b2DistanceJoint.cpp
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上传日期:2022-01-07
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- /*
- * Copyright (c) 2006-2007 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/b2DistanceJoint.h>
- #include <Box2D/Dynamics/b2Body.h>
- #include <Box2D/Dynamics/b2TimeStep.h>
- // 1-D constrained system
- // m (v2 - v1) = lambda
- // v2 + (beta/h) * x1 + gamma * lambda = 0, gamma has units of inverse mass.
- // x2 = x1 + h * v2
- // 1-D mass-damper-spring system
- // m (v2 - v1) + h * d * v2 + h * k *
- // 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
- void b2DistanceJointDef::Initialize(b2Body* b1, b2Body* b2,
- const b2Vec2& anchor1, const b2Vec2& anchor2)
- {
- bodyA = b1;
- bodyB = b2;
- localAnchorA = bodyA->GetLocalPoint(anchor1);
- localAnchorB = bodyB->GetLocalPoint(anchor2);
- b2Vec2 d = anchor2 - anchor1;
- length = d.Length();
- }
- b2DistanceJoint::b2DistanceJoint(const b2DistanceJointDef* def)
- : b2Joint(def)
- {
- m_localAnchor1 = def->localAnchorA;
- m_localAnchor2 = def->localAnchorB;
- m_length = def->length;
- m_frequencyHz = def->frequencyHz;
- m_dampingRatio = def->dampingRatio;
- m_impulse = 0.0f;
- m_gamma = 0.0f;
- m_bias = 0.0f;
- }
- void b2DistanceJoint::InitVelocityConstraints(const b2TimeStep& step)
- {
- b2Body* b1 = m_bodyA;
- b2Body* b2 = m_bodyB;
- // Compute the effective mass matrix.
- b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
- b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
- m_u = b2->m_sweep.c + r2 - b1->m_sweep.c - r1;
- // Handle singularity.
- float32 length = m_u.Length();
- if (length > b2_linearSlop)
- {
- m_u *= 1.0f / length;
- }
- else
- {
- m_u.Set(0.0f, 0.0f);
- }
- float32 cr1u = b2Cross(r1, m_u);
- float32 cr2u = b2Cross(r2, m_u);
- float32 invMass = b1->m_invMass + b1->m_invI * cr1u * cr1u + b2->m_invMass + b2->m_invI * cr2u * cr2u;
- m_mass = invMass != 0.0f ? 1.0f / invMass : 0.0f;
- if (m_frequencyHz > 0.0f)
- {
- float32 C = length - m_length;
- // Frequency
- float32 omega = 2.0f * b2_pi * m_frequencyHz;
- // Damping coefficient
- float32 d = 2.0f * m_mass * m_dampingRatio * omega;
- // Spring stiffness
- float32 k = m_mass * omega * omega;
- // magic formulas
- m_gamma = step.dt * (d + step.dt * k);
- m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
- m_bias = C * step.dt * k * m_gamma;
- m_mass = invMass + m_gamma;
- m_mass = m_mass != 0.0f ? 1.0f / m_mass : 0.0f;
- }
- if (step.warmStarting)
- {
- // Scale the impulse to support a variable time step.
- m_impulse *= step.dtRatio;
- b2Vec2 P = m_impulse * m_u;
- b1->m_linearVelocity -= b1->m_invMass * P;
- b1->m_angularVelocity -= b1->m_invI * b2Cross(r1, P);
- b2->m_linearVelocity += b2->m_invMass * P;
- b2->m_angularVelocity += b2->m_invI * b2Cross(r2, P);
- }
- else
- {
- m_impulse = 0.0f;
- }
- }
- void b2DistanceJoint::SolveVelocityConstraints(const b2TimeStep& step)
- {
- B2_NOT_USED(step);
- b2Body* b1 = m_bodyA;
- b2Body* b2 = m_bodyB;
- b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
- b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
- // Cdot = dot(u, v + cross(w, r))
- b2Vec2 v1 = b1->m_linearVelocity + b2Cross(b1->m_angularVelocity, r1);
- b2Vec2 v2 = b2->m_linearVelocity + b2Cross(b2->m_angularVelocity, r2);
- float32 Cdot = b2Dot(m_u, v2 - v1);
- float32 impulse = -m_mass * (Cdot + m_bias + m_gamma * m_impulse);
- m_impulse += impulse;
- b2Vec2 P = impulse * m_u;
- b1->m_linearVelocity -= b1->m_invMass * P;
- b1->m_angularVelocity -= b1->m_invI * b2Cross(r1, P);
- b2->m_linearVelocity += b2->m_invMass * P;
- b2->m_angularVelocity += b2->m_invI * b2Cross(r2, P);
- }
- bool b2DistanceJoint::SolvePositionConstraints(float32 baumgarte)
- {
- B2_NOT_USED(baumgarte);
- if (m_frequencyHz > 0.0f)
- {
- // There is no position correction for soft distance constraints.
- return true;
- }
- b2Body* b1 = m_bodyA;
- b2Body* b2 = m_bodyB;
- b2Vec2 r1 = b2Mul(b1->GetTransform().R, m_localAnchor1 - b1->GetLocalCenter());
- b2Vec2 r2 = b2Mul(b2->GetTransform().R, m_localAnchor2 - b2->GetLocalCenter());
- b2Vec2 d = b2->m_sweep.c + r2 - b1->m_sweep.c - r1;
- float32 length = d.Normalize();
- float32 C = length - m_length;
- C = b2Clamp(C, -b2_maxLinearCorrection, b2_maxLinearCorrection);
- float32 impulse = -m_mass * C;
- m_u = d;
- b2Vec2 P = impulse * m_u;
- b1->m_sweep.c -= b1->m_invMass * P;
- b1->m_sweep.a -= b1->m_invI * b2Cross(r1, P);
- b2->m_sweep.c += b2->m_invMass * P;
- b2->m_sweep.a += b2->m_invI * b2Cross(r2, P);
- b1->SynchronizeTransform();
- b2->SynchronizeTransform();
- return b2Abs(C) < b2_linearSlop;
- }
- b2Vec2 b2DistanceJoint::GetAnchorA() const
- {
- return m_bodyA->GetWorldPoint(m_localAnchor1);
- }
- b2Vec2 b2DistanceJoint::GetAnchorB() const
- {
- return m_bodyB->GetWorldPoint(m_localAnchor2);
- }
- b2Vec2 b2DistanceJoint::GetReactionForce(float32 inv_dt) const
- {
- b2Vec2 F = (inv_dt * m_impulse) * m_u;
- return F;
- }
- float32 b2DistanceJoint::GetReactionTorque(float32 inv_dt) const
- {
- B2_NOT_USED(inv_dt);
- return 0.0f;
- }