I played around with the perimeter from skimage.measure.

In the documentation, the neighborhood is described as:

"neighbourhood : 4 or 8, optional
Neighborhood connectivity for border pixel determination."

There is an imprecision because this is different from the connectivity of the resulting contour. If N4 is chosen, then the contour is 8-connected, and conversely. The documentation could have this precision.

yann

-- 
Yann GAVET
Assistant Professor - Ecole Nationale Supérieure des Mines de Saint-Etienne
158 Cours Fauriel, CS 62362, 42023 SAINT-ETIENNE cedex 2
Tel: (33) - 4 7742 0170