title: How do I create an alpha only bitmap context
body: <p>Apple tech note <a href="http://developer.apple.com/library/mac/#qa/qa1037/_index.html">QA1037</a> says "To create an alpha-only bitmap context simply pass NULL for the colorspace parameter."</p>
  
  <p>However, I am not sure what to pass for the <code>bitmapInfo</code> parameter. </p>
  
  <p>I was trying something like this, with kCGImageAlphaOnly: </p>
  
  <pre><code>CGContextRef ctx = CGBitmapContextCreate(NULL, scaledSize.width, scaledSize.height, 8, scaledSize.width, NULL, kCGImageAlphaOnly);
  </code></pre>
  
  <p>But this gives me an warning about the enum being wrong. What should I put there instead?</p>