A simple method for obtaining the convex hull of a shape, which genera
tes an octagonal approximation, is based on repeatedly filling the cen
tral pixel in all 3 x 3 neighborhoods which show a concavity. We propo
se an extension of this technique that uses larger windows and yields
improved results. The performance of our method is demonstrated both o
n specially generated objects (simulations) as well as on shapes obtai
ned from ultrasound images of ovarian cysts. (C) 1997 Pattern Recognit
ion Society. Published by Elsevier Science Ltd.