title: Swift 2&3: How to Use CGBitmapContextCreate
body: <p>I have the following code:</p>
  
  <pre><code>let ref:CGContextRef = CGBitmapContextCreate(nil , 200, 130, CGImageGetBitsPerComponent(self.CGImage), 0, CGImageGetColorSpace(self.CGImage), CGImageGetBitmapInfo(self.CGImage))
  </code></pre>
  
  <p>The problem is that I get this error:</p>
  
  <p>Expression does not conform to type 'NilLieralConvertible'</p>
  
  <p>What should I pass in instead of nil for the first argument and why?</p>