code: private float distance(int i, float x, float y, float z) {
          return frustum[i][0] * x + frustum[i][1] * y + frustum[i][2] * z + frustum[i][3];
      }
docs: Compute the distance between a point and the given plane.
  
  @param i The id of the plane
  @param x The x coordinate of the point
  @param y The y coordinate of the point
  @param z The z coordinate of the point
  @return The distance