[GCM]

I have a question regarding to Hirt's VOF method. In Hirt's original JCP paper, the slop of the interface has been explicitly computed based on F function. So the interface is not composed of vertical or horizontal straight lines. however, when I read another paper by Ruman, in his paper, he pointed out that Hirt's method reconstructed the interface by the straight lines parallel to the co-ordinate axes and this method has been improved by PLIC method later. Could somebody give me some explanation for this difference between these two papers and what the Hirt's method should like.
Thanks a lot