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DynamicsB2JointMouse.go
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DynamicsB2JointMouse.go
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package box2d
import (
"fmt"
)
/// Mouse joint definition. This requires a world target point,
/// tuning parameters, and the time step.
type B2MouseJointDef struct {
B2JointDef
/// The initial world target point. This is assumed
/// to coincide with the body anchor initially.
Target B2Vec2
/// The maximum constraint force that can be exerted
/// to move the candidate body. Usually you will express
/// as some multiple of the weight (multiplier * mass * gravity).
MaxForce float64
/// The response speed.
FrequencyHz float64
/// The damping ratio. 0 = no damping, 1 = critical damping.
DampingRatio float64
}
func MakeB2MouseJointDef() B2MouseJointDef {
res := B2MouseJointDef{
B2JointDef: MakeB2JointDef(),
}
res.Type = B2JointType.E_mouseJoint
res.Target.Set(0.0, 0.0)
res.MaxForce = 0.0
res.FrequencyHz = 5.0
res.DampingRatio = 0.7
return res
}
/// A mouse joint is used to make a point on a body track a
/// specified world point. This a soft constraint with a maximum
/// force. This allows the constraint to stretch and without
/// applying huge forces.
/// NOTE: this joint is not documented in the manual because it was
/// developed to be used in the testbed. If you want to learn how to
/// use the mouse joint, look at the testbed.
type B2MouseJoint struct {
*B2Joint
M_localAnchorB B2Vec2
M_targetA B2Vec2
M_frequencyHz float64
M_dampingRatio float64
M_beta float64
// Solver shared
M_impulse B2Vec2
M_maxForce float64
M_gamma float64
// Solver temp
M_indexA int
M_indexB int
M_rB B2Vec2
M_localCenterB B2Vec2
M_invMassB float64
M_invIB float64
M_mass B2Mat22
M_C B2Vec2
}
/// The mouse joint does not support dumping.
func (def *B2MouseJoint) Dump() {
fmt.Printf("Mouse joint dumping is not supported.\n")
}
// p = attached point, m = mouse point
// C = p - m
// Cdot = v
// = v + cross(w, r)
// J = [I r_skew]
// Identity used:
// w k % (rx i + ry j) = w * (-ry i + rx j)
func MakeB2MouseJoint(def *B2MouseJointDef) *B2MouseJoint {
res := B2MouseJoint{
B2Joint: MakeB2Joint(def),
}
B2Assert(def.Target.IsValid())
B2Assert(B2IsValid(def.MaxForce) && def.MaxForce >= 0.0)
B2Assert(B2IsValid(def.FrequencyHz) && def.FrequencyHz >= 0.0)
B2Assert(B2IsValid(def.DampingRatio) && def.DampingRatio >= 0.0)
res.M_targetA = def.Target
res.M_localAnchorB = B2TransformVec2MulT(res.M_bodyB.GetTransform(), res.M_targetA)
res.M_maxForce = def.MaxForce
res.M_impulse.SetZero()
res.M_frequencyHz = def.FrequencyHz
res.M_dampingRatio = def.DampingRatio
res.M_beta = 0.0
res.M_gamma = 0.0
return &res
}
func (joint *B2MouseJoint) SetTarget(target B2Vec2) {
if target != joint.M_targetA {
joint.M_bodyB.SetAwake(true)
joint.M_targetA = target
}
}
func (joint B2MouseJoint) GetTarget() B2Vec2 {
return joint.M_targetA
}
func (joint *B2MouseJoint) SetMaxForce(force float64) {
joint.M_maxForce = force
}
func (joint B2MouseJoint) GetMaxForce() float64 {
return joint.M_maxForce
}
func (joint *B2MouseJoint) SetFrequency(hz float64) {
joint.M_frequencyHz = hz
}
func (joint B2MouseJoint) GetFrequency() float64 {
return joint.M_frequencyHz
}
func (joint *B2MouseJoint) SetDampingRatio(ratio float64) {
joint.M_dampingRatio = ratio
}
func (joint B2MouseJoint) GetDampingRatio() float64 {
return joint.M_dampingRatio
}
func (joint *B2MouseJoint) InitVelocityConstraints(data B2SolverData) {
joint.M_indexB = joint.M_bodyB.M_islandIndex
joint.M_localCenterB = joint.M_bodyB.M_sweep.LocalCenter
joint.M_invMassB = joint.M_bodyB.M_invMass
joint.M_invIB = joint.M_bodyB.M_invI
cB := data.Positions[joint.M_indexB].C
aB := data.Positions[joint.M_indexB].A
vB := data.Velocities[joint.M_indexB].V
wB := data.Velocities[joint.M_indexB].W
qB := MakeB2RotFromAngle(aB)
mass := joint.M_bodyB.GetMass()
// Frequency
omega := 2.0 * B2_pi * joint.M_frequencyHz
// Damping coefficient
d := 2.0 * mass * joint.M_dampingRatio * omega
// Spring stiffness
k := mass * (omega * omega)
// magic formulas
// gamma has units of inverse mass.
