internal static void calculateDiffAxisAngleQuaternion( ref btQuaternion orn0, ref btQuaternion orn1a, out btVector3 axis, out double angle )
{
btQuaternion orn1;
orn0.nearest( ref orn1a, out orn1 );
btQuaternion dorn;
btQuaternion tmp;
orn0.inverse( out tmp );
orn1.Mult( ref tmp, out dorn );
angle = dorn.getAngle();
axis.x = dorn.x; axis.y = dorn.y; axis.z = dorn.z; axis.w = 0;
//check for axis length
double len = axis.length2();
if( len < btScalar.SIMD_EPSILON * btScalar.SIMD_EPSILON )
axis = btVector3.xAxis;
else
axis.Div( btScalar.btSqrt( len ), out axis );
}