title: problems low contrast image(contrast stretching) in matplotlib
body: <p>When reading a low-contrast image it automatically take this example:</p>
  
  <pre><code>In [1]: from PIL import Image
  In [2]: import numpy as np
  In [3]: import matplotlib.pyplot as plt
  In [4]: img = Image.open('images/map.jpg')
  In [5]: arr = np.asarray(img)
  In [6]: plt.gray()
  In [7]: plt.imshow(arr)
  Out[7]: &lt;matplotlib.image.AxesImage at 0x7f9c7e88f490&gt;
  In [8]: plt.show()
  </code></pre>
  
  <p>Input</p>
  
  <p><img src="https://i.imgur.com/IJRfl2bs.png" alt="input"> <a href="https://i.imgur.com/IJRfl2b.png" rel="nofollow noreferrer">large</a></p>
  
  <p>Plot (no change this is automatic by matplotlib.)</p>
  
  <p><img src="https://i.imgur.com/lbCn1SDs.jpg" alt="plot"> <a href="https://i.imgur.com/lbCn1SD.jpg" rel="nofollow noreferrer">large</a></p>
  
  <p>Because this input is different from the plot without, modifying anything.</p>
  
  <p>I need low-contrast image to implement an algorithm to contrast stretching</p>
  
  <p>Reading the book <a href="http://amzn.com/013168728X" rel="nofollow noreferrer">amazon</a> Digital Image Processing (Rafael C. Gonzalez, Richard E. Woods)</p>
  
  <p>PS: matplotlib is <strong>converting</strong> the automatic. I do not need. </p>