// beta has units of inverse time.
h := data.Step.Dt
B2Assert(d+h*k > B2_epsilon)
joint.M_gamma = h * (d + h*k)
if joint.M_gamma != 0.0 {
joint.M_gamma = 1.0 / joint.M_gamma
}
joint.M_beta = h * k * joint.M_gamma
// Compute the effective mass matrix.
joint.M_rB = B2RotVec2Mul(qB, B2Vec2Sub(joint.M_localAnchorB, joint.M_localCenterB))
// K = [(1/m1 + 1/m2) * eye(2) - skew(r1) * invI1 * skew(r1) - skew(r2) * invI2 * skew(r2)]
// = [1/m1+1/m2 0 ] + invI1 * [r1.y*r1.y -r1.x*r1.y] + invI2 * [r1.y*r1.y -r1.x*r1.y]
// [ 0 1/m1+1/m2] [-r1.x*r1.y r1.x*r1.x] [-r1.x*r1.y r1.x*r1.x]
var K B2Mat22
K.Ex.X = joint.M_invMassB + joint.M_invIB*joint.M_rB.Y*joint.M_rB.Y + joint.M_gamma
K.Ex.Y = -joint.M_invIB * joint.M_rB.X * joint.M_rB.Y
K.Ey.X = K.Ex.Y
K.Ey.Y = joint.M_invMassB + joint.M_invIB*joint.M_rB.X*joint.M_rB.X + joint.M_gamma
joint.M_mass = K.GetInverse()
joint.M_C = B2Vec2Sub(B2Vec2Add(cB, joint.M_rB), joint.M_targetA)
joint.M_C.OperatorScalarMulInplace(joint.M_beta)
// Cheat with some damping
wB *= 0.98
if data.Step.WarmStarting {
joint.M_impulse.OperatorScalarMulInplace(data.Step.DtRatio)
vB.OperatorPlusInplace(B2Vec2MulScalar(joint.M_invMassB, joint.M_impulse))
wB += joint.M_invIB * B2Vec2Cross(joint.M_rB, joint.M_impulse)
} else {
joint.M_impulse.SetZero()
}
data.Velocities[joint.M_indexB].V = vB
data.Velocities[joint.M_indexB].W = wB
}
func (joint *B2MouseJoint) SolveVelocityConstraints(data B2SolverData) {
vB := data.Velocities[joint.M_indexB].V
wB := data.Velocities[joint.M_indexB].W
// Cdot = v + cross(w, r)
Cdot := B2Vec2Add(vB, B2Vec2CrossScalarVector(wB, joint.M_rB))
impulse := B2Vec2Mat22Mul(joint.M_mass, (B2Vec2Add(B2Vec2Add(Cdot, joint.M_C), B2Vec2MulScalar(joint.M_gamma, joint.M_impulse))).OperatorNegate())
oldImpulse := joint.M_impulse
joint.M_impulse.OperatorPlusInplace(impulse)
maxImpulse := data.Step.Dt * joint.M_maxForce
if joint.M_impulse.LengthSquared() > maxImpulse*maxImpulse {
joint.M_impulse.OperatorScalarMulInplace(maxImpulse / joint.M_impulse.Length())
}
impulse = B2Vec2Sub(joint.M_impulse, oldImpulse)
vB.OperatorPlusInplace(B2Vec2MulScalar(joint.M_invMassB, impulse))
wB += joint.M_invIB * B2Vec2Cross(joint.M_rB, impulse)
data.Velocities[joint.M_indexB].V = vB
data.Velocities[joint.M_indexB].W = wB
}
func (joint *B2MouseJoint) SolvePositionConstraints(data B2SolverData) bool {
return true
}
func (joint B2MouseJoint) GetAnchorA() B2Vec2 {
return joint.M_targetA
}
func (joint B2MouseJoint) GetAnchorB() B2Vec2 {
return joint.M_bodyB.GetWorldPoint(joint.M_localAnchorB)
}
func (joint B2MouseJoint) GetReactionForce(inv_dt float64) B2Vec2 {
return B2Vec2MulScalar(inv_dt, joint.M_impulse)
}
func (joint B2MouseJoint) GetReactionTorque(inv_dt float64) float64 {
return inv_dt * 0.0
}
func (joint *B2MouseJoint) ShiftOrigin(newOrigin B2Vec2) {
joint.M_targetA.OperatorMinusInplace(newOrigin)
}