/
DynamicsB2JointFriction.go
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/
DynamicsB2JointFriction.go
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package box2d
import (
"fmt"
)
/// Friction joint definition.
type B2FrictionJointDef struct {
B2JointDef
/// The local anchor point relative to bodyA's origin.
LocalAnchorA B2Vec2
/// The local anchor point relative to bodyB's origin.
LocalAnchorB B2Vec2
/// The maximum friction force in N.
MaxForce float64
/// The maximum friction torque in N-m.
MaxTorque float64
}
func MakeB2FrictionJointDef() B2FrictionJointDef {
res := B2FrictionJointDef{
B2JointDef: MakeB2JointDef(),
}
res.Type = B2JointType.E_frictionJoint
res.LocalAnchorA.SetZero()
res.LocalAnchorB.SetZero()
res.MaxForce = 0.0
res.MaxTorque = 0.0
return res
}
/// Friction joint. This is used for top-down friction.
/// It provides 2D translational friction and angular friction.
type B2FrictionJoint struct {
*B2Joint
M_localAnchorA B2Vec2
M_localAnchorB B2Vec2
// Solver shared
M_linearImpulse B2Vec2
M_angularImpulse float64
M_maxForce float64
M_maxTorque float64
// Solver temp
M_indexA int
M_indexB int
M_rA B2Vec2
M_rB B2Vec2
M_localCenterA B2Vec2
M_localCenterB B2Vec2
M_invMassA float64
M_invMassB float64
M_invIA float64
M_invIB float64
M_linearMass B2Mat22
M_angularMass float64
}
/// The local anchor point relative to bodyA's origin.
func (joint B2FrictionJoint) GetLocalAnchorA() B2Vec2 {
return joint.M_localAnchorA
}
/// The local anchor point relative to bodyB's origin.
func (joint B2FrictionJoint) GetLocalAnchorB() B2Vec2 {
return joint.M_localAnchorB
}
// 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
func (joint *B2FrictionJointDef) Initialize(bA *B2Body, bB *B2Body, anchor B2Vec2) {
joint.BodyA = bA
joint.BodyB = bB
joint.LocalAnchorA = joint.BodyA.GetLocalPoint(anchor)
joint.LocalAnchorB = joint.BodyB.GetLocalPoint(anchor)
}
func MakeB2FrictionJoint(def *B2FrictionJointDef) *B2FrictionJoint {
res := B2FrictionJoint{
B2Joint: MakeB2Joint(def),
}
res.M_localAnchorA = def.LocalAnchorA
res.M_localAnchorB = def.LocalAnchorB
res.M_linearImpulse.SetZero()
res.M_angularImpulse = 0.0
res.M_maxForce = def.MaxForce
res.M_maxTorque = def.MaxTorque
return &res
}
func (joint *B2FrictionJoint) InitVelocityConstraints(data B2SolverData) {
joint.M_indexA = joint.M_bodyA.M_islandIndex
joint.M_indexB = joint.M_bodyB.M_islandIndex
joint.M_localCenterA = joint.M_bodyA.M_sweep.LocalCenter
joint.M_localCenterB = joint.M_bodyB.M_sweep.LocalCenter
joint.M_invMassA = joint.M_bodyA.M_invMass
joint.M_invMassB = joint.M_bodyB.M_invMass
joint.M_invIA = joint.M_bodyA.M_invI
joint.M_invIB = joint.M_bodyB.M_invI
aA := data.Positions[joint.M_indexA].A
vA := data.Velocities[joint.M_indexA].V
wA := data.Velocities[joint.M_indexA].W
aB := data.Positions[joint.M_indexB].A
vB := data.Velocities[joint.M_indexB].V
wB := data.Velocities[joint.M_indexB].W
qA := MakeB2RotFromAngle(aA)
qB := MakeB2RotFromAngle(aB)
// Compute the effective mass matrix.
joint.M_rA = B2RotVec2Mul(qA, B2Vec2Sub(joint.M_localAnchorA, joint.M_localCenterA))
joint.M_rB = B2RotVec2Mul(qB, B2Vec2Sub(joint.M_localAnchorB, joint.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]
mA := joint.M_invMassA
mB := joint.M_invMassB
iA := joint.M_invIA
iB := joint.M_invIB
var K B2Mat22
K.Ex.X = mA + mB + iA*joint.M_rA.Y*joint.M_rA.Y + iB*joint.M_rB.Y*joint.M_rB.Y
K.Ex.Y = -iA*joint.M_rA.X*joint.M_rA.Y - iB*joint.M_rB.X*joint.M_rB.Y
K.Ey.X = K.Ex.Y
K.Ey.Y = mA + mB + iA*joint.M_rA.X*joint.M_rA.X + iB*joint.M_rB.X*joint.M_rB.X
joint.M_linearMass = K.GetInverse()
joint.M_angularMass = iA + iB
if joint.M_angularMass > 0.0 {
joint.M_angularMass = 1.0 / joint.M_angularMass
}
if data.Step.WarmStarting {
// Scale impulses to support a variable time step.
