Blobs.perimeter_hull

Blobs.perimeter_hull(**kwargs: Any) → Any

Convex hull of blob’s perimeter

Parameters:

clockwise (bool) – direction of travel for computing the hull, defaults to clockwise

Returns:

Perimeter, one point per column

Return type:

ndarray(2,N) or list of ndarray(2,N)

The result is a convex perimeter that minimally contains the blob.

Example:

>>> from machinevisiontoolbox import Image
>>> im = Image.Read('shark2.png')
>>> blobs = im.blobs()
>>> blobs[0].perimeter.shape
(2, 471)
>>> blobs[0].perimeter_hull(5).shape
(2, 21)
>>> np.set_printoptions(threshold=10)
>>> blobs[0].perimeter_hull()
array([[367, 361, 358, ..., 444, 442, 370],
       [300, 303, 305, ..., 369, 304, 300]], shape=(2, 21), dtype=int32)

To compute parameters of the area enclosed by the convex hull we can first convert it to a Polygon2 object:

>>> from spatialmath import Polygon2
>>> poly = Polygon2(blobs[0].perimeter_hull(), close=True)
>>> poly.area()
np.float64(11056.5)
>>> poly.moment(1, 0)  # first moment
np.float64(-4180916.3333333335)

(Source code, png, hires.png, pdf)

Note

The perimeter is not closed, that is, the first and last point are not the same.

Seealso:

plot_perimeter perimeter perimeter_approx perimeter_approx polar cv2.convexHull