b2FrictionJoint.cpp
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上传日期:2022-01-07
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- /*
- * Copyright (c) 2006-2009 Erin Catto http://www.gphysics.com
- *
- * 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.
- */
- #include <Box2D/Dynamics/Joints/b2FrictionJoint.h>
- #include <Box2D/Dynamics/b2Body.h>
- #include <Box2D/Dynamics/b2TimeStep.h>
- // Point-to-point constraint
- // Cdot = v2 - v1
- // = v2 + cross(w2, r2) - v1 - cross(w1, r1)
- // J = [-I -r1_skew I r2_skew ]
- // Identity used:
- // w k % (rx i + ry j) = w * (-ry i + rx j)
- // Angle constraint
- // Cdot = w2 - w1
- // J = [0 0 -1 0 0 1]
- // K = invI1 + invI2
- void b2FrictionJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor)
- {
- bodyA = bA;
- bodyB = bB;
- localAnchorA = bodyA->GetLocalPoint(anchor);
- localAnchorB = bodyB->GetLocalPoint(anchor);
- }
- b2FrictionJoint::b2FrictionJoint(const b2FrictionJointDef* def)
- : b2Joint(def)
- {
- m_localAnchorA = def->localAnchorA;
- m_localAnchorB = def->localAnchorB;
- m_linearImpulse.SetZero();
- m_angularImpulse = 0.0f;
- m_maxForce = def->maxForce;
- m_maxTorque = def->maxTorque;
- }
- void b2FrictionJoint::InitVelocityConstraints(const b2TimeStep& step)
- {
- b2Body* bA = m_bodyA;
- b2Body* bB = m_bodyB;
- // Compute the effective mass matrix.
- b2Vec2 rA = b2Mul(bA->GetTransform().R, m_localAnchorA - bA->GetLocalCenter());
- b2Vec2 rB = b2Mul(bB->GetTransform().R, m_localAnchorB - bB->GetLocalCenter());
- // J = [-I -r1_skew I r2_skew]
- // [ 0 -1 0 1]
- // r_skew = [-ry; rx]
- // Matlab
- // K = [ mA+r1y^2*iA+mB+r2y^2*iB, -r1y*iA*r1x-r2y*iB*r2x, -r1y*iA-r2y*iB]
- // [ -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB, r1x*iA+r2x*iB]
- // [ -r1y*iA-r2y*iB, r1x*iA+r2x*iB, iA+iB]
- float32 mA = bA->m_invMass, mB = bB->m_invMass;
- float32 iA = bA->m_invI, iB = bB->m_invI;
- b2Mat22 K1;
- K1.col1.x = mA + mB; K1.col2.x = 0.0f;
- K1.col1.y = 0.0f; K1.col2.y = mA + mB;
- b2Mat22 K2;
- K2.col1.x = iA * rA.y * rA.y; K2.col2.x = -iA * rA.x * rA.y;
- K2.col1.y = -iA * rA.x * rA.y; K2.col2.y = iA * rA.x * rA.x;
- b2Mat22 K3;
- K3.col1.x = iB * rB.y * rB.y; K3.col2.x = -iB * rB.x * rB.y;
- K3.col1.y = -iB * rB.x * rB.y; K3.col2.y = iB * rB.x * rB.x;
- b2Mat22 K = K1 + K2 + K3;
- m_linearMass = K.GetInverse();
- m_angularMass = iA + iB;
- if (m_angularMass > 0.0f)
- {
- m_angularMass = 1.0f / m_angularMass;
- }
- if (step.warmStarting)
- {
- // Scale impulses to support a variable time step.
- m_linearImpulse *= step.dtRatio;
- m_angularImpulse *= step.dtRatio;
- b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
- bA->m_linearVelocity -= mA * P;
- bA->m_angularVelocity -= iA * (b2Cross(rA, P) + m_angularImpulse);
- bB->m_linearVelocity += mB * P;
- bB->m_angularVelocity += iB * (b2Cross(rB, P) + m_angularImpulse);
- }
- else
- {
- m_linearImpulse.SetZero();
- m_angularImpulse = 0.0f;
- }
- }
- void b2FrictionJoint::SolveVelocityConstraints(const b2TimeStep& step)
- {
- B2_NOT_USED(step);
- b2Body* bA = m_bodyA;
- b2Body* bB = m_bodyB;
- b2Vec2 vA = bA->m_linearVelocity;
- float32 wA = bA->m_angularVelocity;
- b2Vec2 vB = bB->m_linearVelocity;
- float32 wB = bB->m_angularVelocity;
- float32 mA = bA->m_invMass, mB = bB->m_invMass;
- float32 iA = bA->m_invI, iB = bB->m_invI;
- b2Vec2 rA = b2Mul(bA->GetTransform().R, m_localAnchorA - bA->GetLocalCenter());
- b2Vec2 rB = b2Mul(bB->GetTransform().R, m_localAnchorB - bB->GetLocalCenter());
- // Solve angular friction
- {
- float32 Cdot = wB - wA;
- float32 impulse = -m_angularMass * Cdot;
- float32 oldImpulse = m_angularImpulse;
- float32 maxImpulse = step.dt * m_maxTorque;
- m_angularImpulse = b2Clamp(m_angularImpulse + impulse, -maxImpulse, maxImpulse);
- impulse = m_angularImpulse - oldImpulse;
- wA -= iA * impulse;
- wB += iB * impulse;
- }
- // Solve linear friction
- {
- b2Vec2 Cdot = vB + b2Cross(wB, rB) - vA - b2Cross(wA, rA);
- b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
- b2Vec2 oldImpulse = m_linearImpulse;
- m_linearImpulse += impulse;
- float32 maxImpulse = step.dt * m_maxForce;
- if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
- {
- m_linearImpulse.Normalize();
- m_linearImpulse *= maxImpulse;
- }
- impulse = m_linearImpulse - oldImpulse;
- vA -= mA * impulse;
- wA -= iA * b2Cross(rA, impulse);
- vB += mB * impulse;
- wB += iB * b2Cross(rB, impulse);
- }
- bA->m_linearVelocity = vA;
- bA->m_angularVelocity = wA;
- bB->m_linearVelocity = vB;
- bB->m_angularVelocity = wB;
- }
- bool b2FrictionJoint::SolvePositionConstraints(float32 baumgarte)
- {
- B2_NOT_USED(baumgarte);
- return true;
- }
- b2Vec2 b2FrictionJoint::GetAnchorA() const
- {
- return m_bodyA->GetWorldPoint(m_localAnchorA);
- }
- b2Vec2 b2FrictionJoint::GetAnchorB() const
- {
- return m_bodyB->GetWorldPoint(m_localAnchorB);
- }
- b2Vec2 b2FrictionJoint::GetReactionForce(float32 inv_dt) const
- {
- return inv_dt * m_linearImpulse;
- }
- float32 b2FrictionJoint::GetReactionTorque(float32 inv_dt) const
- {
- return inv_dt * m_angularImpulse;
- }
- void b2FrictionJoint::SetMaxForce(float32 force)
- {
- b2Assert(b2IsValid(force) && force >= 0.0f);
- m_maxForce = force;
- }
- float32 b2FrictionJoint::GetMaxForce() const
- {
- return m_maxForce;
- }
- void b2FrictionJoint::SetMaxTorque(float32 torque)
- {
- b2Assert(b2IsValid(torque) && torque >= 0.0f);
- m_maxTorque = torque;
- }
- float32 b2FrictionJoint::GetMaxTorque() const
- {
- return m_maxTorque;
- }