def phi_y(alpha)
      # If the ray is orthogonal on the y axis, the evaluation will yield NaN, and
      # we return the index j which corresponds to the source's y position instead.
      (((@p2.y - @p1.y == 0 ? @p1.y : py(alpha)) - by) / @vs.delta_y).floor
    end
def phi_y(alpha)
      # If the ray is orthogonal on the y axis, the evaluation will yield NaN, and
      # we return the index j which corresponds to the source's y position instead.
      (((@p2.y - @p1.y == 0 ? @p1.y : py(alpha)) - by) / @vs.delta_y).floor
    end
Calculates a voxel plane y index (j) 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 (j)