问iOS: Swift:如何使用CGImageCreateWithImageInRect获得合适的图像质量？

EN

 func retriveCroppedImage(){
    let yratio: CGFloat = imgviewrect.size.height / chosenImage.size.height
    let xratio: CGFloat = imgviewrect.size.width / chosenImage.size.width

    var cliprect = CGRectMake(centerpoint.x - vWidth/2, centerpoint.y - vHeight/2, vWidth, vHeight)
    print("cliprect top  \(cliprect.size)")
    cliprect.size.height =  cliprect.size.height / xratio;
    cliprect.size.width =  cliprect.size.width / xratio;
    cliprect.origin.x = cliprect.origin.x / xratio + imgviewrect.origin.x  / xratio
    cliprect.origin.y = cliprect.origin.y / yratio - imgviewrect.origin.y  / xratio

    print("cliprect  On Image \(cliprect)")

    let imageRef =  CGImageCreateWithImageInRect(chosenImage.CGImage, cliprect )

    croppedImg  = UIImage(CGImage: imageRef!, scale:  UIScreen.mainScreen().scale, orientation: chosenImage.imageOrientation)
    print("Operation complete");
}