How to save PNG file from NSImage (retina issues)

NSImage is resolution aware and uses a HiDPI graphics context when you lockFocus on a system with retina screen.
The image dimensions you pass to your NSBitmapImageRep initializer are in points (not pixels). An 150.0 point-wide image therefore uses 300 horizontal pixels in a @2x context.

You could use convertRectToBacking: or backingScaleFactor: to compensate for the @2x context. (I didn't try that), or you can use the following NSImage category, that creates a drawing context with explicit pixel dimensions:

@interface NSImage (SSWPNGAdditions)

- (BOOL)writePNGToURL:(NSURL*)URL outputSizeInPixels:(NSSize)outputSizePx error:(NSError*__autoreleasing*)error;

@end

@implementation NSImage (SSWPNGAdditions)

- (BOOL)writePNGToURL:(NSURL*)URL outputSizeInPixels:(NSSize)outputSizePx error:(NSError*__autoreleasing*)error
{
    BOOL result = YES;
    NSImage* scalingImage = [NSImage imageWithSize:[self size] flipped:NO drawingHandler:^BOOL(NSRect dstRect) {
        [self drawAtPoint:NSMakePoint(0.0, 0.0) fromRect:dstRect operation:NSCompositeSourceOver fraction:1.0];
        return YES;
    }];
    NSRect proposedRect = NSMakeRect(0.0, 0.0, outputSizePx.width, outputSizePx.height);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
    CGContextRef cgContext = CGBitmapContextCreate(NULL, proposedRect.size.width, proposedRect.size.height, 8, 4*proposedRect.size.width, colorSpace, kCGBitmapByteOrderDefault|kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colorSpace);
    NSGraphicsContext* context = [NSGraphicsContext graphicsContextWithGraphicsPort:cgContext flipped:NO];
    CGContextRelease(cgContext);
    CGImageRef cgImage = [scalingImage CGImageForProposedRect:&proposedRect context:context hints:nil];
    CGImageDestinationRef destination = CGImageDestinationCreateWithURL((__bridge CFURLRef)(URL), kUTTypePNG, 1, NULL);
    CGImageDestinationAddImage(destination, cgImage, nil);
    if(!CGImageDestinationFinalize(destination))
    {
        NSDictionary* details = @{NSLocalizedDescriptionKey:@"Error writing PNG image"};
        [details setValue:@"ran out of money" forKey:NSLocalizedDescriptionKey];
        *error = [NSError errorWithDomain:@"SSWPNGAdditionsErrorDomain" code:10 userInfo:details];
        result = NO;
    }
    CFRelease(destination);
    return result;
}

@end

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];
}

I found this code on web , and it works on retina. Paste here, hope can help someone.

NSImage *computerImage = [NSImage imageNamed:NSImageNameComputer];
NSInteger size = 256;

NSBitmapImageRep *rep = [[NSBitmapImageRep alloc]
                  initWithBitmapDataPlanes:NULL
                                pixelsWide:size
                                pixelsHigh:size
                             bitsPerSample:8
                           samplesPerPixel:4
                                  hasAlpha:YES
                                  isPlanar:NO
                            colorSpaceName:NSCalibratedRGBColorSpace
                               bytesPerRow:0
                              bitsPerPixel:0];
[rep setSize:NSMakeSize(size, size)];

[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:[NSGraphicsContext     graphicsContextWithBitmapImageRep:rep]];
[computerImage drawInRect:NSMakeRect(0, 0, size, size)  fromRect:NSZeroRect operation:NSCompositeCopy fraction:1.0];
[NSGraphicsContext restoreGraphicsState];

NSData *data = [rep representationUsingType:NSPNGFileType properties:nil]; 

Just incase anyone stumbles up on this thread. Here is certainly flawed solution that does the job of saving image at 1x size (image.size) regardless of device in swift

public func writeToFile(path: String, atomically: Bool = true) -> Bool{

    let bitmap = NSBitmapImageRep(bitmapDataPlanes: nil, pixelsWide: Int(self.size.width), pixelsHigh: Int(self.size.height), bitsPerSample: 8, samplesPerPixel: 4, hasAlpha: true, isPlanar: false, colorSpaceName: NSDeviceRGBColorSpace, bytesPerRow: 0, bitsPerPixel: 0)!
    bitmap.size = self.size

    NSGraphicsContext.saveGraphicsState()

    NSGraphicsContext.setCurrentContext(NSGraphicsContext(bitmapImageRep: bitmap))
    self.drawAtPoint(CGPoint.zero, fromRect: NSRect.zero, operation: NSCompositingOperation.CompositeSourceOver, fraction: 1.0)
    NSGraphicsContext.restoreGraphicsState()

    if let imagePGNData = bitmap.representationUsingType(NSBitmapImageFileType.NSPNGFileType, properties: [NSImageCompressionFactor: 1.0]) {
        return imagePGNData.writeToFile((path as NSString).stringByStandardizingPath, atomically: atomically)
    } else {
        return false
    }
}