org.jbox2d.dynamics.joints.DistanceJoint Maven / Gradle / Ivy
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* Copyright (c) 2013, Daniel Murphy
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/*
* JBox2D - A Java Port of Erin Catto's Box2D
*
* JBox2D homepage: http://jbox2d.sourceforge.net/
* Box2D homepage: http://www.box2d.org
*
* This software is provided 'as-is', without any express or implied
* warranty. In no event will the authors be held liable for any damages
* arising from the use of this software.
*
* Permission is granted to anyone to use this software for any purpose,
* including commercial applications, and to alter it and redistribute it
* freely, subject to the following restrictions:
*
* 1. The origin of this software must not be misrepresented; you must not
* claim that you wrote the original software. If you use this software
* in a product, an acknowledgment in the product documentation would be
* appreciated but is not required.
* 2. Altered source versions must be plainly marked as such, and must not be
* misrepresented as being the original software.
* 3. This notice may not be removed or altered from any source distribution.
*/
package org.jbox2d.dynamics.joints;
import org.jbox2d.common.MathUtils;
import org.jbox2d.common.Rot;
import org.jbox2d.common.Settings;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.SolverData;
import org.jbox2d.pooling.IWorldPool;
//C = norm(p2 - p1) - L
//u = (p2 - p1) / norm(p2 - p1)
//Cdot = dot(u, v2 + cross(w2, r2) - v1 - cross(w1, r1))
//J = [-u -cross(r1, u) u cross(r2, u)]
//K = J * invM * JT
//= invMass1 + invI1 * cross(r1, u)^2 + invMass2 + invI2 * cross(r2, u)^2
/**
* A distance joint constrains two points on two bodies to remain at a fixed distance from each
* other. You can view this as a massless, rigid rod.
*/
public class DistanceJoint extends Joint {
private float m_frequencyHz;
private float m_dampingRatio;
private float m_bias;
// Solver shared
private final Vec2 m_localAnchorA;
private final Vec2 m_localAnchorB;
private float m_gamma;
private float m_impulse;
private float m_length;
// Solver temp
private int m_indexA;
private int m_indexB;
private final Vec2 m_u = new Vec2();
private final Vec2 m_rA = new Vec2();
private final Vec2 m_rB = new Vec2();
private final Vec2 m_localCenterA = new Vec2();
private final Vec2 m_localCenterB = new Vec2();
private float m_invMassA;
private float m_invMassB;
private float m_invIA;
private float m_invIB;
private float m_mass;
protected DistanceJoint(IWorldPool argWorld, final DistanceJointDef def) {
super(argWorld, def);
m_localAnchorA = def.localAnchorA.clone();
m_localAnchorB = def.localAnchorB.clone();
m_length = def.length;
m_impulse = 0.0f;
m_frequencyHz = def.frequencyHz;
m_dampingRatio = def.dampingRatio;
m_gamma = 0.0f;
m_bias = 0.0f;
}
public void setFrequency(float hz) {
m_frequencyHz = hz;
}
public float getFrequency() {
return m_frequencyHz;
}
public float getLength() {
return m_length;
}
public void setLength(float argLength) {
m_length = argLength;
}
public void setDampingRatio(float damp) {
m_dampingRatio = damp;
}
public float getDampingRatio() {
return m_dampingRatio;
}
@Override
public void getAnchorA(Vec2 argOut) {
m_bodyA.getWorldPointToOut(m_localAnchorA, argOut);
}
@Override
public void getAnchorB(Vec2 argOut) {
m_bodyB.getWorldPointToOut(m_localAnchorB, argOut);
}
public Vec2 getLocalAnchorA() {
return m_localAnchorA;
}
public Vec2 getLocalAnchorB() {
return m_localAnchorB;
}
/**
* Get the reaction force given the inverse time step. Unit is N.
*/
@Override
public void getReactionForce(float inv_dt, Vec2 argOut) {
argOut.x = m_impulse * m_u.x * inv_dt;
argOut.y = m_impulse * m_u.y * inv_dt;
}
/**
* Get the reaction torque given the inverse time step. Unit is N*m. This is always zero for a
* distance joint.
*/
@Override
public float getReactionTorque(float inv_dt) {
return 0.0f;
}
@Override
public void initVelocityConstraints(final SolverData data) {
m_indexA = m_bodyA.m_islandIndex;
m_indexB = m_bodyB.m_islandIndex;
m_localCenterA.set(m_bodyA.m_sweep.localCenter);
m_localCenterB.set(m_bodyB.m_sweep.localCenter);
m_invMassA = m_bodyA.m_invMass;
m_invMassB = m_bodyB.m_invMass;
m_invIA = m_bodyA.m_invI;
m_invIB = m_bodyB.m_invI;
Vec2 cA = data.positions[m_indexA].c;
float aA = data.positions[m_indexA].a;
Vec2 vA = data.velocities[m_indexA].v;
float wA = data.velocities[m_indexA].w;
Vec2 cB = data.positions[m_indexB].c;
float aB = data.positions[m_indexB].a;
Vec2 vB = data.velocities[m_indexB].v;
float wB = data.velocities[m_indexB].w;
final Rot qA = pool.popRot();
final Rot qB = pool.popRot();
qA.set(aA);
qB.set(aB);
// use m_u as temporary variable
Rot.mulToOutUnsafe(qA, m_u.set(m_localAnchorA).subLocal(m_localCenterA), m_rA);
Rot.mulToOutUnsafe(qB, m_u.set(m_localAnchorB).subLocal(m_localCenterB), m_rB);
m_u.set(cB).addLocal(m_rB).subLocal(cA).subLocal(m_rA);
pool.pushRot(2);
// Handle singularity.
