Get underlying NSData from UIImage

Question

I can create UIImage from NSData using [UIImage imageWithData:] or [UIImage initWithData:] methods.

I wonder i

Answer 1

You can expect that a UIImage is an object formatted for display and so won't be using the original data (which is probably in PNG or JPEG format) but more likely a pixel array or some other internal format. In other words, UIImage(data: foo) will not retain foo.

1. If you just want to use it elsewhere in your program, the original UIImage will do fine (I presume that's not actually the case here)

2. If you want to serialise, UIImagePNGRepresentation(...) will work but will be oversized if the original was a JPEG; UIImageJPEGRepresentation(...) will often result in slightly oversize data and is slightly lossy if your original was PNG. It should be okay to pick one based on the way the image will be displayed and the format you expect to be provided. If you happen to be using PNG in and want PNG out, you should get a good file size and almost identical data, special PNG chunks aside.

3. If you want to get an exact copy of the original data (perhaps to save a file after thumbnailing, or to SHA1 it), then you need to retain it separately. You might do something like:

   var image:UIImage
   var imageData:NSData {
       didSet {
           image = UIImage(data: imageData)
       }
   }