Convex Hull of a given value inside an numpy array - python 2.7

Question

For a given array (as the one suggested below) and a given value (here, 0), I would like to count how many 0 can be associate at a same convex hull.

array :

1 2 4 5 8 9 7
4 0 0 7 5 6 8
6 5 0 4 3 5 2
1 0 0 5 7 0 6
2 3 5 7 8 9 4

For the array suggested, the solution should get : [5, 1] because a first "pattern" of five 0 can be identify between the second and fourth line and the second/third columns. A second pattern with only one 0 exist.

Have you any idea about how to get that ? I know how to count the number of 0. Maybe I can do something with the mask option of numpy array ?

Best regards and thank's for your help,

I think that by "convex hull" you really mean "connected component". If I understand correctly, the first group of 5 zeros would not be a convex hull because the 5 lies between two elements of the group. — Peter
– Peter, Commented Jun 6, 2018 at 8:51
@Peter yes "connected component" is better for what I want to do. So yes, the is a pattern of five 0 connected together and a pattern of one 0. How can I quantify these connected component ? — Lionel
– Lionel, Commented Jun 6, 2018 at 8:55
I found some informations on internet with "connected component" keyword, thank's ! — Lionel
– Lionel, Commented Jun 6, 2018 at 8:57

Peter · Accepted Answer · 2018-06-06 09:16:32Z

2

What you're looking for is the "connected components" in the array. Scipy has a convenient label function for this:

import numpy as np
from scipy.ndimage.measurements import label

a = np.array([
    [1, 2, 4, 5, 8, 9, 7],
    [4, 0, 0, 7, 5, 6, 8],
    [6, 5, 0, 4, 3, 5, 2],
    [1, 0, 0, 5, 7, 0, 6],
    [2, 3, 5, 7, 8, 9, 4],])

labs, n_components = label(a==0)

component_sizes = [np.sum(labs==i) for i in range(1, n_components+1)]

print(component_sizes)

Prints [5, 1]

edited Jun 6, 2018 at 9:16

answered Jun 6, 2018 at 9:06

Peter

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1 Comment

Lionel Over a year ago

Oh thank you so much. I was looking on this topic : stackoverflow.com/questions/46737409/… I was looking how to count the number of component on each labels ! Thank's for your answer, it's working like a charm ! :)

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