in my Cocoa application, I load a .jpg file from disk, manipulate it. Now it needs to be written to disk as a .png file. How can you do that?
Thanks for your help!
Using CGImageDestination
and passing kUTTypePNG
is the correct approach. Here's a quick snippet:
@import MobileCoreServices; // or `@import CoreServices;` on Mac
@import ImageIO;
BOOL CGImageWriteToFile(CGImageRef image, NSString *path) {
CFURLRef url = (__bridge CFURLRef)[NSURL fileURLWithPath:path];
CGImageDestinationRef destination = CGImageDestinationCreateWithURL(url, kUTTypePNG, 1, NULL);
if (!destination) {
NSLog(@"Failed to create CGImageDestination for %@", path);
return NO;
}
CGImageDestinationAddImage(destination, image, nil);
if (!CGImageDestinationFinalize(destination)) {
NSLog(@"Failed to write image to %@", path);
CFRelease(destination);
return NO;
}
CFRelease(destination);
return YES;
}
You'll need to add ImageIO
and CoreServices
(or MobileCoreServices
on iOS) to your project and include the headers.
If you're on iOS and don't need a solution that works on Mac too, you can use a simpler approach:
// `image` is a CGImageRef
// `path` is a NSString with the path to where you want to save it
[UIImagePNGRepresentation([UIImage imageWithCGImage:image]) writeToFile:path atomically:YES];
In my tests, the ImageIO approach was about 10% faster than the UIImage approach on my iPhone 5s. In the simulator, the UIImage approach was faster. It's probably worth testing each for your particular situation on the device if you're really concerned with performance.