title: Blending V.S. offscreen-rendering, which is worse for Core Animation performance?
body: <p>Blending and offscreen-rendering are both expensive in Core Animation.</p>
  
  <p>One can see them in Core Animation instrument in Instruments, with Debug Options:
  <img src="https://i.stack.imgur.com/hYfsE.png" alt="Instruments"></p>
  
  <p>Here is my case:</p>
  
  <p>Display 50x50 PNG images on UIImageViews. I want to round the images with a 6-point corer radius. The first method is to set UIImageView.layer's cornerRadius and masksToBounds which causes offscreen-rendering. The second method is to make PNG image copies with transparent corners which causes blending(because of the alpha channel).</p>
  
  <p>I've tried both, but I can't see significant performance difference. However, I still want to know which is worse in theory and best practices if any.</p>
  
  <p>Thanks a lot!</p>