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Garrett Brown
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// MIT License
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// Copyright (c) 2019 Erin Catto
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// Permission is hereby granted, free of charge, to any person obtaining a copy
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// of this software and associated documentation files (the "Software"), to deal
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// in the Software without restriction, including without limitation the rights
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// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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// copies of the Software, and to permit persons to whom the Software is
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// furnished to do so, subject to the following conditions:
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// The above copyright notice and this permission notice shall be included in all
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// copies or substantial portions of the Software.
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// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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// SOFTWARE.
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#include "box2d/b2_friction_joint.h"
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#include "box2d/b2_body.h"
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#include "box2d/b2_time_step.h"
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// Point-to-point constraint
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// Cdot = v2 - v1
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// = v2 + cross(w2, r2) - v1 - cross(w1, r1)
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// J = [-I -r1_skew I r2_skew ]
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// Identity used:
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// w k % (rx i + ry j) = w * (-ry i + rx j)
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// Angle constraint
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// Cdot = w2 - w1
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// J = [0 0 -1 0 0 1]
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// K = invI1 + invI2
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void b2FrictionJointDef::Initialize(b2Body* bA, b2Body* bB, const b2Vec2& anchor)
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{
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bodyA = bA;
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bodyB = bB;
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localAnchorA = bodyA->GetLocalPoint(anchor);
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localAnchorB = bodyB->GetLocalPoint(anchor);
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}
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b2FrictionJoint::b2FrictionJoint(const b2FrictionJointDef* def)
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: b2Joint(def)
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{
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m_localAnchorA = def->localAnchorA;
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m_localAnchorB = def->localAnchorB;
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m_linearImpulse.SetZero();
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m_angularImpulse = 0.0f;
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m_maxForce = def->maxForce;
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m_maxTorque = def->maxTorque;
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}
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void b2FrictionJoint::InitVelocityConstraints(const b2SolverData& data)
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{
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m_indexA = m_bodyA->m_islandIndex;
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m_indexB = m_bodyB->m_islandIndex;
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m_localCenterA = m_bodyA->m_sweep.localCenter;
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m_localCenterB = m_bodyB->m_sweep.localCenter;
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m_invMassA = m_bodyA->m_invMass;
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m_invMassB = m_bodyB->m_invMass;
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m_invIA = m_bodyA->m_invI;
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m_invIB = m_bodyB->m_invI;
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float aA = data.positions[m_indexA].a;
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b2Vec2 vA = data.velocities[m_indexA].v;
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float wA = data.velocities[m_indexA].w;
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float aB = data.positions[m_indexB].a;
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b2Vec2 vB = data.velocities[m_indexB].v;
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float wB = data.velocities[m_indexB].w;
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b2Rot qA(aA), qB(aB);
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// Compute the effective mass matrix.
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m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
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m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);
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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 = [ mA+r1y^2*iA+mB+r2y^2*iB, -r1y*iA*r1x-r2y*iB*r2x, -r1y*iA-r2y*iB]
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// [ -r1y*iA*r1x-r2y*iB*r2x, mA+r1x^2*iA+mB+r2x^2*iB, r1x*iA+r2x*iB]
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// [ -r1y*iA-r2y*iB, r1x*iA+r2x*iB, iA+iB]
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float mA = m_invMassA, mB = m_invMassB;
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float iA = m_invIA, iB = m_invIB;
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b2Mat22 K;
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K.ex.x = mA + mB + iA * m_rA.y * m_rA.y + iB * m_rB.y * m_rB.y;
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K.ex.y = -iA * m_rA.x * m_rA.y - iB * m_rB.x * m_rB.y;
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K.ey.x = K.ex.y;
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K.ey.y = mA + mB + iA * m_rA.x * m_rA.x + iB * m_rB.x * m_rB.x;
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m_linearMass = K.GetInverse();
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m_angularMass = iA + iB;
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if (m_angularMass > 0.0f)
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{
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m_angularMass = 1.0f / m_angularMass;
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}
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if (data.step.warmStarting)
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{
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// Scale impulses to support a variable time step.
