title: Convert RGBA-image to black and white image in Tensorflow
body: <p><strong>tf.image.decode_png()</strong> can output grayscale, RGB and RGBA image.</p>
  
  <p>But I'd like to convert RGBA to pure black and white in <strong>Tensorflow</strong> (without using <strong>pillow</strong>).</p>
  
  <p>Please give me some advice.</p>