Using UIImagePickerController to get image — how to know whether to save PNG or JPEG?

Question

I\'ve got a UIImagePickerController letting the user pick an image out of the image library, and am getting its results via the didFinishPickingMediaWithI

Answer 1

in the dictionary the key UIImagePickerControllerReferenceURL will give you informations about the type

(gdb) po info

{
    UIImagePickerControllerMediaType = "public.image";
    UIImagePickerControllerOriginalImage = "<UIImage: 0x5cfd00>";
    UIImagePickerControllerReferenceURL = "assets-library://asset/asset.JPG?id=EFC44C3C-9C82-4669-924B-A2B9DE6F1F45&ext=JPG";
}

In this case it is a jpg as shown by ext=JPG

Answer 2

OK, so I figured it out. This may not work for every scenario, but it's sufficient for me:

    CGImageAlphaInfo imgAlpha = CGImageGetAlphaInfo(theImage.CGImage);

    // Is this an image with transparency (i.e. do we need to save as PNG?)
    if ((imgAlpha == kCGImageAlphaNone) || (imgAlpha == kCGImageAlphaNoneSkipFirst) || (imgAlpha == kCGImageAlphaNoneSkipLast)) {
         // save as a JPEG
    } else {
         // save as a PNG
    }

...of course you need to remember which type of image you saved, give it the appropriate file extension, and load the right one back in... but basically this takes care of it. Images with transparency will be saved as PNGs, everything else as JPEGs.

If anyone has any better methods, I'd love to hear them. Thanks!