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m_linearImpulse *= data.step.dtRatio;
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m_angularImpulse *= data.step.dtRatio;
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b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
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vA -= mA * P;
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wA -= iA * (b2Cross(m_rA, P) + m_angularImpulse);
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vB += mB * P;
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wB += iB * (b2Cross(m_rB, P) + m_angularImpulse);
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}
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else
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{
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m_linearImpulse.SetZero();
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m_angularImpulse = 0.0f;
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}
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data.velocities[m_indexA].v = vA;
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data.velocities[m_indexA].w = wA;
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data.velocities[m_indexB].v = vB;
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data.velocities[m_indexB].w = wB;
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}
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void b2FrictionJoint::SolveVelocityConstraints(const b2SolverData& data)
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{
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b2Vec2 vA = data.velocities[m_indexA].v;
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float wA = data.velocities[m_indexA].w;
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b2Vec2 vB = data.velocities[m_indexB].v;
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float wB = data.velocities[m_indexB].w;
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float mA = m_invMassA, mB = m_invMassB;
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float iA = m_invIA, iB = m_invIB;
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float h = data.step.dt;
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// Solve angular friction
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{
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float Cdot = wB - wA;
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float impulse = -m_angularMass * Cdot;
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float oldImpulse = m_angularImpulse;
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float maxImpulse = h * m_maxTorque;
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m_angularImpulse = b2Clamp(m_angularImpulse + impulse, -maxImpulse, maxImpulse);
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impulse = m_angularImpulse - oldImpulse;
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wA -= iA * impulse;
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wB += iB * impulse;
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}
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// Solve linear friction
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{
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b2Vec2 Cdot = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);
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b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
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b2Vec2 oldImpulse = m_linearImpulse;
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m_linearImpulse += impulse;
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float maxImpulse = h * m_maxForce;
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if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
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{
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m_linearImpulse.Normalize();
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m_linearImpulse *= maxImpulse;
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}
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impulse = m_linearImpulse - oldImpulse;
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vA -= mA * impulse;
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wA -= iA * b2Cross(m_rA, impulse);
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vB += mB * impulse;
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wB += iB * b2Cross(m_rB, impulse);
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}
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data.velocities[m_indexA].v = vA;
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data.velocities[m_indexA].w = wA;
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data.velocities[m_indexB].v = vB;
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data.velocities[m_indexB].w = wB;
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}
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bool b2FrictionJoint::SolvePositionConstraints(const b2SolverData& data)
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{
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B2_NOT_USED(data);
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return true;
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}
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b2Vec2 b2FrictionJoint::GetAnchorA() const
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{
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return m_bodyA->GetWorldPoint(m_localAnchorA);
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}
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b2Vec2 b2FrictionJoint::GetAnchorB() const
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{
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return m_bodyB->GetWorldPoint(m_localAnchorB);
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}
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b2Vec2 b2FrictionJoint::GetReactionForce(float inv_dt) const
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{
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return inv_dt * m_linearImpulse;
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}
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float b2FrictionJoint::GetReactionTorque(float inv_dt) const
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{
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return inv_dt * m_angularImpulse;
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}
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void b2FrictionJoint::SetMaxForce(float force)
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{
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b2Assert(b2IsValid(force) && force >= 0.0f);
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m_maxForce = force;
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}
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float b2FrictionJoint::GetMaxForce() const
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{
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return m_maxForce;
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}
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void b2FrictionJoint::SetMaxTorque(float torque)
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{
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b2Assert(b2IsValid(torque) && torque >= 0.0f);
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m_maxTorque = torque;
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}
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float b2FrictionJoint::GetMaxTorque() const
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{
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return m_maxTorque;
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}
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void b2FrictionJoint::Dump()
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{
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int32 indexA = m_bodyA->m_islandIndex;
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int32 indexB = m_bodyB->m_islandIndex;
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b2Dump(" b2FrictionJointDef jd;\n");
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b2Dump(" jd.bodyA = bodies[%d];\n", indexA);
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b2Dump(" jd.bodyB = bodies[%d];\n", indexB);
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b2Dump(" jd.collideConnected = bool(%d);\n", m_collideConnected);
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b2Dump(" jd.localAnchorA.Set(%.9g, %.9g);\n", m_localAnchorA.x, m_localAnchorA.y);
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b2Dump(" jd.localAnchorB.Set(%.9g, %.9g);\n", m_localAnchorB.x, m_localAnchorB.y);
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b2Dump(" jd.maxForce = %.9g;\n", m_maxForce);
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b2Dump(" jd.maxTorque = %.9g;\n", m_maxTorque);
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b2Dump(" joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
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}
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