Image quality loss when reassigning image to UIImageView

I am developing an application which can tint colors in an image and export it.

I have added 3 versions of an image asset to my Assets.xcassets folder. These are 3 different sizes of the same image, namely: image1.png image1@2x.png image1@3x.png with respective sizes: 256x341, 512x683, 768x1024 pixels.

I have created a UIImageView on my storyboard called myImage and assigned image1 to myImage via storyboard->utilities->attributes inspector-> Image View -> Image.

I am trying to use the following tintWithColor function as a UIImage extension to change the color of this image.

 extension UIImage {

    func tintWithColor(color:UIColor)->UIImage {

        UIGraphicsBeginImageContext(self.size)

        let context = UIGraphicsGetCurrentContext()
        self.drawAtPoint(CGPoint(x: 0, y: 0))
        CGContextSaveGState(context)

        // flip the image
        CGContextScaleCTM(context, 1.0, -1.0)
        CGContextTranslateCTM(context, 0.0, -self.size.height)

        // multiply blend mode
        CGContextSetBlendMode(context, CGBlendMode.Multiply)

        let rect = CGRectMake(0, 0, self.size.width, self.size.height)
        CGContextClipToMask(context, rect, self.CGImage)
        color.setFill()
        CGContextFillRect(context, rect)
        CGContextRestoreGState(context)

        // create uiimage
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage   
    }
}

As I am testing this function on my viewDidLoad() to change the color of myImage.image as below, I see that (on whatever device I am using) the width and height of my originalImage is always 256 x 341,5

 override func viewDidLoad() {
    super.viewDidLoad()

    let originalImage =  myImage.image
    print("LOG: original image width: \(originalImage!.size.width) height: \(originalImage!.size.height)")
    let tintedImage = originalImage!.tintWithColor(UIColor(hue: 300/360, saturation: 0.70, brightness: 0.70, alpha: 0.6))
     myImage.image = tintedImage
    // Do any additional setup after loading the view.
}

If I don't apply these changes on my image, the quality of my image on an iPhone 6 (4,7") device looks as below:

If I apply the tintWithColor and then reassign the resulting image to my imageView the quality seems to drop, lines smoothen, as can be seen below:

Is there a way to avoid this quality loss? Eventually I will be exporting the high quality image, so I would like to apply this color tint function on the high quality version of this image.

Thank you.

UIGraphicsBeginImageContextWithOptions(CGSizeMake(self.size.width, self.size.height), false, 3.0)

should work