The solution to do UIImage flipping is with the Objective-C code:
[UIImage imageWithCGImage:img.CGImage scale:1.0 orientation: UIImageOrientationDownMirrored
For me the simplest way was to use the .withHorizontallyFlippedOrientation() instance method of UIImage as follows:
.withHorizontallyFlippedOrientation()
UIImage
let flippedImage = straightImage.withHorizontallyFlippedOrientation()
Simple one-liners always make me happy :)