func (arb *Arbiter) preStep(inv_dt float64) {
const allowedPenetration = 0.01
biasFactor := 0.0
if Settings.PositionCorrection {
biasFactor = 0.2
}
b1 := arb.ShapeA.Body
b2 := arb.ShapeB.Body
for i := 0; i < arb.NumContacts; i++ {
c := &arb.Contacts[i]
c.R1 = vect.Sub(c.Position, b1.Transform.Position)
c.R2 = vect.Sub(c.Position, b2.Transform.Position)
r1 := c.R1
r2 := c.R2
//Precompute normal mass, tangent mass, and bias
rn1 := vect.Dot(r1, c.Normal)
rn2 := vect.Dot(r2, c.Normal)
kNormal := b1.invMass + b2.invMass
kNormal += b1.invI*(vect.Dot(r1, r1)-rn1*rn1) +
b2.invI*(vect.Dot(r2, r2)-rn2*rn2)
c.MassNormal = 1.0 / kNormal
tangent := vect.CrossVF(c.Normal, 1.0)
rt1 := vect.Dot(r1, tangent)
rt2 := vect.Dot(r2, tangent)
kTangent := b1.invMass + b2.invMass
kTangent += b1.invI*(vect.Dot(r1, r1)-rt1*rt1) +
b2.invI*(vect.Dot(r2, r2)-rt2*rt2)
c.MassTangent = 1.0 / kTangent
c.Bias = -biasFactor * inv_dt * math.Min(0.0, c.Separation+allowedPenetration)
if Settings.AccumulateImpulses {
//Apply normal + friction impulse
P := vect.Add(vect.Mult(c.Normal, c.Pn), vect.Mult(tangent, c.Pt))
b1.Velocity.Sub(vect.Mult(P, b1.invMass))
b1.AngularVelocity -= b1.invI * vect.Cross(r1, P)
b2.Velocity.Add(vect.Mult(P, b2.invMass))
b2.AngularVelocity += b2.invI * vect.Cross(r2, P)
}
}
}
func (arb *Arbiter) applyImpulse() {
sA := arb.ShapeA
sB := arb.ShapeB
b1 := sA.Body
b2 := sB.Body
//xfA := b1.Transform
//xfB := b2.Transform
for i := 0; i < arb.NumContacts; i++ {
c := &arb.Contacts[i]
// Relative velocity at contact
dv := vect.Vect{}
{
t1 := vect.Add(b2.Velocity, vect.CrossFV(b2.AngularVelocity, c.R2))
t2 := vect.Sub(b1.Velocity, vect.CrossFV(b1.AngularVelocity, c.R1))
dv = vect.Sub(t1, t2)
}
// Compute normal impulse
vn := vect.Dot(dv, c.Normal)
dPn := c.MassNormal * (-vn + c.Bias)
if Settings.AccumulateImpulses {
// Clamp the accumulated impulse
Pn0 := c.Pn
c.Pn = math.Max(Pn0+dPn, 0.0)
dPn = c.Pn - Pn0
} else {
dPn = math.Max(dPn, 0.0)
}
//Apply contact impulse
Pn := vect.Mult(c.Normal, dPn)
b1.Velocity.Sub(vect.Mult(Pn, b1.invMass))
b1.AngularVelocity -= b1.invI * vect.Cross(c.R1, Pn)
b2.Velocity.Add(vect.Mult(Pn, b2.invMass))
b2.AngularVelocity += b2.invI * vect.Cross(c.R2, Pn)
//Relative velocity at contact
{
t1 := vect.Add(b2.Velocity, vect.CrossFV(b2.AngularVelocity, c.R2))
t2 := vect.Sub(b1.Velocity, vect.CrossFV(b1.AngularVelocity, c.R1))
dv = vect.Sub(t1, t2)
}
tangent := vect.CrossVF(c.Normal, 1.0)
vt := vect.Dot(dv, tangent)
dPt := c.MassTangent * (-vt)
if Settings.AccumulateImpulses {
//Compute friction impulse
maxPt := arb.Friction * c.Pn
//Clamp Friction
oldTangentImpulse := c.Pt
c.Pt = clamp(oldTangentImpulse+dPt, -maxPt, maxPt)
dPt = c.Pt - oldTangentImpulse
} else {
maxPt := arb.Friction * dPn
dPt = clamp(dPt, -maxPt, maxPt)
}
// Apply contact impulse
Pt := vect.Mult(tangent, dPt)
b1.Velocity.Sub(vect.Mult(Pt, b1.invMass))
b1.AngularVelocity -= b1.invI * vect.Cross(c.R1, Pt)
b2.Velocity.Add(vect.Mult(Pt, b2.invMass))
b2.AngularVelocity += b2.invI * vect.Cross(c.R2, Pt)
}
}