title: Gaussian filters create holes in histogram
body: <p>I'm using a simple gaussian filter to blur the image in the MWE below</p>
  
  <pre><code>import imageio
  import matplotlib.pyplot as plt
  import numpy as np
  from scipy.ndimage.filters import gaussian_filter
  
  image = imageio.imread('imageio:camera.png')
  image_blurred = np.copy(image).astype(np.float)
  image_blurred /= 255
  image_blurred = gaussian_filter(image_blurred, sigma=3)
  image_blurred *= 255
  image_blurred = np.round(image_blurred).astype(np.uint8)
  
  plt.close('all')
  fig, ax = plt.subplots(2,2, figsize=(6,8))
  
  ax[0][0].imshow(image, cmap='gray')
  ax[0][0].set_title('Original')
  ax[1][0].hist(image.ravel(), 256, density=True)
  
  ax[0][1].imshow(image_blurred, cmap='gray')
  ax[0][1].set_title('Blur 3')
  ax[1][1].hist(image_blurred.ravel(), 256, density=True)
  plt.show()
  </code></pre>
  
  <p>The problem is, the image after a gaussian filter shows some holes in historgram as we can see below in the histogram on right</p>
  
  <p><a href="https://i.stack.imgur.com/wxKBG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wxKBG.png" alt="enter image description here"></a></p>
  
  <p>Is that expected? If so which operation can I apply on image to smooth the histogram ?</p>