public ContactSolver(TimeStep step, Contact[] contacts, int contactCount)
{
Step = step;
ConstraintCount = contactCount;
Constraints = new ContactConstraint[ConstraintCount];
for (int i = 0; i < ConstraintCount; ++i)
{
Contact contact = contacts[i];
Fixture fixtureA = contact.GetFixtureA();
Fixture fixtureB = contact.GetFixtureB();
Shape shapeA = fixtureA.GetShape();
Shape shapeB = fixtureB.GetShape();
float radiusA = shapeA._radius;
float radiusB = shapeB._radius;
Body bodyA = fixtureA.GetBody();
Body bodyB = fixtureB.GetBody();
Manifold manifold = contact.GetManifold();
float friction = Settings.MixFriction(fixtureA.GetFriction(), fixtureB.GetFriction());
float restitution = Settings.MixRestitution(fixtureA.GetRestitution(), fixtureB.GetRestitution());
Vec2 vA = bodyA._linearVelocity;
Vec2 vB = bodyB._linearVelocity;
float wA = bodyA._angularVelocity;
float wB = bodyB._angularVelocity;
Box2DXDebug.Assert(manifold.PointCount > 0);
WorldManifold worldManifold = new WorldManifold();
worldManifold.Initialize(manifold, bodyA.GetTransform(), radiusA, bodyB.GetTransform(), radiusB);
ContactConstraint cc = new ContactConstraint();
Constraints[i] = cc;
cc.BodyA = bodyA;
cc.BodyB = bodyB;
cc.Manifold = manifold;
cc.Normal = worldManifold.Normal;
cc.PointCount = manifold.PointCount;
cc.Friction = friction;
cc.Restitution = restitution;
cc.LocalPlaneNormal = manifold.LocalPlaneNormal;
cc.LocalPoint = manifold.LocalPoint;
cc.Radius = radiusA + radiusB;
cc.Type = manifold.Type;
for (int j = 0; j < cc.PointCount; ++j)
{
ManifoldPoint cp = manifold.Points[j];
ContactConstraintPoint ccp = cc.Points[j];
ccp.NormalImpulse = cp.NormalImpulse;
ccp.TangentImpulse = cp.TangentImpulse;
ccp.LocalPoint = cp.LocalPoint;
ccp.RA = worldManifold.Points[j] - bodyA._sweep.C;
ccp.RB = worldManifold.Points[j] - bodyB._sweep.C;
float rnA = Vec2.Cross(ccp.RA, cc.Normal);
float rnB = Vec2.Cross(ccp.RB, cc.Normal);
rnA *= rnA;
rnB *= rnB;
float kNormal = bodyA._invMass + bodyB._invMass + bodyA._invI * rnA + bodyB._invI * rnB;
Box2DXDebug.Assert(kNormal > Settings.FLT_EPSILON);
ccp.NormalMass = 1.0f / kNormal;
float kEqualized = bodyA._mass * bodyA._invMass + bodyB._mass * bodyB._invMass;
kEqualized += bodyA._mass * bodyA._invI * rnA + bodyB._mass * bodyB._invI * rnB;
Box2DXDebug.Assert(kEqualized > Settings.FLT_EPSILON);
ccp.EqualizedMass = 1.0f / kEqualized;
Vec2 tangent = Vec2.Cross(cc.Normal, 1.0f);
float rtA = Vec2.Cross(ccp.RA, tangent);
float rtB = Vec2.Cross(ccp.RB, tangent);
rtA *= rtA;
rtB *= rtB;
float kTangent = bodyA._invMass + bodyB._invMass + bodyA._invI * rtA + bodyB._invI * rtB;
Box2DXDebug.Assert(kTangent > Settings.FLT_EPSILON);
ccp.TangentMass = 1.0f / kTangent;
// Setup a velocity bias for restitution.
ccp.VelocityBias = 0.0f;
float vRel = Vec2.Dot(cc.Normal, vB + Vec2.Cross(wB, ccp.RB) - vA - Vec2.Cross(wA, ccp.RA));
if (vRel < -Settings.VelocityThreshold)
{
ccp.VelocityBias = -cc.Restitution * vRel;
}
}
// If we have two points, then prepare the block solver.
if (cc.PointCount == 2)
{
ContactConstraintPoint ccp1 = cc.Points[0];
ContactConstraintPoint ccp2 = cc.Points[1];
float invMassA = bodyA._invMass;
float invIA = bodyA._invI;
float invMassB = bodyB._invMass;
float invIB = bodyB._invI;
float rn1A = Vec2.Cross(ccp1.RA, cc.Normal);
float rn1B = Vec2.Cross(ccp1.RB, cc.Normal);
float rn2A = Vec2.Cross(ccp2.RA, cc.Normal);
float rn2B = Vec2.Cross(ccp2.RB, cc.Normal);
float k11 = invMassA + invMassB + invIA * rn1A * rn1A + invIB * rn1B * rn1B;
float k22 = invMassA + invMassB + invIA * rn2A * rn2A + invIB * rn2B * rn2B;
float k12 = invMassA + invMassB + invIA * rn1A * rn2A + invIB * rn1B * rn2B;
// Ensure a reasonable condition number.
const float k_maxConditionNumber = 100.0f;
if (k11 * k11 < k_maxConditionNumber * (k11 * k22 - k12 * k12))
{
// K is safe to invert.
cc.K.Col1.Set(k11, k12);
cc.K.Col2.Set(k12, k22);
cc.NormalMass = cc.K.Invert();
}
else
{
// The constraints are redundant, just use one.
// TODO_ERIN use deepest?
cc.PointCount = 1;
}
}
}
}
