Search code examples
c++bit-manipulationbitwise-and

C++ Points of Vertices in Cuboid (Bitwise AND)

I'm trying to calculate the points in a cuboid given its centre (which is a Vector3) and the lengths of the sides along the x, y and z axis. I found the following on math.stackexchange.com: https://math.stackexchange.com/questions/107778/simplest-equation-for-drawing-a-cube-based-on-its-center-and-or-other-vertices which says I can use the following formulae:

https://math.stackexchange.com/questions/107778/simplest-equation-for-drawing-a-cube-based-on-its-center-and-or-other-vertices The constructor for the World class is:

World::World(Vector3 o, float d1, float d2, float d3) : origin(o)
{
  // If we consider an edge length to be d, we need to find r such that
  // 2r = d in order to calculate the positions of each vertex in the world.
  float r1 = d1 / 2,
      r2 = d2 / 2,
      r3 = d3 / 2;

  for (int i = 0; i < 8; i++)
  {
    /* Sets up the vertices of the cube.
     *
     * @see http://bit.ly/1cc2RPG
     */
    float x = o.getX() + (std::pow(-1, i&1) * r1),
          y = o.getY() + (std::pow(-1, i&2) * r2),
          z = o.getZ() + (std::pow(-1, i&4) * r3);

    points[i] = Vector3(x, y, z);
    std::cout << points[i] << "\n";
  }
}

And I passing the following parameters to the constructor:

Vector3 o(0, 0, 0);
World w(o, 100.f, 100.f, 100.f);

The coordinates being output for all 8 vertices are:

(50, 50, 50)
(-50, 50, 50)
(50, 50, 50)
(-50, 50, 50)
(50, 50, 50)
(-50, 50, 50)
(50, 50, 50)
(-50, 50, 50)

Which cannot be correct. Any guidance would be very much appreciated!

Solution

  • The problem lies in the bitwise & inside your pow calls: In the y and z components, they always return 0 and 2 or 4, respectively. -1^2 = -1^4 = 1, which is why the sign of these components is always positive. You could try (i&2)!=0 or (i&2) >> 1 for the y component instead. The same goes for the z component.