internal override void InitVelocityConstraints(ref TimeStep step)
{
Body b = _bodyB;
float mass = b.GetMass();
// Frequency
float omega = 2.0f * Settings.b2_pi * _frequencyHz;
// Damping coefficient
float d = 2.0f * mass * _dampingRatio * omega;
// Spring stiffness
float k = mass * (omega * omega);
// magic formulas
// gamma has units of inverse mass.
// beta has units of inverse time.
//Debug.Assert(d + step.dt * k > Settings.b2_epsilon);
_gamma = step.dt * (d + step.dt * k);
if (_gamma != 0.0f)
{
_gamma = 1.0f / _gamma;
}
_beta = step.dt * k * _gamma;
// Compute the effective mass matrix.
Transform xf1;
b.GetTransform(out xf1);
Vector2 r = MathUtils.Multiply(ref xf1.R, _localAnchor - b.GetLocalCenter());
// 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]
float invMass = b._invMass;
float invI = b._invI;
Mat22 K1 = new Mat22(new Vector2(invMass, 0.0f), new Vector2(0.0f, invMass));
Mat22 K2 = new Mat22(new Vector2(invI * r.y * r.y, -invI * r.x * r.y), new Vector2(-invI * r.x * r.y, invI * r.x * r.x));
Mat22 K;
Mat22.Add(ref K1, ref K2, out K);
K.col1.x += _gamma;
K.col2.y += _gamma;
_mass = K.GetInverse();
_C = b._sweep.c + r - _target;
// Cheat with some damping
b._angularVelocity *= 0.98f;
// Warm starting.
_impulse *= step.dtRatio;
b._linearVelocity += invMass * _impulse;
b._angularVelocity += invI * MathUtils.Cross(r, _impulse);
}
