UIImage的质量

Question

I am resizing various images with this func:

    func resizeImage(image: UIImage) -> UIImage {
        var actualHeight: CGFloat = image.size.height
        var actualWidth: CGFloat = image.size.width

        let maxHeight: CGFloat = 600.0
        let maxWidth: CGFloat = text.frame.size.width - 10

        var imgRatio: CGFloat = actualWidth / actualHeight
        let maxRatio: CGFloat = maxWidth / maxHeight
        let compressionQuality: CGFloat = 0.5
        //50 percent compression
        if actualHeight > maxHeight || actualWidth > maxWidth {
            if imgRatio < maxRatio {
                //adjust width according to maxHeight
                imgRatio = maxHeight / actualHeight
                actualWidth = imgRatio * actualWidth
                actualHeight = maxHeight
            }
            else if imgRatio > maxRatio {
                //adjust height according to maxWidth
                imgRatio = maxWidth / actualWidth
                actualHeight = imgRatio * actualHeight
                actualWidth = maxWidth
            }
            else {
                actualHeight = maxHeight
                actualWidth = maxWidth
            }
        }
        let rect: CGRect = CGRectMake(0.0, 0.0, actualWidth, actualHeight)
        UIGraphicsBeginImageContext(rect.size)
        image.drawInRect(rect)
        let img: UIImage = UIGraphicsGetImageFromCurrentImageContext()
        let imageData: NSData = UIImageJPEGRepresentation(img, compressionQuality)!
        UIGraphicsEndImageContext()

        return UIImage(data: imageData)!
    }

But the quality is just terrible....Heres a picture of what I get bellow:

I thought the quality of the image varied on what the compressionQuality was...? I have it at 0.5 now as you can see from my code above, but even if I change it to 1 the quality is still terrible?

Any ideas,

Many thanks.

Answer 1

You should use UIGraphicsBeginImageContextWithOptions(rect.size, false, 0.0) instead. It will use the correct scale for your device. The version you're using (without options), uses a default scale factor of 1.0 . Using the ...WithOptions() variant and passing in 0.0 for the scale, defaults to the native scale of the device. Note that this might not be what you want.

Secondly, you can use AVMakeRectWithAspectRatioInsideRect() to calculate your target size. Thirdly, you don't need to go through the JPEG encoding, you can return the generated image directly.

Combined, this greatly simplifies your code:

func resizeImage(image: UIImage) -> UIImage {
  let maxSize = CGSize(
    width: text.frame.size.width - 10, 
    height: 600.0
  )
  let newSize = AVMakeRectWithAspectRatioInsideRect(
    image.size, 
    CGRect(origin: CGPointZero, size: maxSize)
  ).size
  UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0)
  image.drawInRect(CGRect(origin: CGPointZero, size: newSize))
  let scaled = UIGraphicsGetImageFromCurrentImageContext()
  UIGraphicsEndImageContext()
  return scaled
}

EDIT: This is an extension on UIImage that I use in a project:

extension UIImage {
    /**
     Returns an scaled version of self that fits within the provided bounding box.
     It retains aspect ratio, and also retains the rendering mode.
     - parameter size: The target size, in point
     - parameter scale: Optional target scale. If omitted, uses the scale of input image
     - returns: A scaled image
     */
    func imageFittingSize(size: CGSize, scale: CGFloat? = nil) -> UIImage {
        let newSize = AVMakeRectWithAspectRatioInsideRect(self.size, CGRect(origin: CGPointZero, size: size)).size
        UIGraphicsBeginImageContextWithOptions(newSize, false, scale ?? self.scale)
        self.drawInRect(CGRect(origin: CGPointZero, size: newSize))
        let scaled = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return scaled.imageWithRenderingMode(renderingMode)
    }
}

You can call it on an image like so:

let maxSize = CGSize(width: text.frame.size.width - 10, height: 600.0)
let scaled = image.imageFittingSize(maxSize)