title: Create an image with pypng
body: <p>I'm writing a png image to file using PyPng, the file is created but it's all black when viewed.Here is my code:</p>
  
  <pre><code>pngImage = numpy.uint16(numpy.zeros((NUM_ROWS, NUM_COLS)))
  #code that assigns the pixels
  with open(dataFile + ".png", "wb") as outFile:
          pngWriter = png.Writer(NUM_COLS, NUM_ROWS, greyscale=True, bitdepth=16)
          pngWriter.write(outFile, pngImage)
  </code></pre>
  
  <p>I'm certain that the code that assigns the pixels works fine. I put a break point on the write call and verified that pngImage has nonzero values.
  pngImage is a 2D array with integer value for each pixel (not R,G,B tuple).</p>
  
  <p>Am I missing something? Should I be converting the numpy array or something?</p>