How to save ALAsset image to disk fast on iOS?

Question

I am using ALAsset to retrieve images like that:

[[asset defaultRepresentation] fullResolutionImage]]

This return CGImageRef which I want to save to disk as fast as possible...

Solution 1:

UIImage *currentImage = [UIImage imageWithCGImage:[[asset defaultRepresentation] fullResolutionImage]];
NSData *currentImageData = UIImagePNGRepresentation(currentImage);
[currentImageData writeToFile:filePath atomically:YES];

Solution 2:

CFURLRef url = (__bridge CFURLRef)[NSURL fileURLWithPath:filePath];
CGImageDestinationRef destination = CGImageDestinationCreateWithURL(url, kUTTypePNG, 1, NULL);
CGImageDestinationAddImage(destination, [[asset defaultRepresentation] fullResolutionImage], nil);
CGImageDestinationFinalize(destination);

The problem is that both methods are very slow performing on a device. I takes about 2 seconds per image to perform this. And this is absolutely to long.

Question: How can I speed up this image saving process? Or perhaps is there a better solution for this?

UPDATE: The best performance improvements in both solutions is to save images to JPEG format instead of PNG. So for solution 1 have replaced UIImagePNGRepresentation with UIImageJPEGRepresentation. For solution 2 have replaced kUTTypePNG with kUTTypeJPEG.

Also worth noting that second solution is way more memory efficient that first one.

Answer 1

You could just copy the raw data instead.
This has the advantages of not re-encoding the file, not making the file bigger, not losing quality through additional compression and preserving any meta data in the file. Should also be the fastest possible way.
Assuming you have theAsset and a filepath to save it to.
Should add error handling as well.

long long sizeOfRawDataInBytes = [[theAsset defaultRepresentation] size];
NSMutableData* rawData = [NSMutableData dataWithLength:(NSUInteger) sizeOfRawDataInBytes];
void* bufferPointer = [rawData mutableBytes];
NSError* error=nil;
[[theAsset defaultRepresentation] getBytes:bufferPointer 
                                fromOffset:0
                                    length:sizeOfRawDataInBytes
                                     error:&error];
if (error) 
{
    NSLog(@"Getting bytes failed with error: %@",error);
}
else 
{
    [rawData writeToFile:filepath 
              atomically:YES];
}

Answer 2

That's because of PNG compression is slow process, and takes a while on iPhone's processor, especially for full-size photography.

Answer 3

There is no such a way to speed the process up, but a possible solution would be to use threads, this way you will not block the mainthread and you will offer the user a better experience:

dispatch_queue_t dispatchQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(dispatchQueue, ^(void) {
    // Your code here
});