b2WeldJoint.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/b2WeldJoint.h>
- #include <Box2D/Dynamics/b2Body.h>
- #include <Box2D/Dynamics/b2TimeStep.h>
- // Point-to-point constraint
- // C = p2 - p1
- // 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
- // C = angle2 - angle1 - referenceAngle
- // Cdot = w2 - w1
- // J = [0 0 -1 0 0 1]
- // K = invI1 + invI2
- void b2WeldJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor)
- {
- bodyA = bA;
- bodyB = bB;
- localAnchorA = bodyA->GetLocalPoint(anchor);
- localAnchorB = bodyB->GetLocalPoint(anchor);
- referenceAngle = bodyB->GetAngle() - bodyA->GetAngle();
- }
- b2WeldJoint::b2WeldJoint(const b2WeldJointDef* def)
- : b2Joint(def)
- {
- m_localAnchorA = def->localAnchorA;
- m_localAnchorB = def->localAnchorB;
- m_referenceAngle = def->referenceAngle;
- m_impulse.SetZero();
- }
- void b2WeldJoint::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;
- m_mass.col1.x = mA + mB + rA.y * rA.y * iA + rB.y * rB.y * iB;
- m_mass.col2.x = -rA.y * rA.x * iA - rB.y * rB.x * iB;
- m_mass.col3.x = -rA.y * iA - rB.y * iB;
- m_mass.col1.y = m_mass.col2.x;
- m_mass.col2.y = mA + mB + rA.x * rA.x * iA + rB.x * rB.x * iB;
- m_mass.col3.y = rA.x * iA + rB.x * iB;
- m_mass.col1.z = m_mass.col3.x;
- m_mass.col2.z = m_mass.col3.y;
- m_mass.col3.z = iA + iB;
- if (step.warmStarting)
- {
- // Scale impulses to support a variable time step.
- m_impulse *= step.dtRatio;
- b2Vec2 P(m_impulse.x, m_impulse.y);
- bA->m_linearVelocity -= mA * P;
- bA->m_angularVelocity -= iA * (b2Cross(rA, P) + m_impulse.z);
- bB->m_linearVelocity += mB * P;
- bB->m_angularVelocity += iB * (b2Cross(rB, P) + m_impulse.z);
- }
- else
- {
- m_impulse.SetZero();
- }
- }
- void b2WeldJoint::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 point-to-point constraint
- b2Vec2 Cdot1 = vB + b2Cross(wB, rB) - vA - b2Cross(wA, rA);
- float32 Cdot2 = wB - wA;
- b2Vec3 Cdot(Cdot1.x, Cdot1.y, Cdot2);
- b2Vec3 impulse = m_mass.Solve33(-Cdot);
- m_impulse += impulse;
- b2Vec2 P(impulse.x, impulse.y);
- vA -= mA * P;
- wA -= iA * (b2Cross(rA, P) + impulse.z);
- vB += mB * P;
- wB += iB * (b2Cross(rB, P) + impulse.z);
- bA->m_linearVelocity = vA;
- bA->m_angularVelocity = wA;
- bB->m_linearVelocity = vB;
- bB->m_angularVelocity = wB;
- }
- bool b2WeldJoint::SolvePositionConstraints(float32 baumgarte)
- {
- B2_NOT_USED(baumgarte);
- b2Body* bA = m_bodyA;
- b2Body* bB = m_bodyB;
- 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());
- b2Vec2 C1 = bB->m_sweep.c + rB - bA->m_sweep.c - rA;
- float32 C2 = bB->m_sweep.a - bA->m_sweep.a - m_referenceAngle;
- // Handle large detachment.
- const float32 k_allowedStretch = 10.0f * b2_linearSlop;
- float32 positionError = C1.Length();
- float32 angularError = b2Abs(C2);
- if (positionError > k_allowedStretch)
- {
- iA *= 1.0f;
- iB *= 1.0f;
- }
- m_mass.col1.x = mA + mB + rA.y * rA.y * iA + rB.y * rB.y * iB;
- m_mass.col2.x = -rA.y * rA.x * iA - rB.y * rB.x * iB;
- m_mass.col3.x = -rA.y * iA - rB.y * iB;
- m_mass.col1.y = m_mass.col2.x;
- m_mass.col2.y = mA + mB + rA.x * rA.x * iA + rB.x * rB.x * iB;
- m_mass.col3.y = rA.x * iA + rB.x * iB;
- m_mass.col1.z = m_mass.col3.x;
- m_mass.col2.z = m_mass.col3.y;
- m_mass.col3.z = iA + iB;
- b2Vec3 C(C1.x, C1.y, C2);
- b2Vec3 impulse = m_mass.Solve33(-C);
- b2Vec2 P(impulse.x, impulse.y);
- bA->m_sweep.c -= mA * P;
- bA->m_sweep.a -= iA * (b2Cross(rA, P) + impulse.z);
- bB->m_sweep.c += mB * P;
- bB->m_sweep.a += iB * (b2Cross(rB, P) + impulse.z);
- bA->SynchronizeTransform();
- bB->SynchronizeTransform();
- return positionError <= b2_linearSlop && angularError <= b2_angularSlop;
- }
- b2Vec2 b2WeldJoint::GetAnchorA() const
- {
- return m_bodyA->GetWorldPoint(m_localAnchorA);
- }
- b2Vec2 b2WeldJoint::GetAnchorB() const
- {
- return m_bodyB->GetWorldPoint(m_localAnchorB);
- }
- b2Vec2 b2WeldJoint::GetReactionForce(float32 inv_dt) const
- {
- b2Vec2 P(m_impulse.x, m_impulse.y);
- return inv_dt * P;
- }
- float32 b2WeldJoint::GetReactionTorque(float32 inv_dt) const
- {
- return inv_dt * m_impulse.z;
- }