pocketpy/3rd/box2d/src/dynamics/b2_friction_joint.cpp

// MIT License

// Copyright (c) 2019 Erin Catto

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include "box2d/b2_friction_joint.h"
#include "box2d/b2_body.h"
#include "box2d/b2_time_step.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 b2SolverData& data)
{
	m_indexA = m_bodyA->m_islandIndex;
	m_indexB = m_bodyB->m_islandIndex;
	m_localCenterA = m_bodyA->m_sweep.localCenter;
	m_localCenterB = 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;

	float aA = data.positions[m_indexA].a;
	b2Vec2 vA = data.velocities[m_indexA].v;
	float wA = data.velocities[m_indexA].w;

	float aB = data.positions[m_indexB].a;
	b2Vec2 vB = data.velocities[m_indexB].v;
	float wB = data.velocities[m_indexB].w;

	b2Rot qA(aA), qB(aB);

	// Compute the effective mass matrix.
	m_rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
	m_rB = b2Mul(qB, m_localAnchorB - m_localCenterB);

	// 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]

	float mA = m_invMassA, mB = m_invMassB;
	float iA = m_invIA, iB = m_invIB;

	b2Mat22 K;
	K.ex.x = mA + mB + iA * m_rA.y * m_rA.y + iB * m_rB.y * m_rB.y;
	K.ex.y = -iA * m_rA.x * m_rA.y - iB * m_rB.x * m_rB.y;
	K.ey.x = K.ex.y;
	K.ey.y = mA + mB + iA * m_rA.x * m_rA.x + iB * m_rB.x * m_rB.x;

	m_linearMass = K.GetInverse();

	m_angularMass = iA + iB;
	if (m_angularMass > 0.0f)
	{
		m_angularMass = 1.0f / m_angularMass;
	}

	if (data.step.warmStarting)
	{
		// Scale impulses to support a variable time step.
		m_linearImpulse *= data.step.dtRatio;
		m_angularImpulse *= data.step.dtRatio;

		b2Vec2 P(m_linearImpulse.x, m_linearImpulse.y);
		vA -= mA * P;
		wA -= iA * (b2Cross(m_rA, P) + m_angularImpulse);
		vB += mB * P;
		wB += iB * (b2Cross(m_rB, P) + m_angularImpulse);
	}
	else
	{
		m_linearImpulse.SetZero();
		m_angularImpulse = 0.0f;
	}

	data.velocities[m_indexA].v = vA;
	data.velocities[m_indexA].w = wA;
	data.velocities[m_indexB].v = vB;
	data.velocities[m_indexB].w = wB;
}

void b2FrictionJoint::SolveVelocityConstraints(const b2SolverData& data)
{
	b2Vec2 vA = data.velocities[m_indexA].v;
	float wA = data.velocities[m_indexA].w;
	b2Vec2 vB = data.velocities[m_indexB].v;
	float wB = data.velocities[m_indexB].w;

	float mA = m_invMassA, mB = m_invMassB;
	float iA = m_invIA, iB = m_invIB;

	float h = data.step.dt;

	// Solve angular friction
	{
		float Cdot = wB - wA;
		float impulse = -m_angularMass * Cdot;

		float oldImpulse = m_angularImpulse;
		float maxImpulse = h * 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, m_rB) - vA - b2Cross(wA, m_rA);

		b2Vec2 impulse = -b2Mul(m_linearMass, Cdot);
		b2Vec2 oldImpulse = m_linearImpulse;
		m_linearImpulse += impulse;

		float maxImpulse = h * m_maxForce;

		if (m_linearImpulse.LengthSquared() > maxImpulse * maxImpulse)
		{
			m_linearImpulse.Normalize();
			m_linearImpulse *= maxImpulse;
		}

		impulse = m_linearImpulse - oldImpulse;

		vA -= mA * impulse;
		wA -= iA * b2Cross(m_rA, impulse);

		vB += mB * impulse;
		wB += iB * b2Cross(m_rB, impulse);
	}

	data.velocities[m_indexA].v = vA;
	data.velocities[m_indexA].w = wA;
	data.velocities[m_indexB].v = vB;
	data.velocities[m_indexB].w = wB;
}

bool b2FrictionJoint::SolvePositionConstraints(const b2SolverData& data)
{
	B2_NOT_USED(data);

	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(float inv_dt) const
{
	return inv_dt * m_linearImpulse;
}

float b2FrictionJoint::GetReactionTorque(float inv_dt) const
{
	return inv_dt * m_angularImpulse;
}

void b2FrictionJoint::SetMaxForce(float force)
{
	b2Assert(b2IsValid(force) && force >= 0.0f);
	m_maxForce = force;
}

float b2FrictionJoint::GetMaxForce() const
{
	return m_maxForce;
}

void b2FrictionJoint::SetMaxTorque(float torque)
{
	b2Assert(b2IsValid(torque) && torque >= 0.0f);
	m_maxTorque = torque;
}

float b2FrictionJoint::GetMaxTorque() const
{
	return m_maxTorque;
}

void b2FrictionJoint::Dump()
{
	int32 indexA = m_bodyA->m_islandIndex;
	int32 indexB = m_bodyB->m_islandIndex;

	b2Dump("  b2FrictionJointDef jd;\n");
	b2Dump("  jd.bodyA = bodies[%d];\n", indexA);
	b2Dump("  jd.bodyB = bodies[%d];\n", indexB);
	b2Dump("  jd.collideConnected = bool(%d);\n", m_collideConnected);
	b2Dump("  jd.localAnchorA.Set(%.9g, %.9g);\n", m_localAnchorA.x, m_localAnchorA.y);
	b2Dump("  jd.localAnchorB.Set(%.9g, %.9g);\n", m_localAnchorB.x, m_localAnchorB.y);
	b2Dump("  jd.maxForce = %.9g;\n", m_maxForce);
	b2Dump("  jd.maxTorque = %.9g;\n", m_maxTorque);
	b2Dump("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}