The following code block shows how gray-level opening and closing can remove salt-and-pepper noise from a grayscale image, and how the successive application of opening and closing removes salt-and-pepper (impulse) noise from the input, a noisy mandrill grayscale image:

from scipy import ndimage
im = rgb2gray(imread('../images/mandrill_spnoise_0.1.jpg'))
im_o = ndimage.grey_opening(im, size=(2,2))
im_c = ndimage.grey_closing(im, size=(2,2))
im_oc = ndimage.grey_closing(ndimage.grey_opening(im, size=(2,2)), size=(2,2))
pylab.figure(figsize=(20,20))
pylab.subplot(221), pylab.imshow(im), pylab.title('original', size=20), pylab.axis('off')
pylab.subplot(222), pylab.imshow(im_o), pylab.title('opening (removes salt)', size=20), pylab.axis('off')
pylab.subplot(223), pylab.imshow(im_c), pylab.title('closing (removes pepper)', size=20),pylab.axis('off')
pylab.subplot(224), pylab.imshow(im_oc), pylab.title('opening + closing (removes salt + pepper)', size=20)
pylab.axis('off')
pylab.show()

The following shows the output of the previous code—how opening and closing removes the salt and the pepper noise from the grayscale noisy image of a mandrill: