title: CALayer alpha mask not working
body: <p>I'm trying to create a simple example of drawing an image to a layer, and then setting that layer's alpha mask.  I added the code below to my sample app's viewDidLoad.  without setting the mask, i can see the image of the leaf.  after i set the mask, i see nothing where my sublayer was.  what am i doing wrong?  </p>
  
  <p>Here are the two images i'm using (just samples i found on the net)
  <a href="http://sorenworlds.netfirms.com/Alpha/leaf.jpg" rel="nofollow">http://sorenworlds.netfirms.com/Alpha/leaf.jpg</a></p>
  
  <p><a href="http://sorenworlds.netfirms.com/Alpha/leaf-alfa.jpg" rel="nofollow">http://sorenworlds.netfirms.com/Alpha/leaf-alfa.jpg</a></p>
  
  <pre><code>self.view.layer.backgroundColor = [UIColor orangeColor].CGColor;
  self.view.layer.cornerRadius = 20.0;
  self.view.layer.frame = CGRectInset(self.view.layer.frame, 20, 20);
  
  
  CALayer *sublayer = [CALayer layer];
  sublayer.backgroundColor = [UIColor blueColor].CGColor;
  sublayer.shadowOffset = CGSizeMake(0, 3);
  sublayer.shadowRadius = 5.0;
  sublayer.shadowColor = [UIColor blackColor].CGColor;
  sublayer.shadowOpacity = 0.8;
  sublayer.frame = CGRectMake(30, 30, 128, 192);
  [self.view.layer addSublayer:sublayer];
  
  CGImageRef img = [UIImage imageNamed:@"leaf.jpg"].CGImage;
  sublayer.borderColor = [UIColor blackColor].CGColor;
  sublayer.borderWidth = 2.0;
  sublayer.contents = img;
  
  CGImageRef imgAlpha = [UIImage imageNamed:@"leaf_alpha.jpg"].CGImage;   
  CALayer *alphaLayer = [CALayer layer];
  alphaLayer.contents = (id)imgAlpha;
  sublayer.mask = alphaLayer;
  </code></pre>
  
  <p>Any help is greatly appreciated.  </p>