def phi_x(alpha)
      # If the ray is orthogonal on the x axis, the evaluation will yield NaN, and
      # we return the index i which corresponds to the source's x position instead.
      (((@p2.x - @p1.x == 0 ? @p1.x : px(alpha)) - bx) / @vs.delta_x).floor
    end
def phi_x(alpha)
      # If the ray is orthogonal on the x axis, the evaluation will yield NaN, and
      # we return the index i which corresponds to the source's x position instead.
      (((@p2.x - @p1.x == 0 ? @p1.x : px(alpha)) - bx) / @vs.delta_x).floor
    end
Computes a hash code for this object.

 @note Two objects with the same attributes will have the same hash code.

 @return [Fixnum] the object's hash code

 Calculates a voxel plane x index (i) from an alpha value.

 @param [Float] alpha a fraction of the ray's movement between source and target
 @return [Integer] the calculated voxel plane index (i)