public void Execute(int i)
{
int k = 0;
float maximumMass = 10000;
coms[i] = float4.zero;
float4x4 Apq = float4x4.zero, Rpq = float4x4.zero;
// calculate shape mass, center of mass, and moment matrix:
for (int j = 0; j < numIndices[i]; ++j)
{
k = particleIndices[firstIndex[i] + j];
float mass = maximumMass;
if (invMasses[k] > 1.0f / maximumMass)
{
mass = 1.0f / invMasses[k];
}
coms[i] += positions[k] * mass;
float4x4 particleR = orientations[k].toMatrix();
float4x4 particleRT = restOrientations[k].toMatrix();
particleR[3][3] = 0;
particleRT[3][3] = 0;
Rpq += math.mul(particleR,
math.mul(math.rcp(invInertiaTensors[k] + new float4(BurstMath.epsilon)).asDiagonal(),
math.transpose(particleRT))
);
float4 restPosition = restPositions[k];
restPosition[3] = 0;
Apq += mass * BurstMath.multrnsp4(positions[k], restPosition);
}
if (restComs[i][3] < BurstMath.epsilon)
{
return;
}
coms[i] /= restComs[i][3];
// subtract global shape moment:
float4 restCom = restComs[i];
restCom[3] = 0;
Apq -= restComs[i][3] * BurstMath.multrnsp4(coms[i], restCom);
// calculate optimal transform including plastic deformation:
float4x4 Apq_def = Rpq + math.mul(Apq, math.transpose(deformation[i]));
// extract rotation from transform matrix, using warmstarting and few iterations:
constraintOrientations[i] = BurstMath.ExtractRotation(Apq_def, constraintOrientations[i], 2);
// finally, obtain rotation matrix:
float4x4 R = constraintOrientations[i].toMatrix();
R[3][3] = 0;
// calculate particle orientations:
if (explicitGroup[i] > 0)
{
// if the group is explicit, set the orientation for all particles:
for (int j = 0; j < numIndices[i]; ++j)
{
k = particleIndices[firstIndex[i] + j];
orientations[k] = math.mul(constraintOrientations[i], restOrientations[k]);
}
}
else
{
// set orientation of center particle only:
int centerIndex = particleIndices[firstIndex[i]];
orientations[centerIndex] = math.mul(constraintOrientations[i], restOrientations[centerIndex]);
}
// calculate and accumulate particle goal positions:
float4 goal;
float4x4 transform = math.mul(R, deformation[i]);
for (int j = 0; j < numIndices[i]; ++j)
{
k = particleIndices[firstIndex[i] + j];
goal = coms[i] + math.mul(transform, restPositions[k] - restComs[i]);
deltas[k] += (goal - positions[k]) * shapeMaterialParameters[i * 5];
counts[k]++;
}
// update plastic deformation:
float plastic_yield = shapeMaterialParameters[i * 5 + 1];
float plastic_creep = shapeMaterialParameters[i * 5 + 2];
float plastic_recovery = shapeMaterialParameters[i * 5 + 3];
float max_deform = shapeMaterialParameters[i * 5 + 4];
// if we are allowed to absorb deformation:
if (plastic_creep > 0)
{
R[3][3] = 1;
Apq_def[3][3] = 1;
// get scale matrix (A = RS so S = Rt * A) and its deviation from the identity matrix:
float4x4 deform_matrix = math.mul(math.transpose(R), math.mul(Apq_def, Aqq[i])) - float4x4.identity;
// if the amount of deformation exceeds the yield threshold:
float norm = deform_matrix.frobeniusNorm();
if (norm > plastic_yield)
{
// deform the shape permanently:
deformation[i] = math.mul(float4x4.identity + plastic_creep * deform_matrix, deformation[i]);
// clamp deformation so that it does not exceed a percentage;
deform_matrix = deformation[i] - float4x4.identity;
norm = deform_matrix.frobeniusNorm();
if (norm > max_deform)
{
deformation[i] = float4x4.identity + max_deform * deform_matrix / norm;
}
// if we cannot recover from plastic deformation, recalculate rest shape now:
if (plastic_recovery == 0)
{
RecalculateRestData(i,
ref particleIndices,
ref firstIndex,
ref restComs,
ref Aqq,
ref deformation,
ref numIndices,
ref invMasses,
ref restPositions,
ref restOrientations,
ref invInertiaTensors);
}
}
}
// if we can recover from plastic deformation, lerp towards non-deformed shape and recalculate rest shape:
if (plastic_recovery > 0)
{
deformation[i] += (float4x4.identity - deformation[i]) * math.min(plastic_recovery * deltaTime, 1.0f);
RecalculateRestData(i,
ref particleIndices,
ref firstIndex,
ref restComs,
ref Aqq,
ref deformation,
ref numIndices,
ref invMasses,
ref restPositions,
ref restOrientations,
ref invInertiaTensors);
}
}