void btBox2dShape::calculateLocalInertia(btScalar mass, btVector3& inertia) const
{
//btScalar margin = btScalar(0.);
btVector3 halfExtents = getHalfExtentsWithMargin();
btScalar lx = btScalar(2.) * (halfExtents.x());
btScalar ly = btScalar(2.) * (halfExtents.y());
btScalar lz = btScalar(2.) * (halfExtents.z());
inertia.setValue(mass / (btScalar(12.0)) * (ly * ly + lz * lz),
mass / (btScalar(12.0)) * (lx * lx + lz * lz),
mass / (btScalar(12.0)) * (lx * lx + ly * ly));
}
void btCylinderShape::calculateLocalInertia(btScalar mass,btVector3& inertia) const
{
//approximation of box shape, todo: implement cylinder shape inertia before people notice ;-)
btVector3 halfExtents = getHalfExtentsWithMargin();
btScalar lx=btScalar(2.)*(halfExtents.x());
btScalar ly=btScalar(2.)*(halfExtents.y());
btScalar lz=btScalar(2.)*(halfExtents.z());
inertia.setValue(mass/(btScalar(12.0)) * (ly*ly + lz*lz),
mass/(btScalar(12.0)) * (lx*lx + lz*lz),
mass/(btScalar(12.0)) * (lx*lx + ly*ly));
}
void btCylinderShape::calculateLocalInertia(btScalar mass,btVector3& inertia) const
{
//Until Bullet 2.77 a box approximation was used, so uncomment this if you need backwards compatibility
//#define USE_BOX_INERTIA_APPROXIMATION 1
#ifndef USE_BOX_INERTIA_APPROXIMATION
/*
cylinder is defined as following:
*
* - principle axis aligned along y by default, radius in x, z-value not used
* - for btCylinderShapeX: principle axis aligned along x, radius in y direction, z-value not used
* - for btCylinderShapeZ: principle axis aligned along z, radius in x direction, y-value not used
*
*/
btScalar radius2; // square of cylinder radius
btScalar height2; // square of cylinder height
btVector3 halfExtents = getHalfExtentsWithMargin(); // get cylinder dimension
btScalar div12 = mass / 12.f;
btScalar div4 = mass / 4.f;
btScalar div2 = mass / 2.f;
int idxRadius, idxHeight;
switch (m_upAxis) // get indices of radius and height of cylinder
{
case 0: // cylinder is aligned along x
idxRadius = 1;
idxHeight = 0;
break;
case 2: // cylinder is aligned along z
idxRadius = 0;
idxHeight = 2;
break;
default: // cylinder is aligned along y
idxRadius = 0;
idxHeight = 1;
}
// calculate squares
radius2 = halfExtents[idxRadius] * halfExtents[idxRadius];
height2 = btScalar(4.) * halfExtents[idxHeight] * halfExtents[idxHeight];
// calculate tensor terms
btScalar t1 = div12 * height2 + div4 * radius2;
btScalar t2 = div2 * radius2;
switch (m_upAxis) // set diagonal elements of inertia tensor
{
case 0: // cylinder is aligned along x
inertia.setValue(t2,t1,t1);
break;
case 2: // cylinder is aligned along z
inertia.setValue(t1,t1,t2);
break;
default: // cylinder is aligned along y
inertia.setValue(t1,t2,t1);
}
#else //USE_BOX_INERTIA_APPROXIMATION
//approximation of box shape
btVector3 halfExtents = getHalfExtentsWithMargin();
btScalar lx=btScalar(2.)*(halfExtents.x());
btScalar ly=btScalar(2.)*(halfExtents.y());
btScalar lz=btScalar(2.)*(halfExtents.z());
inertia.setValue(mass/(btScalar(12.0)) * (ly*ly + lz*lz),
mass/(btScalar(12.0)) * (lx*lx + lz*lz),
mass/(btScalar(12.0)) * (lx*lx + ly*ly));
#endif //USE_BOX_INERTIA_APPROXIMATION
}