joint.M_linearImpulse.OperatorScalarMulInplace(data.Step.DtRatio)
joint.M_angularImpulse *= data.Step.DtRatio
P := MakeB2Vec2(joint.M_linearImpulse.X, joint.M_linearImpulse.Y)
vA.OperatorMinusInplace(B2Vec2MulScalar(mA, P))
wA -= iA * (B2Vec2Cross(joint.M_rA, P) + joint.M_angularImpulse)
vB.OperatorPlusInplace(B2Vec2MulScalar(mB, P))
wB += iB * (B2Vec2Cross(joint.M_rB, P) + joint.M_angularImpulse)
} else {
joint.M_linearImpulse.SetZero()
joint.M_angularImpulse = 0.0
}
data.Velocities[joint.M_indexA].V = vA
data.Velocities[joint.M_indexA].W = wA
data.Velocities[joint.M_indexB].V = vB
data.Velocities[joint.M_indexB].W = wB
}
func (joint *B2FrictionJoint) SolveVelocityConstraints(data B2SolverData) {
vA := data.Velocities[joint.M_indexA].V
wA := data.Velocities[joint.M_indexA].W
vB := data.Velocities[joint.M_indexB].V
wB := data.Velocities[joint.M_indexB].W
mA := joint.M_invMassA
mB := joint.M_invMassB
iA := joint.M_invIA
iB := joint.M_invIB
h := data.Step.Dt
// Solve angular friction
{
Cdot := wB - wA
impulse := -joint.M_angularMass * Cdot
oldImpulse := joint.M_angularImpulse
maxImpulse := h * joint.M_maxTorque
joint.M_angularImpulse = B2FloatClamp(joint.M_angularImpulse+impulse, -maxImpulse, maxImpulse)
impulse = joint.M_angularImpulse - oldImpulse
wA -= iA * impulse
wB += iB * impulse
}
// Solve linear friction
{
Cdot := B2Vec2Sub(B2Vec2Sub(B2Vec2Add(vB, B2Vec2CrossScalarVector(wB, joint.M_rB)), vA), B2Vec2CrossScalarVector(wA, joint.M_rA))
impulse := B2Vec2Mat22Mul(joint.M_linearMass, Cdot).OperatorNegate()
oldImpulse := joint.M_linearImpulse
joint.M_linearImpulse.OperatorPlusInplace(impulse)
maxImpulse := h * joint.M_maxForce
if joint.M_linearImpulse.LengthSquared() > maxImpulse*maxImpulse {
joint.M_linearImpulse.Normalize()
joint.M_linearImpulse.OperatorScalarMulInplace(maxImpulse)
}
impulse = B2Vec2Sub(joint.M_linearImpulse, oldImpulse)
vA.OperatorMinusInplace(B2Vec2MulScalar(mA, impulse))
wA -= iA * B2Vec2Cross(joint.M_rA, impulse)
vB.OperatorPlusInplace(B2Vec2MulScalar(mB, impulse))
wB += iB * B2Vec2Cross(joint.M_rB, impulse)
}
data.Velocities[joint.M_indexA].V = vA
data.Velocities[joint.M_indexA].W = wA
data.Velocities[joint.M_indexB].V = vB
data.Velocities[joint.M_indexB].W = wB
}
func (joint *B2FrictionJoint) SolvePositionConstraints(data B2SolverData) bool {
return true
}
func (joint B2FrictionJoint) GetAnchorA() B2Vec2 {
return joint.M_bodyA.GetWorldPoint(joint.M_localAnchorA)
}
func (joint B2FrictionJoint) GetAnchorB() B2Vec2 {
return joint.M_bodyB.GetWorldPoint(joint.M_localAnchorB)
}
func (joint B2FrictionJoint) GetReactionForce(inv_dt float64) B2Vec2 {
return B2Vec2MulScalar(inv_dt, joint.M_linearImpulse)
}
func (joint B2FrictionJoint) GetReactionTorque(inv_dt float64) float64 {
return inv_dt * joint.M_angularImpulse
}
func (joint *B2FrictionJoint) SetMaxForce(force float64) {
B2Assert(B2IsValid(force) && force >= 0.0)
joint.M_maxForce = force
}
func (joint B2FrictionJoint) GetMaxForce() float64 {
return joint.M_maxForce
}
func (joint *B2FrictionJoint) SetMaxTorque(torque float64) {
B2Assert(B2IsValid(torque) && torque >= 0.0)
joint.M_maxTorque = torque
}
func (joint B2FrictionJoint) GetMaxTorque() float64 {
return joint.M_maxTorque
}
func (joint *B2FrictionJoint) Dump() {
indexA := joint.M_bodyA.M_islandIndex
indexB := joint.M_bodyB.M_islandIndex
fmt.Printf(" b2FrictionJointDef jd;\n")
fmt.Printf(" jd.bodyA = bodies[%d];\n", indexA)
fmt.Printf(" jd.bodyB = bodies[%d];\n", indexB)
fmt.Printf(" jd.collideConnected = bool(%v);\n", joint.M_collideConnected)
fmt.Printf(" jd.localAnchorA.Set(%.15f, %.15f);\n", joint.M_localAnchorA.X, joint.M_localAnchorA.Y)
fmt.Printf(" jd.localAnchorB.Set(%.15f, %.15f);\n", joint.M_localAnchorB.X, joint.M_localAnchorB.Y)
fmt.Printf(" jd.maxForce = %.15f;\n", joint.M_maxForce)
fmt.Printf(" jd.maxTorque = %.15f;\n", joint.M_maxTorque)
fmt.Printf(" joints[%d] = m_world.CreateJoint(&jd);\n", joint.M_index)
}