <value></value>
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public float MotorTorque
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{
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get { return _motorImpulse; }
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set
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{
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WakeBodies();
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_motorImpulse = value;
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}
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}
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public override Vector2 GetReactionForce(float inv_dt)
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{
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return inv_dt * new Vector2(_impulse.X, _impulse.Y);
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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 * _impulse.Z;
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}
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internal override void InitVelocityConstraints(ref TimeStep step)
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{
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Body b1 = BodyA;
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if (_enableMotor || _enableLimit)
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{
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// You cannot create a rotation limit between bodies that
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// both have fixed rotation.
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Debug.Assert(b1.InvI > 0.0f /* || b2._invI > 0.0f*/);
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}
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// Compute the effective mass matrix.
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Transform xf1;
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b1.GetTransform(out xf1);
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Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
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Vector2 r2 = _worldAnchor; // MathUtils.Multiply(ref xf2.R, LocalAnchorB - b2.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 = [ m1+r1y^2*i1+m2+r2y^2*i2, -r1y*i1*r1x-r2y*i2*r2x, -r1y*i1-r2y*i2]
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// [ -r1y*i1*r1x-r2y*i2*r2x, m1+r1x^2*i1+m2+r2x^2*i2, r1x*i1+r2x*i2]
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// [ -r1y*i1-r2y*i2, r1x*i1+r2x*i2, i1+i2]
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float m1 = b1.InvMass;
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const float m2 = 0;
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float i1 = b1.InvI;
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const float i2 = 0;
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_mass.Col1.X = m1 + m2 + r1.Y * r1.Y * i1 + r2.Y * r2.Y * i2;
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_mass.Col2.X = -r1.Y * r1.X * i1 - r2.Y * r2.X * i2;
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_mass.Col3.X = -r1.Y * i1 - r2.Y * i2;
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_mass.Col1.Y = _mass.Col2.X;
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_mass.Col2.Y = m1 + m2 + r1.X * r1.X * i1 + r2.X * r2.X * i2;
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_mass.Col3.Y = r1.X * i1 + r2.X * i2;
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_mass.Col1.Z = _mass.Col3.X;
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_mass.Col2.Z = _mass.Col3.Y;
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_mass.Col3.Z = i1 + i2;
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_motorMass = i1 + i2;
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if (_motorMass > 0.0f)
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{
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_motorMass = 1.0f / _motorMass;
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}
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if (_enableMotor == false)
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{
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_motorImpulse = 0.0f;
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}
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if (_enableLimit)
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{
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float jointAngle = 0 - b1.Sweep.A - ReferenceAngle;
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if (Math.Abs(_upperAngle - _lowerAngle) < 2.0f * Settings.AngularSlop)
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{
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_limitState = LimitState.Equal;
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}
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else if (jointAngle <= _lowerAngle)
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{
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if (_limitState != LimitState.AtLower)
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{
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_impulse.Z = 0.0f;
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}
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_limitState = LimitState.AtLower;
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}
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else if (jointAngle >= _upperAngle)
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{
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if (_limitState != LimitState.AtUpper)
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{
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_impulse.Z = 0.0f;
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}
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_limitState = LimitState.AtUpper;
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}
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else
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{
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_limitState = LimitState.Inactive;
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_impulse.Z = 0.0f;
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}
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}
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else
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{
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_limitState = LimitState.Inactive;
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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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_impulse *= step.dtRatio;
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_motorImpulse *= step.dtRatio;
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Vector2 P = new Vector2(_impulse.X, _impulse.Y);
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b1.LinearVelocityInternal -= m1 * P;
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b1.AngularVelocityInternal -= i1 * (MathUtils.Cross(r1, P) + _motorImpulse + _impulse.Z);
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}
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else
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{
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_impulse = Vector3.Zero;
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_motorImpulse = 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 b1 = BodyA;
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Vector2 v1 = b1.LinearVelocityInternal;
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float w1 = b1.AngularVelocityInternal;
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Vector2 v2 = Vector2.Zero;
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const float w2 = 0;
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float m1 = b1.InvMass;
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float i1 = b1.InvI;
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// Solve motor constraint.
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if (_enableMotor && _limitState != LimitState.Equal)
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{
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float Cdot = w2 - w1 - _motorSpeed;
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float impulse = _motorMass * (-Cdot);
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float oldImpulse = _motorImpulse;
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float maxImpulse = step.dt * _maxMotorTorque;
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_motorImpulse = MathUtils.Clamp(_motorImpulse + impulse, -maxImpulse, maxImpulse);
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impulse = _motorImpulse - oldImpulse;
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w1 -= i1 * impulse;
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}
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// Solve limit constraint.
