crossmat method

Mat3 crossmat()

Get the cross product matrix a_cross from a vector, such that a x b = a_cross * b = c

See {@link https://www8.cs.umu.se/kurser/TDBD24/VT06/lectures/Lecture6.pdf Umeå University Lecture}

Implementation

Mat3 crossmat(){
  return Mat3([0, -z, y, z, 0, -x, -y, x, 0]);
}