Hello Zhu
Actually, all of the shape metrics in Fragstats describe the irregularity of the patch shape, in most cases relative to a standard of a simple circular or square shape. The differences among the shape metrics are in the particular perspective on shape and the mathematical algorithm, but most of likely to be highly empirically correlated in real landscapes, so the choice may be more of one of familiarity or ease of describing and interpreting the results. Good luck.
Kevin
