title: Image.fromarray just produces black image
body: <p>I'm trying to save a numpy matrix as a grayscale image using <code>Image.fromarray</code>. It seems to work on a random matrix, but not on a particular one (where there should appear a circle). Can anyone explain what I'm doing wrong?</p>
  
  <pre><code>from PIL import Image
  import numpy as np
  radius = 0.5
  size = 10
  x,y = np.meshgrid(np.linspace(-1,1,size),np.linspace(-1,1,size))
  f = np.vectorize(lambda x,y: ( 1.0 if x*x + y*y &lt; radius*radius else 0.0))
  z = f(x,y)
  print(z)
  zz = np.random.random((size,size))
  img = Image.fromarray(zz,mode='L') #replace z with zz and it will just produce a black image
  img.save('my_pic.png')
  </code></pre>