The notion of watershed, used in morphological segmentation, has only
a digital definition. In this paper, we propose to extend this definit
ion to the continuous plane. Using this continuous definition, we pres
ent the watershed differences with classical edge detectors. We then e
xhibit a metric in the plane for which the watershed is a skeleton by
influence zones and show the lower semicontinuous behaviour of the ass
ociated skeleton. This theoretical approach suggests an algorithm for
solving the eikonal equation: \\del f\\ = g. Finally, we end with some
new watershed algorithms, which present the advantage of allowing the
use of markers and/or anchor points, thus opening the way towards gre
y-tone skeletons.