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if (_enableLimit && _limitState != LimitState.Inactive)
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{
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Transform xf1;
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b1.GetTransform(out xf1);
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Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
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Vector2 r2 = _worldAnchor;
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// Solve point-to-point constraint
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Vector2 Cdot1 = v2 + MathUtils.Cross(w2, r2) - v1 - MathUtils.Cross(w1, r1);
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float Cdot2 = w2 - w1;
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Vector3 Cdot = new Vector3(Cdot1.X, Cdot1.Y, Cdot2);
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Vector3 impulse = _mass.Solve33(-Cdot);
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if (_limitState == LimitState.Equal)
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{
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_impulse += impulse;
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}
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else if (_limitState == LimitState.AtLower)
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{
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float newImpulse = _impulse.Z + impulse.Z;
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if (newImpulse < 0.0f)
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{
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Vector2 reduced = _mass.Solve22(-Cdot1);
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impulse.X = reduced.X;
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impulse.Y = reduced.Y;
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impulse.Z = -_impulse.Z;
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_impulse.X += reduced.X;
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_impulse.Y += reduced.Y;
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_impulse.Z = 0.0f;
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}
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}
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else if (_limitState == LimitState.AtUpper)
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{
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float newImpulse = _impulse.Z + impulse.Z;
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if (newImpulse > 0.0f)
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{
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Vector2 reduced = _mass.Solve22(-Cdot1);
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impulse.X = reduced.X;
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impulse.Y = reduced.Y;
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impulse.Z = -_impulse.Z;
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_impulse.X += reduced.X;
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_impulse.Y += reduced.Y;
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_impulse.Z = 0.0f;
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}
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}
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Vector2 P = new Vector2(impulse.X, impulse.Y);
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v1 -= m1 * P;
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w1 -= i1 * (MathUtils.Cross(r1, P) + impulse.Z);
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}
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else
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{
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Transform xf1;
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b1.GetTransform(out xf1);
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Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
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Vector2 r2 = _worldAnchor;
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// Solve point-to-point constraint
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Vector2 Cdot = v2 + MathUtils.Cross(w2, r2) - v1 - MathUtils.Cross(w1, r1);
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Vector2 impulse = _mass.Solve22(-Cdot);
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_impulse.X += impulse.X;
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_impulse.Y += impulse.Y;
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v1 -= m1 * impulse;
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w1 -= i1 * MathUtils.Cross(r1, impulse);
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}
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b1.LinearVelocityInternal = v1;
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b1.AngularVelocityInternal = w1;
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}
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internal override bool SolvePositionConstraints()
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{
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// TODO_ERIN block solve with limit. COME ON ERIN
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Body b1 = BodyA;
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float angularError = 0.0f;
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float positionError;
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// Solve angular limit constraint.
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if (_enableLimit && _limitState != LimitState.Inactive)
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{
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float angle = 0 - b1.Sweep.A - ReferenceAngle;
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float limitImpulse = 0.0f;
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if (_limitState == LimitState.Equal)
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{
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// Prevent large angular corrections
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float C = MathUtils.Clamp(angle - _lowerAngle, -Settings.MaxAngularCorrection,
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Settings.MaxAngularCorrection);
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limitImpulse = -_motorMass * C;
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angularError = Math.Abs(C);
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}
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else if (_limitState == LimitState.AtLower)
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{
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float C = angle - _lowerAngle;
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angularError = -C;
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// Prevent large angular corrections and allow some slop.
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C = MathUtils.Clamp(C + Settings.AngularSlop, -Settings.MaxAngularCorrection, 0.0f);
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limitImpulse = -_motorMass * C;
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}
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else if (_limitState == LimitState.AtUpper)
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{
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float C = angle - _upperAngle;
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angularError = C;
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// Prevent large angular corrections and allow some slop.
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C = MathUtils.Clamp(C - Settings.AngularSlop, 0.0f, Settings.MaxAngularCorrection);
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limitImpulse = -_motorMass * C;
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}
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b1.Sweep.A -= b1.InvI * limitImpulse;
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b1.SynchronizeTransform();
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}
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// Solve point-to-point constraint.
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{
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Transform xf1;
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b1.GetTransform(out xf1);
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Vector2 r1 = MathUtils.Multiply(ref xf1.R, LocalAnchorA - b1.LocalCenter);
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Vector2 r2 = _worldAnchor;
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Vector2 C = Vector2.Zero + r2 - b1.Sweep.C - r1;
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positionError = C.Length();
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float invMass1 = b1.InvMass;
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const float invMass2 = 0;
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float invI1 = b1.InvI;
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const float invI2 = 0;
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// Handle large detachment.
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const float k_allowedStretch = 10.0f * Settings.LinearSlop;
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if (C.LengthSquared() > k_allowedStretch * k_allowedStretch)
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{
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// Use a particle solution (no rotation).
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Vector2 u = C;
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u.Normalize();
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float k = invMass1 + invMass2;
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Debug.Assert(k > Settings.Epsilon);
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float m = 1.0f / k;
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Vector2 impulse2 = m * (-C);
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const float k_beta = 0.5f;
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b1.Sweep.C -= k_beta * invMass1 * impulse2;
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C = Vector2.Zero + r2 - b1.Sweep.C - r1;
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}
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Mat22 K1 = new Mat22(new Vector2(invMass1 + invMass2, 0.0f), new Vector2(0.0f, invMass1 + invMass2));
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Mat22 K2 = new Mat22(new Vector2(invI1 * r1.Y * r1.Y, -invI1 * r1.X * r1.Y),
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new Vector2(-invI1 * r1.X * r1.Y, invI1 * r1.X * r1.X));
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Mat22 K3 = new Mat22(new Vector2(invI2 * r2.Y * r2.Y, -invI2 * r2.X * r2.Y),
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new Vector2(-invI2 * r2.X * r2.Y, invI2 * r2.X * r2.X));
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Mat22 Ka;
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Mat22.Add(ref K1, ref K2, out Ka);
|
|
|
|
Mat22 K;
|
|
Mat22.Add(ref Ka, ref K3, out K);
|
|
|
|
Vector2 impulse = K.Solve(-C);
|
|
|
|
b1.Sweep.C -= b1.InvMass * impulse;
|
|
b1.Sweep.A -= b1.InvI * MathUtils.Cross(r1, impulse);
|
|
|
|
b1.SynchronizeTransform();
|
|
}
|
|
|
|
return positionError <= Settings.LinearSlop && angularError <= Settings.AngularSlop;
|
|
}
|
|
}
|
|
} |