How to save PNG file from NSImage (retina issues)

I'm doing some operations on images, and after I'm done, I want to save the image as PNG on the disk. I'm doing the following:

+ (void)saveImage:(NSImage *)image atPath:(NSString *)path {
        
    [image lockFocus] ;
    NSBitmapImageRep *imageRepresentation = [[NSBitmapImageRep alloc] initWithFocusedViewRect:NSMakeRect(0.0, 0.0, image.size.width, image.size.height)] ;
    [image unlockFocus] ;
        
    NSData *data = [imageRepresentation representationUsingType:NSPNGFileType properties:nil];
    [data writeToFile:path atomically:YES];
}

This code is working, but there's a problem on Macs with Retina screens; if I print the NSBitmapImageRep object, I get a different size and pixels rectangle, and when my image is saved on the disk, it's twice the size:

$0 = 0x0000000100413890 NSBitmapImageRep 0x100413890 Size={300, 300} ColorSpace=sRGB IEC61966-2.1 colorspace BPS=8 BPP=32 Pixels=600x600 Alpha=YES Planar=NO Format=0 CurrentBacking=<CGImageRef: 0x100414830>

I tried to force the pixel size to not care about the Retina scale, as I want to preserve the original size:

imageRepresentation.pixelsWide = image.size.width;
imageRepresentation.pixelsHigh = image.size.height;

This time I get the right size when I print the NSBitmapImageRep object, but when I save the file, I still get the same issue:

$0 = 0x0000000100413890 NSBitmapImageRep 0x100413890 Size={300, 300} ColorSpace=sRGB IEC61966-2.1 colorspace BPS=8 BPP=32 Pixels=300x300 Alpha=YES Planar=NO Format=0 CurrentBacking=<CGImageRef: 0x100414830>

Any idea how to fix this, and preserve the original pixel size?

If you have an NSImage and want to save it as an image file to the filesystem, you should never use lockFocus! lockFocus creates a new image which is determined for getting shown an the screen and nothing else. Therefore lockFocus uses the properties of the screen: 72 dpi for normal screens and 144 dpi for retina screens. For what you want I propose the following code:

+ (void)saveImage:(NSImage *)image atPath:(NSString *)path {

    CGImageRef cgRef = [image CGImageForProposedRect:NULL
                                             context:nil
                                               hints:nil];

    NSBitmapImageRep *newRep = [[NSBitmapImageRep alloc] initWithCGImage:cgRef];
    [newRep setSize:[image size]]; // if you want the same resolution
    NSData *pngData = [newRep representationUsingType:NSPNGFileType properties:nil];
    [pngData writeToFile:path atomically:YES];
    [newRep autorelease];
}