iPhone saved png is different after I load it back

Question

I want to modify a .png image and save it to a folder.

1. convert the uiimage to NSData
2. modify some pixel
3. create a new UIImage from the modified NSData
4. save this UIImage to a folder
5. set this same UIImage to an ImageView's image property

If I check the UIImage's image, the pixel data is correct, but if I load the saved image, the pixel data is a little different from the modified value.

For example:

• 107 -76 -43 -1 //one pixel from the original image in RGBA888 format
• 93 -89 -56 -1 // the modified pixel value
• 93 -89 -56 -1 //the pixel value, after I set it to the imageView's image and read it back
• 95 -91 -57 -1 //the pixel value, after I load it back from the stroage

My image saving method:

- (void) saveImage: (UIImage*) image toFolder: (NSString*) folder

{

[self.library saveImage: image toAlbum: folder withCompletionBlock:^(NSError *error) {
    if (error!=nil) {
        NSLog(@"Error: %@", [error description]);

        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error"
                                                        message: [error description]
                                                       delegate:nil
                                              cancelButtonTitle:@"OK"
                                              otherButtonTitles:nil];
        [alert show];
    }else{
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Image saved!"
                                                        message: nil
                                                       delegate:nil
                                              cancelButtonTitle:@"OK"
                                              otherButtonTitles:nil];
        [alert show];
    }
}];

}

The pixel modification method:

CGImageRef cRef = CGImageRetain(_imageView.image.CGImage);
NSData* pixelData = (NSData*) CFBridgingRelease(CGDataProviderCopyData(CGImageGetDataProvider(cRef)));
// image raw data    
signed char* pixelBytes = (signed char *)[pixelData bytes];

// MODIFY SOME PIXEL HERE

NSData* newPixelData = [NSData dataWithBytes:pixelBytes length:[pixelData length]];
// cast NSData as CFDataRef
CFDataRef imgData = (CFDataRef)CFBridgingRetain(newPixelData);
// Make a data provider from CFData
CGDataProviderRef imgDataProvider = CGDataProviderCreateWithCFData(imgData);

// get PNG properties from cRef
size_t width = CGImageGetWidth(cRef);
size_t height = CGImageGetHeight(cRef);
size_t bitsPerComponent = CGImageGetBitsPerComponent(cRef);
size_t bitsPerPixel = CGImageGetBitsPerPixel(cRef);
size_t bytesPerRow = CGImageGetBytesPerRow(cRef);
CGColorSpaceRef colorSpace = CGImageGetColorSpace(cRef);
CGBitmapInfo info = CGImageGetBitmapInfo(cRef);
CGFloat *decode = NULL;
BOOL shouldInteroplate = NO;
CGColorRenderingIntent intent = CGImageGetRenderingIntent(cRef);

// cRef PNG properties + imgDataProvider's data
CGImageRef throughCGImage = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpace, info, imgDataProvider, decode, shouldInteroplate, intent);
CGDataProviderRelease(imgDataProvider);
// make UIImage with CGImage
UIImage* newImage = [UIImage imageWithCGImage:throughCGImage];
CGImageRelease(throughCGImage);

[self saveImage:newImage toFolder:@"DEMOFOLDER"];

_imageView.image = newImage;

Answer 1

My bad, the iOS default saves the image as JPG.