Resizing an NSImage on retina Macs for output as a 1x image

gistfile1.txt

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];

I want to know how to get a 2x image as also.