How to find top_left, top_right, bottom_left, right coordinates in 2d mask where cell has specified value?

Question

I have 2D numpy array which is a mask from an image. Each cell has 0 or 1 value. So I would like to find top:left,right, bottom:left,right in an array

Answer 1

Use transpose and nonzero from numpy, like:

im=np.array([[0,0,0,0,0],
[0,1,1,1,0],
[0,1,1,0,0],
[0,0,0,0,0]])

print(np.transpose(np.nonzero(im)))

yields:

array([[1, 1],
       [1, 2],
       [1, 3],
       [2, 1],
       [2, 2]])

Update: Still not perfect, but as long as the mask is continuous within its rows, you could evaluate np.diff() to get an idea where the 0->1 and 1->0 transitions are:

leftedge=np.transpose(np.nonzero(np.diff(im,prepend=0)==1))
rightedge=np.transpose(np.nonzero(np.diff(im,append=0)==-1))

top_left     = leftedge[0]
bottom_left  = leftedge[-1]
bottom_right = rightedge[-1]
top_right    = rightedge[0]

pts=[list(x) for x in [top_left,top_right,bottom_left,bottom_right]]

yields: [[1, 1], [1, 3], [2, 1], [2, 2]]

I'd suggest to use Chris' answer instead.