The erosion operation can be used to extract the boundary of a binary image—we just need to subtract the eroded image from the input binary image to extract the boundary. The following code block implements this:

from skimage.morphology import binary_erosion
im = rgb2gray(imread('../images/horse-dog.jpg'))
threshold = 0.5
im[im < threshold] = 0
im[im >= threshold] = 1
boundary = im - binary_erosion(im)
plot_images_horizontally(im, boundary, 'boundary',sz=(18,9))

The following screenshot shows the output of the previous code block: