pocketpy/3rd/box2d/src/dynamics/b2_weld_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_body.h"
#include "box2d/b2_time_step.h"
#include "box2d/b2_weld_joint.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_stiffness = def->stiffness;
	m_damping = def->damping;

	m_impulse.SetZero();
}

void b2WeldJoint::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);

	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;

	b2Mat33 K;
	K.ex.x = mA + mB + m_rA.y * m_rA.y * iA + m_rB.y * m_rB.y * iB;
	K.ey.x = -m_rA.y * m_rA.x * iA - m_rB.y * m_rB.x * iB;
	K.ez.x = -m_rA.y * iA - m_rB.y * iB;
	K.ex.y = K.ey.x;
	K.ey.y = mA + mB + m_rA.x * m_rA.x * iA + m_rB.x * m_rB.x * iB;
	K.ez.y = m_rA.x * iA + m_rB.x * iB;
	K.ex.z = K.ez.x;
	K.ey.z = K.ez.y;
	K.ez.z = iA + iB;

	if (m_stiffness > 0.0f)
	{
		K.GetInverse22(&m_mass);

		float invM = iA + iB;

		float C = aB - aA - m_referenceAngle;

		// Damping coefficient
		float d = m_damping;

		// Spring stiffness
		float k = m_stiffness;

		// magic formulas
		float h = data.step.dt;
		m_gamma = h * (d + h * k);
		m_gamma = m_gamma != 0.0f ? 1.0f / m_gamma : 0.0f;
		m_bias = C * h * k * m_gamma;

		invM += m_gamma;
		m_mass.ez.z = invM != 0.0f ? 1.0f / invM : 0.0f;
	}
	else if (K.ez.z == 0.0f)
	{
		K.GetInverse22(&m_mass);
		m_gamma = 0.0f;
		m_bias = 0.0f;
	}
	else
	{
		K.GetSymInverse33(&m_mass);
		m_gamma = 0.0f;
		m_bias = 0.0f;
	}

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

		b2Vec2 P(m_impulse.x, m_impulse.y);

		vA -= mA * P;
		wA -= iA * (b2Cross(m_rA, P) + m_impulse.z);

		vB += mB * P;
		wB += iB * (b2Cross(m_rB, P) + m_impulse.z);
	}
	else
	{
		m_impulse.SetZero();
	}

	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 b2WeldJoint::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;

	if (m_stiffness > 0.0f)
	{
		float Cdot2 = wB - wA;

		float impulse2 = -m_mass.ez.z * (Cdot2 + m_bias + m_gamma * m_impulse.z);
		m_impulse.z += impulse2;

		wA -= iA * impulse2;
		wB += iB * impulse2;

		b2Vec2 Cdot1 = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);

		b2Vec2 impulse1 = -b2Mul22(m_mass, Cdot1);
		m_impulse.x += impulse1.x;
		m_impulse.y += impulse1.y;

		b2Vec2 P = impulse1;

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

		vB += mB * P;
		wB += iB * b2Cross(m_rB, P);
	}
	else
	{
		b2Vec2 Cdot1 = vB + b2Cross(wB, m_rB) - vA - b2Cross(wA, m_rA);
		float Cdot2 = wB - wA;
		b2Vec3 Cdot(Cdot1.x, Cdot1.y, Cdot2);

		b2Vec3 impulse = -b2Mul(m_mass, Cdot);
		m_impulse += impulse;

		b2Vec2 P(impulse.x, impulse.y);

		vA -= mA * P;
		wA -= iA * (b2Cross(m_rA, P) + impulse.z);

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

	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 b2WeldJoint::SolvePositionConstraints(const b2SolverData& data)
{
	b2Vec2 cA = data.positions[m_indexA].c;
	float aA = data.positions[m_indexA].a;
	b2Vec2 cB = data.positions[m_indexB].c;
	float aB = data.positions[m_indexB].a;

	b2Rot qA(aA), qB(aB);

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

	b2Vec2 rA = b2Mul(qA, m_localAnchorA - m_localCenterA);
	b2Vec2 rB = b2Mul(qB, m_localAnchorB - m_localCenterB);

	float positionError, angularError;

	b2Mat33 K;
	K.ex.x = mA + mB + rA.y * rA.y * iA + rB.y * rB.y * iB;
	K.ey.x = -rA.y * rA.x * iA - rB.y * rB.x * iB;
	K.ez.x = -rA.y * iA - rB.y * iB;
	K.ex.y = K.ey.x;
	K.ey.y = mA + mB + rA.x * rA.x * iA + rB.x * rB.x * iB;
	K.ez.y = rA.x * iA + rB.x * iB;
	K.ex.z = K.ez.x;
	K.ey.z = K.ez.y;
	K.ez.z = iA + iB;

	if (m_stiffness > 0.0f)
	{
		b2Vec2 C1 =  cB + rB - cA - rA;

		positionError = C1.Length();
		angularError = 0.0f;

		b2Vec2 P = -K.Solve22(C1);

		cA -= mA * P;
		aA -= iA * b2Cross(rA, P);

		cB += mB * P;
		aB += iB * b2Cross(rB, P);
	}
	else
	{
		b2Vec2 C1 =  cB + rB - cA - rA;
		float C2 = aB - aA - m_referenceAngle;

		positionError = C1.Length();
		angularError = b2Abs(C2);

		b2Vec3 C(C1.x, C1.y, C2);
	
		b2Vec3 impulse;
		if (K.ez.z > 0.0f)
		{
			impulse = -K.Solve33(C);
		}
		else
		{
			b2Vec2 impulse2 = -K.Solve22(C1);
			impulse.Set(impulse2.x, impulse2.y, 0.0f);
		}

		b2Vec2 P(impulse.x, impulse.y);

		cA -= mA * P;
		aA -= iA * (b2Cross(rA, P) + impulse.z);

		cB += mB * P;
		aB += iB * (b2Cross(rB, P) + impulse.z);
	}

	data.positions[m_indexA].c = cA;
	data.positions[m_indexA].a = aA;
	data.positions[m_indexB].c = cB;
	data.positions[m_indexB].a = aB;

	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(float inv_dt) const
{
	b2Vec2 P(m_impulse.x, m_impulse.y);
	return inv_dt * P;
}

float b2WeldJoint::GetReactionTorque(float inv_dt) const
{
	return inv_dt * m_impulse.z;
}

void b2WeldJoint::Dump()
{
	int32 indexA = m_bodyA->m_islandIndex;
	int32 indexB = m_bodyB->m_islandIndex;

	b2Dump("  b2WeldJointDef 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.referenceAngle = %.9g;\n", m_referenceAngle);
	b2Dump("  jd.stiffness = %.9g;\n", m_stiffness);
	b2Dump("  jd.damping = %.9g;\n", m_damping);
	b2Dump("  joints[%d] = m_world->CreateJoint(&jd);\n", m_index);
}