flightgear/src/FDM/YASim/RigidBody.cpp

#include "Math.hpp"
#include "RigidBody.hpp"
namespace yasim {

RigidBody::RigidBody()
{
    // Allocate space for 16 masses initially.  More space will be
    // allocated dynamically.
    _nMasses = 0;
    _massesAlloced = 16;
    _masses = new Mass[_massesAlloced];
    _gyro[0] = _gyro[1] = _gyro[2] = 0;
    _spin[0] = _spin[1] = _spin[2] = 0;
}

RigidBody::~RigidBody()
{
    delete[] _masses;
}

int RigidBody::addMass(float mass, float* pos)
{
    // If out of space, reallocate twice as much
    if(_nMasses == _massesAlloced) {
        _massesAlloced *= 2;
        Mass *m2 = new Mass[_massesAlloced];
        for(int i=0; i<_nMasses; i++)
            m2[i] = _masses[i];
        delete[] _masses;
        _masses = m2;
    }

    _masses[_nMasses].m = mass;
    Math::set3(pos, _masses[_nMasses].p);
    return _nMasses++;
}

void RigidBody::setMass(int handle, float mass)
{
    _masses[handle].m = mass;
}

void RigidBody::setMass(int handle, float mass, float* pos)
{
    _masses[handle].m = mass;
    Math::set3(pos, _masses[handle].p);
}

int RigidBody::numMasses()
{
    return _nMasses;
}

float RigidBody::getMass(int handle)
{
    return _masses[handle].m;
}

void RigidBody::getMassPosition(int handle, float* out)
{
    out[0] = _masses[handle].p[0];
    out[1] = _masses[handle].p[1];
    out[2] = _masses[handle].p[2];
}

float RigidBody::getTotalMass()
{
    return _totalMass;
}

// Calcualtes the rotational velocity of a particular point.  All
// coordinates are local!
void RigidBody::pointVelocity(float* pos, float* rot, float* out)
{
    Math::sub3(pos, _cg, out);   //  out = pos-cg
    Math::cross3(rot, out, out); //      = rot cross (pos-cg)
}

void RigidBody::setGyro(float* angularMomentum)
{
    Math::set3(angularMomentum, _gyro);
}

void RigidBody::recalc()
{
    // Calculate the c.g and total mass:
    _totalMass = 0;
    _cg[0] = _cg[1] = _cg[2] = 0;
    for(int i=0; i<_nMasses; i++) {
        float m = _masses[i].m;
        _totalMass += m;
        _cg[0] += m * _masses[i].p[0];
        _cg[1] += m * _masses[i].p[1];
        _cg[2] += m * _masses[i].p[2];
    }
    Math::mul3(1/_totalMass, _cg, _cg);

    // Now the inertia tensor:
    for(int i=0; i<9; i++)
	_I[i] = 0;

    for(int i=0; i<_nMasses; i++) {
	float m = _masses[i].m;

	float x = _masses[i].p[0] - _cg[0];
	float y = _masses[i].p[1] - _cg[1];
	float z = _masses[i].p[2] - _cg[2];

	float xy = m*x*y; float yz = m*y*z; float zx = m*z*x;
	float x2 = m*x*x; float y2 = m*y*y; float z2 = m*z*z;

	_I[0] += y2+z2;  _I[1] -=    xy;  _I[2] -=    zx;
	_I[3] -=    xy;  _I[4] += x2+z2;  _I[5] -=    yz;
	_I[6] -=    zx;	 _I[7] -=    yz;  _I[8] += x2+y2;
    }

    // And its inverse
    Math::invert33(_I, _invI);
}

void RigidBody::reset()
{
    _torque[0] = _torque[1] = _torque[2] = 0;
    _force[0] = _force[1] = _force[2] = 0;
}

void RigidBody::addForce(float* force)
{
    Math::add3(_force, force, _force);
}

void RigidBody::addTorque(float* torque)
{
    Math::add3(_torque, torque, _torque);
}

void RigidBody::addForce(float* pos, float* force)
{
    addForce(force);
    
    // For a force F at position X, the torque about the c.g C is:
    // torque = F cross (C - X)
    float v[3], t[3];
    Math::sub3(_cg, pos, v);
    Math::cross3(force, v, t);
    addTorque(t);
}

void RigidBody::setBodySpin(float* rotation)
{
    Math::set3(rotation, _spin);
}

void RigidBody::getCG(float* cgOut)
{
    Math::set3(_cg, cgOut);
}

void RigidBody::getAccel(float* accelOut)
{
    Math::mul3(1/_totalMass, _force, accelOut);
}

void RigidBody::getAccel(float* pos, float* accelOut)
{
    getAccel(accelOut);

    // Turn the "spin" vector into a normalized spin axis "a" and a
    // radians/sec scalar "rate".
    float a[3];
    float rate = Math::mag3(_spin);
    Math::set3(_spin, a);
    Math::mul3(1/rate, a, a);

    float v[3];
    Math::sub3(_cg, pos, v);             // v = cg - pos
    Math::mul3(Math::dot3(v, a), a, a);  // a = a * (v dot a)
    Math::add3(v, a, v);                 // v = v + a

    // Now v contains the vector from pos to the rotation axis.
    // Multiply by the square of the rotation rate to get the linear
    // acceleration.
    Math::mul3(rate*rate, v, v);
    Math::add3(v, accelOut, accelOut);
}

void RigidBody::getAngularAccel(float* accelOut)
{
    // Compute "tau" as the externally applied torque, plus the
    // counter-torque due to the internal gyro.
    float tau[3]; // torque
    Math::cross3(_gyro, _spin, tau);
    Math::add3(_torque, tau, tau);

    // Now work the equation of motion.  Use "v" as a notational
    // shorthand, as the value isn't an acceleration until the end.
    float *v = accelOut;
    Math::vmul33(_I, _spin, v);  // v = I*omega
    Math::cross3(_spin, v, v);   // v = omega X I*omega
    Math::add3(tau, v, v);       // v = tau + (omega X I*omega)
    Math::vmul33(_invI, v, v);   // v = invI*(tau + (omega X I*omega))
}

}; // namespace yasim