title: why rgb2gray function 's output in skimage.color values is between 0 and 1?
body: <p>I have some rgb images and I want to change them to gray. I used skimage.color.rgb2gay function. my color images have values between 0 and 255 but after changing them to gray the value are between 0 and 1 and it seems they normalize. could you please tell me what is the problem? the input image's type is <code>uint8</code> and I expect the gray image has <code>unit8</code> type not <code>float64</code>! this is the code I used.</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  from skimage import data
  from skimage.color import rgb2gray
  
  original = data.astronaut()
  grayscale1 = rgb2gray(original)
  
  fig, axes = plt.subplots(1, 2, figsize=(8, 4))
  ax = axes.ravel()
  
  ax[0].imshow(original)
  ax[0].set_title("Original")
  ax[1].imshow(grayscale1, cmap=plt.cm.gray)
  ax[1].set_title("Grayscale")
  
  fig.tight_layout()
  plt.show()
  </code></pre>