float length = m_u.length();
if (length > Settings.linearSlop) {
m_u.x *= 1.0f / length;
m_u.y *= 1.0f / length;
} else {
m_u.set(0.0f, 0.0f);
}
float crAu = Vec2.cross(m_rA, m_u);
float crBu = Vec2.cross(m_rB, m_u);
float invMass = m_invMassA + m_invIA * crAu * crAu + m_invMassB + m_invIB * crBu * crBu;
// Compute the effective mass matrix.
m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
if (m_frequencyHz > 0.0f) {
float C = length - m_length;
// Frequency
float omega = 2.0f * MathUtils.PI * m_frequencyHz;
// Damping coefficient
float d = 2.0f * m_mass * m_dampingRatio * omega;
// Spring stiffness
float k = m_mass * omega * omega;
// magic formulas
float h = data.step.dt;
m_gamma = h * (d + h * k);
m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
m_bias = C * h * k * m_gamma;
invMass += m_gamma;
m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
} else {
m_gamma = 0.0f;
m_bias = 0.0f;
}
if (data.step.warmStarting) {
// Scale the impulse to support a variable time step.
m_impulse *= data.step.dtRatio;
Vec2 P = pool.popVec2();
P.set(m_u).mulLocal(m_impulse);
vA.x -= m_invMassA * P.x;
vA.y -= m_invMassA * P.y;
wA -= m_invIA * Vec2.cross(m_rA, P);
vB.x += m_invMassB * P.x;
vB.y += m_invMassB * P.y;
wB += m_invIB * Vec2.cross(m_rB, P);
pool.pushVec2(1);
} else {
m_impulse = 0.0f;
}
// data.velocities[m_indexA].v.set(vA);
data.velocities[m_indexA].w = wA;
// data.velocities[m_indexB].v.set(vB);
data.velocities[m_indexB].w = wB;
}
@Override
public void solveVelocityConstraints(final SolverData data) {
Vec2 vA = data.velocities[m_indexA].v;
float wA = data.velocities[m_indexA].w;
Vec2 vB = data.velocities[m_indexB].v;
float wB = data.velocities[m_indexB].w;
final Vec2 vpA = pool.popVec2();
final Vec2 vpB = pool.popVec2();
// Cdot = dot(u, v + cross(w, r))
Vec2.crossToOutUnsafe(wA, m_rA, vpA);
vpA.addLocal(vA);
Vec2.crossToOutUnsafe(wB, m_rB, vpB);
vpB.addLocal(vB);
float Cdot = Vec2.dot(m_u, vpB.subLocal(vpA));
float impulse = -m_mass * (Cdot + m_bias + m_gamma * m_impulse);
m_impulse += impulse;
float Px = impulse * m_u.x;
float Py = impulse * m_u.y;
vA.x -= m_invMassA * Px;
vA.y -= m_invMassA * Py;
wA -= m_invIA * (m_rA.x * Py - m_rA.y * Px);
vB.x += m_invMassB * Px;
vB.y += m_invMassB * Py;
wB += m_invIB * (m_rB.x * Py - m_rB.y * Px);
// data.velocities[m_indexA].v.set(vA);
data.velocities[m_indexA].w = wA;
// data.velocities[m_indexB].v.set(vB);
data.velocities[m_indexB].w = wB;
pool.pushVec2(2);
}
@Override
public boolean solvePositionConstraints(final SolverData data) {
if (m_frequencyHz > 0.0f) {
return true;
}
final Rot qA = pool.popRot();
final Rot qB = pool.popRot();
final Vec2 rA = pool.popVec2();
final Vec2 rB = pool.popVec2();
final Vec2 u = pool.popVec2();
Vec2 cA = data.positions[m_indexA].c;
float aA = data.positions[m_indexA].a;
Vec2 cB = data.positions[m_indexB].c;
float aB = data.positions[m_indexB].a;
qA.set(aA);
qB.set(aB);
Rot.mulToOutUnsafe(qA, u.set(m_localAnchorA).subLocal(m_localCenterA), rA);
Rot.mulToOutUnsafe(qB, u.set(m_localAnchorB).subLocal(m_localCenterB), rB);
u.set(cB).addLocal(rB).subLocal(cA).subLocal(rA);
float length = u.normalize();
float C = length - m_length;
C = MathUtils.clamp(C, -Settings.maxLinearCorrection, Settings.maxLinearCorrection);
float impulse = -m_mass * C;
float Px = impulse * u.x;
float Py = impulse * u.y;
cA.x -= m_invMassA * Px;
cA.y -= m_invMassA * Py;
aA -= m_invIA * (rA.x * Py - rA.y * Px);
cB.x += m_invMassB * Px;
cB.y += m_invMassB * Py;
aB += m_invIB * (rB.x * Py - rB.y * Px);
// data.positions[m_indexA].c.set(cA);
data.positions[m_indexA].a = aA;
// data.positions[m_indexB].c.set(cB);
data.positions[m_indexB].a = aB;
pool.pushVec2(3);
pool.pushRot(2);
return MathUtils.abs(C) < Settings.linearSlop;
}
}
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